Talk:Joule–Thomson effect

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Why is there no citation of the paper which described the effect originally? It is mentioned in the beginning of the article but not cited. I think the paper may have been: "On the Thermal Effects experienced by Air in rushing through small Apertures" published to The London, Edinburgh and Dublin Philosophical Magazine, Ser, 1852. Perhaps someone more motivated than I could find either a full text or an ISBN number for the magazine. (talk) 19:19, 14 September 2008 (UTC)

The first responder to someone's new comment should enter the response just beneath the new comment (instead of using the above + tab) and indent the response by starting with a colon like this :. Any second responder, indent further by starting with two colons like this :: and any third responder, start with three colons like this ::: and so forth. If we don't follow these practices, the result is jumbled mess.

I don't really see any connection between this article on the Joule-Thomson effect and your Hydrogen WikiProject. But, to each his own I guess. - mbeychok 23:08, 4 September 2006 (UTC)

Speaking as someone new to this topic, the "proof" seems imcomplete. After appling the first law, the proof states that it implies constant enthalpy. This seems more like a statement than a proof. Maybe it is obvious to those schooled in the art, but that article should be written for those who are not. Any chance a qualified person could add a few lines to bridsge the gap? —Preceding unsigned comment added by (talk) 12:29, 31 October 2008 (UTC)

Enthalpy is defined as E + P V. You can rewrite the equation as

E1 + P1 V1 = E2 + P2 V2


H1 = H2

Count Iblis (talk) 13:44, 31 October 2008 (UTC)

Reasons for revision[edit]

My initial rewrite was in a form suitable for a thermodynamics textbook. I have completely rewritten it in an attempt to make it suitable for an encyclopedia. Retired Pchem Prof (talk) 17:31, 12 January 2016 (UTC)

I rewrote the section on "Physical mechanism". The first paragraph was OK and I left it in. The second dealt with an iso-energetic process, not an iso-enthalpic one, so I deleted much of it; what remains is in the 4th paragraph of the revised section. I deleted the 3rd and 4th paragraphs. They were confusing, unhelpful, and almost certainly wrong. I need to add links to the new text; I will do that when I figure out how. Retired Pchem Prof (talk) 21:15, 9 January 2016 (UTC)

The article was extensively revised for a number of reasons, including:

  • It did not differentiate between real gases and ideal gases which is necessary.
  • It repeatedly used the terms "free expansion" and "freely expanded" without a precise definition of what was meant.
  • It used the term "adiabatic" quite loosely and failed to explain that the Joule-Thomson effect was a contant enthalpy (i.e., enthalpic) process which applies only to real gases (as differentiated from ideal gases.
  • It contained some rather involved and tedious paragraphs purportedly explaining the Joule-Thomson effect in terms of molecular collisions and intermolecular attractive electromagnetic forces. I don't think such explanations are necessary and would simply confuse the large majority of readers (who are not physicists or thermodynamicists). I have left that explanation as a comment section on the Edit page in case someone wants to try and make it simpler and more understandable.
  • Some parts of the discussion were re-arranged so as to hopefully improve the flow and readability of the article.
  • Some links were added to the "See also" section and a reference book was added to the "Bibliography".
  • Some other minor typos were fixed.

I hope that the overall revision has resulted in a much improved article. - mbeychok 01:36, 11 June 2006 (UTC)

Thanks for making the improvements. I strongly disagree with the removal of the explanation of the effect however. What is the point of an article on a physical phenomenon that doesn't explain the reason behind it? Rracecarr 19:59, 11 June 2006 (UTC)
Rracecarr: I am not going to delete your reversion of the explanation of the physical mechanism. However, I would like to discuss some changes that you might make. But first let me say that, in my opinion, most readers of this article will be seeking information as to how to use the Joule-Thomson effect. Only a few will be seeking the physics explanation of the J-T effect. With that in mind, here are some changes/improvements that I would suggest:
  • Quoting from the physical mechanism writeup: In the process of free expansion, work is done against intermolecular attractive forces, .... The article has earlier explained that the J-T effect involves an isenthalpic expansion (with no heat transferred into or from the gas and no work extracted from the gas) and here the reader sees "... free expansion, work is done against ..." which might confuse him/her, don't you think? Could you somehow eliminate that possible confusion by explaining what you mean by a free expansion (which I frankly don't understand myself) and explaining how doing work against intermolecular attractive forces does not constitute extraction of work from the gas? Again, keep in mind the readers who are not advanced physics students.
  • Quoting again from the physical mechanism writeup: As density decreases, there is a drop .... The question here is why does the density decrease? Could you change that to read "As density decreases because the gas pressure decreases during expansion of a gas, there is a drop..."?
  • A final quote: ... dominates, and free expansion causes .... Again, the use of free expansion with no explanation. Wouldn't that sentence be just as true without the word "free"?
If you could accomplish the above suggestions, I think the physical mechanism explanation would be much improved. One final trivial point on another subject. You deleted my link of the words "real gas" to an article on "ideal gas" because presumably you thought it incorrect to use the ideal gas article to explain the meaning of real gas. The reason I used that link was that after a lengthy search through Wikipedia, surprisingly that was the only article I could find that even discussed the difference between an ideal gas and a real gas. I have threfore replaced that link even though admittedly it would be better if there were an article devoted to "real gas". In point of fact, when the Wikipedia search function is used on "Real gas", it is re-directed to "Ideal gas". Regards, mbeychok 21:07, 11 June 2006 (UTC)
I have re-written the section in an attempt to address your concerns. Let me know what you think. Rracecarr 09:46, 12 June 2006 (UTC)
Rracecarr: I think your re-write is excellent. Thanks for the cooperation. - mbeychok 15:55, 12 June 2006 (UTC)

Need link to Refrigeration[edit]

Hello folks. This is a nice article, but shouldn't it have some link to the general article on refrigeration? After all, that is what the Joule-Thomson effect is used for in practice. Thanks.

You're right, it does need a link so I added it in the "See also" section. In the future, PLEASE provide a heading for your discussions (which I added for you) and also sign with your name and the date. Thanks, mbeychok 08:12, 17 November 2006 (UTC)
What it really needs is a link to an article on the Linde process. But it seem there is no such article, and I am not qualified to write it. Retired Pchem Prof (talk) 17:29, 19 January 2016 (UTC)
Turns out there is an article on the Linde process, but using a name I never heard before (Hampson–Linde cycle). I put that link in, along with some other refrigeration related links. Retired Pchem Prof (talk) 15:25, 2 February 2016 (UTC)
User:Tevildo has kindly created two new titles: Linde process and Linde cycle. (They both lead the reader to the article on the Hampson-Linde cycle.) Dolphin (t) 09:41, 11 February 2016 (UTC)
Thanks for the mention! Just a question of converting a couple of piped links to proper redirects. Tevildo (talk) 19:13, 11 February 2016 (UTC)

Why the word "fluid" added by User: was deleted[edit]

The Joule-Thomson effect applies primarily to non-ideal real gases. To a great many people, the word "fluid" means a liquid (rather than a real gas) and therefore I felt that it should be deleted so as not to mislead many people. - mbeychok 16:11, 1 May 2007 (UTC)

Why I am reverting Rracecarr's wording changes again[edit]

I think that perhaps you are confusing the noun heat with the verb heat. Yes, there is no heat (the noun) transferred during a Joule-Thomson expansion. But in those cases where the Joule-Thomson expansion results in raising the temperature, the gas has been heated (the verb).

I have looked in 3 different dictionaries and they all say that the verb "heat" means to make warm or hot, or become warm or hot. Raising the temperature makes the gas warmer or hotter and therefore the verb "heat" is just as appropriate as the verb "warm" ... and your statement that it is obviously inapproprate to use the verb "heat" is incorrect.

To the vast majority of people, when you raise the temperature of something you have heated (verb) that something ... whether or not you added any heat (noun) in order to raise the temperature of that something. Therefore, I am reverting your changes. Regards, mbeychok (talk) 03:30, 11 March 2008 (UTC)

To heat means to add heat to. Of course people use the word in a sloppy way in non-technical settings. But I think an article on the Joule-Thompson effect, which your "vast majority of people" have never heard of, qualifies as a technical setting. Anyway, why introduce the confusion? "Warms" has the same meaning you claim "heats" does, and does not imply that the energy content has changed. Please stop being unreasonable. Rracecarr (talk) 19:18, 11 March 2008 (UTC)
I am going to stop, but only because I don't have the time or inclination to engage in a reversion war with someone who is too stubborn to listen to reason. I did not start the confusion ... you started the confusion when you first reverted my original use of the verb heat. As I pointed out above, the verb heat means to raise the temperature whether or not heat is added. But you simply dismiss dictionary definitions as "sloppy" and "non-technical". I suggest you read the Wiktionary definition at which clearly defines the verb heat as: To cause an increase in temperature of an object or space; to cause something to become hot. Perhaps you should also re-write that Wiktionary definition??
In the book Chemical Thermodynamics:Principles and Applications(2000), ISBN 978-0-12-530990-5, on page 141, is this wording: ... is negative at high temperatures and pressures. Therefore, a gas heats up as it expands under these conditions. Is that book's wording also "sloppy" and "non-technical"?
From the looks of your user Talk page (which you have "conveniently" deleted against Wikipedia policy but which is still available in your Talk page history), you have been involved in editing wars a few times before. It is people with attitudes such as yours who drive people away from Wikipedia. I strongly suggest that you supplement your degree in oceanography with a course in grammar. Have a nice day, mbeychok (talk) 21:38, 11 March 2008 (UTC)
Wow. Where to start. Let's see,
  1. The wording I objected to was "heats", not "heats up", which is better, but not a good as "warms".
  2. YOU started the confusion (read: edit war) by pointlessly reverting by edit. Mine was not a revert as you claim, since it didn't say "warms" before.
  3. Clearing my talk page is certainly not against wikipedia policy. Archiving is "preferred" but it's not worth the trouble.
  4. Your own grasp of grammar makes your jab a bit ironic.
I notice you claim on your user page to have "rewritten completely and expanding" this article, a claim which the history of the page doesn't back up. I also notice that you rated the article as "A" class (which it's not), and the importance within physics as "high" (which is laughable). I am glad you've stopped reverting my edits. I will only add that if you think there's a problem with my attitude, perhaps you should consider discussing good faith edits before reverting in the future. Rracecarr (talk) 22:16, 11 March 2008 (UTC)
Wow, indeed! I note you have no answer for the either the Wiktionary definition or the quote from Chemical Thermodynamics:Principles and Applications(2000), ISBN 978-0-12-530990-5, other than saying that you find a distinction between "heats" and "heats up". Now you are grasping at straws, aren't you?
As I said before, you "conveniently" deleted your Talk page probably to hide your history of edit warring, and now you claim "its not worth the trouble" to archive it.
As for my early participation in this article, the editing I performed on June 10th, 2006 was indeed a major rewrite and expansion. In fact, if you scroll up on this page, you will find that you commented on June 11th, 2006 and you thanked me for making the improvements ... and then we went on to work out the revision of your write-up on the physical mechanism. Have you forgotten that?
You may have the last word if you so wish. As for me, I have wasted enough time on this already and you are still unwilling to admit that changing "heats" to "warms" is just silly, pedantic nonsense. mbeychok (talk) 00:04, 12 March 2008 (UTC)

What a wonderful article![edit]

Usually people just read various WikiPedia articles, and some are OK, others are meh, and when they read a masterpiece like this, they are happy but noone bothers to comment. Well, I decided this one was GREAT and wanted to give props to the author(s). The pedagogic methodology exhibited here is of the finest quality. I wish I knew what other material the author(s) has/have contributed. If anyone who reads this knows the author(s), please kindly extend my compliments. —Preceding unsigned comment added by (talk) 01:00, 14 March 2008 (UTC)

Thanks for the kind words. Read my user page at User:Mbeychok to learn more about me. mbeychok (talk) 02:35, 14 March 2008 (UTC)


I fixed the typography, them moved the article again, to use the en dash to clarify the Joule and Thomson are two equal names joined, as opposed to a compound last name, which would use a hyphen. This is how it appears in carefully edited book (like this and this and this), but like most such subtleties, it probably appears wrong more often than right. Does anyone think this is not the best way to do it? Dicklyon (talk) 20:32, 15 March 2008 (UTC)

I have reverted the changes to the article, but not the move. There was at least one typo introduced, a missing letter I think, and a bunch of unnecessary blank space added. The title should probably either be changed back, or the hyphens should be changed back to dashes (but without reintroducing the typos). As I indicated in the edit summary, I don't have a strong preference, but a quick web search definitely seems to turn up far more hyphens than dashes. Rracecarr (talk) 20:54, 16 March 2008 (UTC)
Sorry about the dropped letter typo; fixed. It's good that you don't care about the typography, as that means it will OK to do it correctly. And I already stated that it is more commonly done wrong than right, but that's not a good reason to avoid the useful cue that doing it right provides to literate readers. You are correct that blank lines before paragraphs are not always necessary, but they are always allowed, and make it much easier to spot headings when editing. Dicklyon (talk) 21:31, 16 March 2008 (UTC)
Looks ok to me... Rracecarr (talk) 23:26, 16 March 2008 (UTC)

Reasons for B class[edit]

This one is much better than Henry's Law, so list won't be as long. I won't relist the FA criteria, other than mention an A class articles should be of Feature-quality (and by I mean as good as it can be, not whether or not it could achieve FA status).

The things that are missing before it can be considered A-Class

  • An engaging lead, with perhaps a few historical details such as time of discovery, typical occurrences of JT Effect (and which fields uses it and to what end)
  • Fix appearance problems.
  • Some sections seems to be in written in bullet "list" format, without being bullet lists (especially "Description").
  • Useless words (on the other hand), and prose (Expanded in this manner, though the temperature of an ideal gas would remain constant, the temperature of a real gas may either increase or decrease, depending on the initial temperature and pressure.). Rewrite things so there's less going through loops to figure out what the sentence means. This is not a book by Nathaniel Hawthorne :P.
  • Heavy use of jargon, wikilink things such as "enthalpy".
  • A picture would add a little something to this article. I think the section Physical mechanism is the one that would benefit from one. Not a show stopper if there isn't one tho.
  • <math> seems overkill for complexity of the things written in the table.
  • Consider centering tables within the page.
  • The "flow" could be improved. Many sections feels like simple stubs (although not as much as in Henry's Law), and a collection of stubs does not make an A-class articles.
  • Some sections are unreferenced. Namely:
  • Physical mechanism
  • Applications

These needs to be addressed before it's considered A-Class IMO. Headbomb {ταλκκοντριβςWP Physics} 23:13, 15 June 2008 (UTC)

Actually, there are so many inaccuracies, that it would be wiser to rewrite the article first and then look at the prose. I just corrected a few things. The most obscene statement was: "the gas does no work" which was clearly mentioned in context of the Joule-Thompson process, not the free expansion case. It almost made me sick to read that :( Count Iblis (talk) 00:59, 16 June 2008 (UTC)

Feedback for the Isenthalic Joule-Thompson throttling proccess or whatever.[edit]

I've picked up my Reif (Fundamentals of Statistical and Thermal Physics). Here's what it says about:

  • "Free expansion experiment":

The fact that the internal energy E of a gas does not depend on its volume (if the gas is sufficiently dilute that it can be considered ideal) was verified in a classical experiment by Joule [where] he made use of the "free expansion" of an ideal gas.

A container consisting of two compartments separated by a valve is immersed in water. Initially, the valve is closed, and one compartment is filled with the gas under investigation, while the other is evacuated. Suppose that the valve is now opened so that the gas is free to expand and fill both compartments. In this process no works gets done by the system consisting of the gas and container. (The container walls are rigid and nothing moves.) Hence one can say, by the first law [of thermodynamics], that the heat Q absorbed by this system equals its increase in thermal energy,

Assume that the internal energy change of the (thin-walled) container is negligibly small. Then the ΔE measures simply the energy change of the gas.

Joule found that the temperature of the water did not change in this experiment. (Because of the large heat capacity of the water, any anticipated temperature change is, however, quite small; Joule's actual sensitivity of temperature measurement was, in retrospect, rather inadequate.) Thus the water absorbed no heat from the gas; consequently, the heat Q absorbed by the gas also vanished. All that happens in the experiment is that the temperature of the gas remains unchanged while its volume changes from its initial value Vi to its final value Vf. Since Q = 0, Joule's experiment leads [...] to the conclusion

which verifies that E(T,V) is independent of the volume V.
  • Throttling (or Joule-Thomson) Process (selective quoting):

[Long description of the experiment]...

In this process, the mass of the gas also does work ... but no heat is absorbed ... we arrive at the result that in a throttling process the gas passes through the constriction in such a way that its enthalpy remains constant.

The difference [with the free expansion] is that the gas does work in the throttling process, so that the enthalpy rather than the internal energy is the quantity that is conserved.

  • Joule-Thomson effect (withing the JT process section):

The Joule-Thomson effect constitutes a practical method for cooling gases and is often used in processes designed to liquefy gases. In order to achieve a lower temperature as a result of throttling a gas, it is necessary to work in that region of pressure and temperature where μ > 0, in particular, the initial temperature must be less than the maximum temperature on the inversion curve....

Hope this helps Headbomb {ταλκκοντριβςWP Physics} 15:30, 17 June 2008 (UTC)

Thank's very much for typing in these texts from F. Reif's book (the best book on Statistical Mechanics ever written, b.t.w.)! Count Iblis (talk) 15:38, 17 June 2008 (UTC)

Reif's the shit :P. Everyone hated that book back then, but it's the only thing that rescued me from the horrible teacher we had back then. And by horrible, I really do mean horrible. Stuff like you had to argue that ln(0) was not equal to 0, and that ln(1) was not equal to 1, and then when she couldn't argue anymore, she hand-waved it away "Well it's just conventions anyway" or "Well those are approximations". ln(0)=0 an approximation... Yeah... Or gave us exams problems such as "Find the density of the atmosphere as a function of height. Assume that the atmosphere is a dilute gas (i.e a gas where gravity is negligible)".

Anyway Reif is an awesome book, by far the best I've seen on statistical and classical thermodynamics. Very thorough in its treatment of concepts, and never falls back on abhorrent "obtaining this formula is trivial so it is left as an exercise to the reader"-s or the "it can be demonstrated that..."-s. Headbomb {ταλκκοντριβςWP Physics} 15:47, 17 June 2008 (UTC)


I want to congratulate those who have succeeded in revising a fairly simple article about the Joule-Thomson effect into an extensive thermodynamic thesis. I am removing this article from my watchlist. It is not worth watching any more. - mbeychok (talk) 18:28, 17 June 2008 (UTC)

Yeah, a "fairly simple article" that contained the statement: "the gas does no work" and now it just explains that if the gas moves through the valve, it will perform work and if you add to that the internal energy change and the condition that no heat has been absorbed, the enthalpy will stay the same. That's high school level physics (at least it used to be when I was in high school). The appendix is technical, but then that's why that technical stuff is delegated to the appendix. Count Iblis (talk) 18:58, 17 June 2008 (UTC)


Articles don't have appendices. Renaming the section to "derivation of the JT coefficient" or something like that would be better. Headbomb {ταλκκοντριβςWP Physics} 20:51, 17 June 2008 (UTC)

I see! This must then also be done in these articles:

Stefan–Boltzmann law

Planck's law

Count Iblis (talk) 21:46, 17 June 2008 (UTC)

Yuppers. Headbomb {ταλκκοντριβςWP Physics} 01:10, 18 June 2008 (UTC)

I've fixed it here. -- SamuelWantman 03:53, 31 March 2009 (UTC)

What a tortuous article this has become[edit]

In the first paragraph, the well-known Joule-Thomson effect which has been used for decades has suddenly been named the "throttling process".

Then the following description section starts out by describing an entirely different process ... an isentropic expansion during which work is done (such as occurs when a gas is expanded in a turbo-expander) and the temperature decreases. That is correct but it does not describe the Joule-Thomson effect which is what this article is meant to be about.

Then it discusses a "free expansion" and is linked to an incomprehensible stub of an article consisting of only three sentences.

Then it finally begins an equally tortuous discussion of the Joule-Thomson effect.

The very first two sentences should read:

The Joule-Thomson effect or Joule-Kelvin effect is the increase or decrease in the temperature of a real gas (as differentiated from an ideal gas) when it is allowed to expand freely through a valve or other throttling device while kept insulated so that no heat is transferred to or from the gas, and no external mechanical work is extracted from the gas. The Joule-Thomson effect is an isenthalpic process, meaning that the enthalpy of the gas is constant (i.e., does not change) during the expansion.

Count Iblis and Headbomb, have either of you ever heard of KISS ... meaning Keep It Simple Sam? --Pchemist (talk) 21:04, 1 September 2008 (UTC)Actually "KISS" stands for "Keep it simple STUPID."

Have either of you ever designed an industrial large-scale low-temperature distillation system using the J-T effect? I have designed and operated at least 5 such systems during my chemical engineering career and those systems are currently operating well.

This article was meant to simply and clearly explain what a Joule-Thomson effect is and how to use it. You might read the comment posted above by some reader on March 14th, 2008 entitled "What a wonderful article!

You two have turned an excellent, clearly written explanation of the J-T effect into a messy, tortuous thermodynamic treatise. What you have ended up with is an excellent example of the old saying: Those who can, do ... and those who cannot, teach. mbeychok (talk) 08:12, 22 June 2008 (UTC)

Mbeychok, the reason why it was necessary to make some changes to the article was because the previous version was misleading. Now the bold faced quote is technically correct, but the old version said something like "the gas does no work" if I remember correctly, which is false if you take it literally. It is only correct if you define that work done is the work extracted from the system. But then, how is a reader, whose background may not be engineering, going to guess that work done by or on the gas does not count unless it turns a turbine if you don't mention that?

I agree that the article can be written up more clearly. However, it has to be accssible to people who are not engineers, who do not automatically "see that no turbine is extracting work" when they only read about a gas moving through a valve and no turbine is explicitely mentioned.

And is giving an explanation for why enthalpy is conserved really that complicated? It is all based on the fact that pressure times change in volume is work, which is taught in high school. How can giving that explanation have turned the article into a "tortuous thermodynamic treatise"? Perhaps you are referring to the Appendix. But then that's why that section is called "Appendix".

Oh, and didn't Einstein say that:

Everything should be made as simple as possible, but not simpler

Count Iblis (talk) 13:23, 22 June 2008 (UTC)

Before you jumped in with two feet and started complicating the article and added an "Appendix" (which I have never before seen in a Wikipedia article), did you bother to read the lead-in paragraph and the following description section as it was back in say early 2007? You will find that is was very similar to my above lead-in (in bold letters) that you now admit is "technically correct". After all, this is a technical article and the lead-in should be "technically correct" should it not?
The reason the article is now so tortuous to read is that you two and others before you have given it a bad case of "Wikipediatics" by either being overly pedantic (like you two) or completely unknowledgeable.
One final point, using expressions something like "Riefs book is the (expletive)" is rather juvenile on th part of Headbomb, is it not? - mbeychok (talk) 17:04, 22 June 2008 (UTC)
Could you please withdraw your accusations of being pedantic? All I did here is provide quotes from what is IMO, THE reference for thermodynamics, about JT Effect and throttling process. There was a debate about it, I came in with the refs and things got settled immediately. I don't even know what the article was before, nor do I know what the debate was exactly about. Then someone objected to B-class rating, so I gave my reasons (or perhaps that was before, the order isn't very important) for it, and what it would take for this article to reach A class. If you think there's a better way to phrase things, say so, but do so with civility. Headbomb {ταλκWP Physics: PotW} 20:28, 22 June 2008 (UTC)

JT-Effect definition[edit]

The JT effect refers to the temperature drop (perhaps this was later generalized to temperature change, but originally it referred to the drop) due to a fluid's expansion when its expansion is constrained (i.e pressure difference is kept constant, a porous plug is one way of achieving this, but not the only one). Having a thermally insulated environment is an experimental and theoretical consideration to isolate the effects of heat exchange. If the environment is insulated, the process is isenthalpic, otherwise it's not. Headbomb {ταλκWP Physics: PotW} 15:48, 26 June 2008 (UTC)

Later in the article, it says that temperature change can be of either sign. Self-consistency is probably a good thing. Usually, the JT effect is used to cool gases because most gases are cooled by JT expansion at normal temperatures, and because there are much easier ways to heat gas. Rracecarr (talk) 16:03, 26 June 2008 (UTC)

The temperature change can be of either sign, but the JT effect is when the temperature drops. Headbomb {ταλκWP Physics: PotW} 16:06, 26 June 2008 (UTC)

Can you quote a reference to that effect? Rracecarr (talk) 17:51, 26 June 2008 (UTC)
I'm disappointed that someone would claim that the effect is not called the JT effect when the temperature rises without saying what the effect actually is called in that case. Is he hoping to have it named after himself?
For a reference that uses "JT effect" for both cooling and warming see [1], which is one of the earliest papers to attempt to measure the JT effect for hydrogen in both cases. Even the title uses "JT effect" for both cases!
[1] Herrick L. Johnston, Irving I. Bezman, and Charles B. Hood, Joule-Thomson Effects in Hydrogen at Liquid Air and at Room Temperatures, J. Am. Chem. Soc., 1946, 68 (11), pp 2367–2373 (Nov. 1946)
In the intervening seven decades nothing has changed in that regard, as witnessed by slide 3 of this 2015 presentation bearing on the very recent introduction of refueling fuel cell cars to the US, one of which I bought as soon as it became available, whence my sudden acute interest in this topic. Vaughan Pratt (talk) 04:02, 13 July 2016 (UTC)

Storage of hydrogen[edit]

I remember reading somewhere that storage of hydrogen under pressure at room temperature is dangerous because the inversion temperature of hydrogen is below room temperature. In case of a leak (hydrogen can leak more easily than other gasses), the hydrogen that escapes will get an increased temperature causing it to burn, which may cause the entire storage vessel to rupture and explode.

The wiki article on hydrogen storage does not mention this. Perhaps if we find some source we can mention this here.

Count Iblis (talk) 16:42, 26 June 2008 (UTC)

Use Castellan's Physical Chemistry as I provided it here. It's exactly there, same chapter and everything. Same thing happens for Helium, but Helium doesn't go BOOM!, so it's not dangerous. Headbomb {ταλκWP Physics: PotW} 17:16, 26 June 2008 (UTC)

The Joule-Thompson heating of H2 is only a few degrees; not nearly enough to ignite it. Storing H2 is potentially dangerous because if it leaks and mixes with air there are any number of things that can make it go boom. Retired Pchem Prof (talk) 17:25, 19 January 2016 (UTC)

T w.r.t. P[edit]

The partial derivative of T w.r.t. P at constant H can be computed by expressing the differential of the enthalpy dH in terms of dT and dP, and equating the resulting expression to zero and solving for the ratio of dT and dP.

What? Headbomb {ταλκWP Physics: PotW} 17:24, 26 June 2008 (UTC)

With Respect To? Rracecarr (talk) 17:51, 26 June 2008 (UTC)
Oh... yeah that was a brain fart. Nonetheless it should be written in full. Headbomb {ταλκWP Physics: PotW} 18:31, 26 June 2008 (UTC)
Agree.Rracecarr (talk) 18:49, 26 June 2008 (UTC)

Addition by Pchemist[edit]

Joule-Thomson expansion, the process by which a gas at constant pressure P1 is caused to flow adiabatically into a region of lower constant pressure P2 via a valve or porous plug such that P1V1 = P2V2. Since the pressures on both sides of the porous plug are each kept constant the compressive work "done on" the high pressure side of the porous plug equals the expansion work "done by" the low pressure side of the system. Accordingly the change in energy of the system equals this work. Since there is no exchange of heat with the environment (adiabatic process) the enthalpy (H = U + PV) remains constant: E2 - E1 = P1V1 - P2V2 or H2 = H1(see Appendix).

P1V1 = P2V2? Looks nonsensical to me, because that would mean that the internal energy should is constant, which is definitely not the case in the JT process (unless the gas is ideal). Count Iblis (talk) 03:18, 3 September 2008 (UTC)

Your right. I must admitt I got involved here in haste. I saw many things that needed correction and did a patch job. Here is what should be done:

The discussion about different types of expansion should be removed because each one could be discussed more rigorously and with greater attention to accuracy. The JT effect should be carfully be framed as an expansion of a gas at constant enthalpy, period. Given that all else falls out. q = 0, energy change = work. Now you can look at the work done a lot of different ways. One way it should not be looked at is from the perspective of gas expanding from a compressed gas cylinder. It should also not be looked at from the perspective of how the JT coeficient is actually measured i.e. by measuring the isothermal JT coef. Further confounding the issues I have with Racecarr's text is that the temperatur of the gas on either side of the porous plug are not the same, but both the expansion and the compression each occur isothermally. Accordingly consider the following revision:

Joule-Thomson expansion, the process by which a gas at constant pressure P1 is caused to flow under adiabatic conditions into a region of lower constant pressure P2 via a valve or porous plug such that the enthalpy remains constant. Under such conditions (q = 0)the internal energy change is only the work done, ΔU = w. Since the pressures on both sides of the porous plug are each kept constant, the compressive work "done on" the high pressure side of the porous plug plus the expansion work "done by" the low pressure side of the system is this "total" work, w = P1V1 - P2V2. The measurement of the requisite temperature change that accompanyies this expansion is the Joule Thomson effect. The Joule Thomson effect is quantified as the ratio of this temperature change over the pressure difference found under conditions of constant enthalpy.(see Appendix). --Pchemist (talk) 15:29, 3 September 2008 (UTC)

This looks better. But there are still some problems here. It is not clear why the condition:

Since the pressures on both sides of the porous plug are each kept constant

is necessary. I think that you are looking at all of the gas on both sides and you want to look at a steady state situation. But that can better be mentioned explicitely, like in the current version that mentions:

under steady state conditions and without change in kinetic energy

We need the steady state condition, e.g consider this counterexample: Consider filling an evacuated chamber with an ideal gas on the outside of the chamber via a porous plug (the usual JT process, but now in a non-steady state situaton). This was actually a practice problem I gave to my students. The problem was to show that when the pressure in the chamber has equalized with the outside pressure, the temperature in the chamber is cp/cv times the temperature of the gas on the outside.
The other condition that the kinetic energy of the gas doesn't change is also necessary. In case of adiabatic flow you always have that 1/2 v^2 + h = const, where h is the specific enthalpy and v the flow velocity. If the gas could move through the plug without friction, it's kinetic energy would increase (it would be accelerated by the pressure drop). The whole process would then be reversible. Instead, what happens is that the kinetic energy which the gas could gain in theory, is lost due to friction. This is why you need the porous plug or some other narrow opening in the first place. Count Iblis (talk) 16:01, 3 September 2008 (UTC)

Section Physical mechanism[edit]

I dissagree with description in the section Physical mechanism. Energy of gas is not constant (enthalpy is constant), process is irreversible so entrophy increases E=H-TS, energy increases below and decreases above inversion point. —Preceding unsigned comment added by Trifold (talkcontribs) 17:01, 6 September 2008 (UTC)

You're thinking of Gibbs free energy there with the H-TS. —Preceding unsigned comment added by (talk) 06:06, 19 October 2010

Please explain[edit]

The article says, "During gas molecule collisions, kinetic energy is temporarily converted into potential energy." This is not linked to any other article, and an explanation of how this happens is not obvious to me or my partner (I went to MIT and he went to CalTech). Our best guess for the meaning of this sentence is that for a brief instant, when they are changing direction, the particles have no velocity and thus no kinetic energy. If we are correct, this should be added to the description, with links. Please remember that these articles should be written for people who don't know much about the topic! -- SamuelWantman 03:46, 31 March 2009 (UTC)

That's basically right--think of a mass bouncing off one end of a spring, the other end of which is fixed. As it moves toward the spring, the mass has a certain amount of kinetic energy. The spring compresses, momentarily bringing the mass to a stop. The kinetic energy has been converted into potential energy. Then the spring accelerates the ball (in the opposite direction) converting the potential energy back to kinetic. The exact same thing happens in a collision between gas molecules, except that the intermolecular force is provided by Coulomb repulsion, rather than a spring. Rracecarr (talk) 04:09, 31 March 2009 (UTC)


As I always learned it, enthalpy is the same before and after the throttling process, but cannot be described during it due to gas passing through non-equilibrium states, the article presents it differently claiming that enthalpy is constant throughout the process. Either my memory or the article should really be updated to reflect the truth. (talk) 05:59, 19 October 2010 (UTC)

Confused and redundant[edit]

The following section has problems:

  • By using the framework of the Lennard-Jones potential, insight can be gained as to what causes gases to cool or warm as they are expanded. The Lennard-Jones potential takes into account the repulsive (steric) forces of physical interaction as well as the attractive (electrochemical) forces. Initially, repulsive forces dominate the Lennard-Jones potential, and so due to the steric hindrance of the particles, energy is released upon expansion which causes the gas to cool. This same principle can be used to explain the fact that hydrogen and helium warm upon expansion. Because these particles are close to ideal repulsive forces are much weaker and attractive forces dominate at room temperature; therefore, energy is absorbed, as expected with conservation, as the particles are now further apart, and this absorption of energy causes it to heat.

Some points:

  • "the framework of the Lennard-Jones potential" There is no reason highlight the Lennard-Jones potential.
  • "attractive (electrochemical) forces" This misuses the term "electrochemical".
  • "steric hindrance of the particles" This misuses the term "steric hindrance" (from reaction kinetics).
  • "hydrogen and helium warm upon expansion" As the next section explains, warming v. cooling depends on the pressure.
  • "energy is released upon expansion which causes the gas to cool" Backward.
  • "absorption of energy causes it to heat" Backward.

I removed this section, leaving the one that follows, which is unconfused, correct, and complete in itself. — Preceding unsigned comment added by (talk) 19:23, 3 November 2011 (UTC)

I agree with deletion of the paragraph. The article has been improved.
At present, the section says the total energy of the gas remains the same, and it links to Conservation of energy. Would that be more correctly linked to Continuity equation?
The article is conspicuously deficient in in-line citations to allow independent verification of its contents. The section Physical mechanism has never had an in-line citation. Are you able to identify a suitable source for the contents of the section? Dolphin (t) 21:36, 3 November 2011 (UTC)

Poorly written article[edit]

This article is poorly written. Consider the following sentence. "In the Joule experiment, the gas expands in a vacuum and the temperature drop of the system is zero, if the gas were ideal." This illiterate phrase demonstrates a lack of coherent thought. Now consider a revised version. "An ideal gas will exhibit no change in temperature as it expands in a vacuum." Or since an ideal gas is a theoretical concept. "In Joule's analysis, an ideal gas will exhibit no change in temperature as it expands in a vacuum." The fact that real world behavior differs and why it differs should be introduced immediately after Joule's analysis of the ideal gas. Presenting the math to quantify the effect before mentioning why the effect occurs is nonsensical and abusive to the reader. (talk) 19:44, 29 June 2013 (UTC)

Diagram needed[edit]

A throttling process proceeds along a constant-enthalpy curve in the direction of decreasing pressure, which means that the process occurs from right to left on a Temperature-Pressure diagram.

The article continues with a detailed description of this diagram, but the displayed diagram appears to be about a mathematically analogous but different process. And it is not a temperature-pressure diagram. As a result, it is not labelled in a way that makes it the right diagram.

It would be great to have the correct diagram!

Here is a sample: [1] But where is one that can added to the article? (talk) 18:06, 14 March 2015 (UTC)

I think I can make one for steam. An open source MATLAB code for steam has been made available that would allow me to construct it. Also, can the Joule-Thomson effect be modeled by the Van der Waal equation? If so, I think I can model that one for you, provided we can generate an energy equation of state -- it doesn't have to be accurate, just free from copyvio suspicion. Give me a few days to finish another project first. --Guy vandegrift (talk) 14:50, 15 March 2015 (UTC)
Sorry, but unless I am doing something wrong, we cannot use the steam functions that are available at:
It appears that we need to go well above 800 bars of pressure, which is the code's upper limit for the temperatures we need.--Guy vandegrift (talk) 02:31, 19 March 2015 (UTC)
OK, I think I can get the data I need at It will be for water, and I will use the Joule-Thomson coefficient. This will take me a few days because I have to do it almost point by point to get a half dozen contours. It seems like we need it in this article.--Guy vandegrift (talk) 06:32, 19 March 2015 (UTC)

Hanging subsection "Thermodynamic Interpretation of the experiment"[edit]

The subsection "Thermodynamic Interpretation of the experiment" is inconclusive. It starts to set up the thermodynamic model, then just stops. What gives?? (talk) 02:27, 15 March 2015 (UTC)

Welcome to Wikipedia.--Guy vandegrift (talk) 14:52, 15 March 2015 (UTC)

New image: T vs P with coutours of h for inversion curve[edit]

As per #Diagram_needed. After failing to find the curve on commons and failing to use the matlab code XSteam to generate the curves, I made a generic curve. I will eventually attempt to put it into the article, unless somebody asks me not to, or unless somebody does it for me. --Guy vandegrift (talk) 00:24, 20 March 2015 (UTC)

FYI I did try to make a proper inversion curve using the matlab program Xsteam. Unfortunately it does not evaluate at the high temperatures and pressures required to capture the curve, if it indeed exists for water. The "hypothetical" substance is indeed water, or at least my guess as to where the inversion exists, if it even exists for water.--Guy vandegrift (talk) 00:46, 20 March 2015 (UTC)

This is really nice. It should definitely go up in place of the current one, a T-S diagram as I recall. (BTW I'm the IP guy that originally posted about the diagram :-) .) (talk) 22:40, 11 April 2015 (UTC)
I decided I din't have the expertise in this field to add the image myself. And although I prefer lots of images, some people might feel that one image should replace the other, and that takes me even further out of my comfort zone. I "discovered" the Joule-Thomson effect just recently after googling enthalpy while working on an Engineering Thermodynamics course. We had little need for enthalpy when I did plasma physics back in the 1980s, but late last night it occurred to me that the Joule-Thomson effect is just a shock wave "jump" condition, as stated in this article: Hoover, Wm G., Carol G. Hoover, and Karl P. Travis. "Shock-Wave Compression and Joule-Thomson Expansion." Physical review letters 112.14 (2014): 144504
This connection does belong in the article.--Guy vandegrift (talk) 12:46, 12 April 2015 (UTC)
The article could use an image of this sort, especially considering the inappropriate one already there. But I have a strong distaste for hypothetical substances. I will try making one for N2, but it will have to be isotherms on an H vs. P plot, since that is the data format I can easily get from the NIST WebBook. I think it will look much the same. Retired Pchem Prof (talk) 23:23, 19 January 2016 (UTC)

Proof that the specific enthalpy remains constant: fluid kinetic energy[edit]

I reverted an edit in Joule–Thomson_effect#Proof_that_the_specific_enthalpy_remains_constant because it lacks precision. I am under the impression that there is a term on each side of the equation. If that is true, we neglect fluid kinetic energy either because it is small, or because the flow rate is the same on each side. The fluid does accelerate and decelerate through the plug. I agree that something needs to be said, if what is said is both accurate and brief -- the editor should try to rephrase.--Guy vandegrift (talk) 23:06, 4 April 2015 (UTC)

One option is to present the material at a bit higher level in another section. See for example section 12-3 of this article:

Proposed deletion from the introduction[edit]

The final sentence of the introduction reads "The physical mechanism associated with the Joule–Thomson effect is closely related to that of a shock wave". I propose to delete it since it seems to serve no purpose. Retired Pchem Prof (talk) 17:17, 19 January 2016 (UTC)

Thank you for discussing this. There is something on wikipedia called WP:BRD which means "Bold, Revert, Discuss" Sometimes it is good to make "Bold" edits like the one you propose. If another editor disagrees then they will "Revert" the edit. If you really want to redo the edit, then it is usually a good idea to discuss it here before redoing, but sometimes edit summaries can be enough.
One reason that another editor may disagree with your removal, is that it is referenced. However it is a PRL paper which is a WP:PRIMARY source. There are no secondary sources cited that pick up and this idea, that establish the theory's WP:NOTABILITY. Given the coverage of the JT effect in third party sources, this appears like a WP:FRINGE theory, i.e. it has little prominence in a very large subject. Hence it does not belong in the lead, and probably not in the article, so I suggest you be bold and remove the sentence. Martin451 22:14, 19 January 2016 (UTC)
One reason I decided not to be bold is that it is referenced. Another is that it appears to be correct, is certainly interesting, and might be considered notable by someone who knows more about shock waves than I do. So after reflection I have decided to be semi-bold and move it to the section on application, where it will not seem nearly so out of place. Retired Pchem Prof (talk) 23:15, 19 January 2016 (UTC)

Derivation of the Joule–Thomson (Kelvin) coefficient[edit]

This derivation is not very clear. How did he get from the 2nd equation to the 3rd? I am in the process of rewriting it, following the standard textbook approach via the isothermal Joult-Thomson coefficient. It seems that some people don't like seeing math, but I see no harm as long as it is near the end. But it ought to be clear and correct. Retired Pchem Prof (talk) 20:44, 19 January 2016 (UTC)

I have prepared a draft rewrite of this section. It is now on my user page for comment.Retired Pchem Prof (talk) 01:36, 20 January 2016 (UTC)
I see your draft re-write at User:Retired Pchem Prof/sandbox02#Derivation of the Joule–Thomson coefficient. This is a commendable piece of work! I will examine it closely over the next day or two and give you my comments. Dolphin (t) 07:08, 20 January 2016 (UTC)
I have now replaced the original section with my rewrite. Retired Pchem Prof (talk) 20:22, 28 January 2016 (UTC)

Changes to Description, Physical Mechanism, and Applications[edit]

Under "Description", I made minor edits to the 2nd and 3rd bullet points. I replaced the figure, which was not the correct one to use, as noted above under #Diagram needed and #New image: T vs P with coutours of h for inversion curve. I kept the figure simple, without the isenthalpic curves, which have so little curvature that the maxima can not really be seen. I also replaced the accompanying text, which was confusing and contained some errors. I removed the last paragraph and incorporated some of the content into the section on "Applications" along with some other minor edits. I largely rewrote the section on "Physical mechanism" to tighten up the text, make the style more suitable to an encyclopedia, remove some material that belonged in the article on Joule expansion, and included more references. Retired Pchem Prof (talk) 19:34, 1 February 2016 (UTC)

Assessment comment[edit]

The comment(s) below were originally left at Talk:Joule–Thomson effect/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

I have rated the Joule-Thomson effect article as Class=A and Importance=High. I would first like to say that this article is, in my opinion, most important to Engineers and Chemists as well as to Physicists. I would not have categorized it as solely a Physics article. It is of very high importance to chemists and engineers who work in a great many industries. I have classified it as Class A because it has been well-written, organized well as per Wikipedia practice and includes an excellent Bibliography of pertinent reference texts. - mbeychok 03:05, 29 November 2006 (UTC)

Substituted at 01:12, 22 May 2016 (UTC)

Less Clear and Simple than before[edit]

I'm writing as a unqualified learner (too old to be called a student), and I found this article less clear than it used to be.

A few years ago I read this article and it explained this effect in terms of what was happening to the atoms or molecules (attractive and repulsive forces), I thought that was a great simple and easy to grasp explanation .... and no maths needed.

So I went back to lookup that description and found it in the archived version of this article, dated 10th March 2008

Whether I'm right or wrong, I expect this to be an encyclopedia article that introduces me to what the JT Effect is and why it arises I'm tempted to put back in as the "Simple folks explanation of the Physical Mechanism" That version contains the following description under the heading "Physical Mechanism" - If it is a valid description, I think it should be added back into the article ...

As a gas expands, the average distance between molecules grows. Because of intermolecular attractive forces, expansion causes an increase in the potential energy of the gas. If no external work is extracted in the process (“free expansion”) and no heat is transferred, the total energy of the gas remains the same because of the conservation of energy. The increase in potential energy thus means a decrease in kinetic energy and therefore in temperature.

A second mechanism has the opposite effect. During gas molecule collisions, kinetic energy is temporarily converted into potential energy. As the average intermolecular distance increases, there is a drop in the number of collisions per time unit, which causes a decrease in average potential energy. Again, total energy is conserved, so this leads to an increase in kinetic energy (temperature). Below the Joule-Thompson inversion temperature, the former effect (work done internally against intermolecular attractive forces) dominates, and free expansion causes a decrease in temperature. Above the inversion temperature, the latter effect (reduced collisions causing a decrease in the average potential energy) dominates, and free expansion causes a temperature increase.

I arrived at this article today when I followed a link from the CryoCoolers article, I was hoping to get a better understanding of how a JT Expansion works in the real world. — Preceding unsigned comment added by NicholasB54 (talkcontribs) 02:37, 15 August 2016 (UTC)

      • Correcting myself ***

It seems it was an edit on 9th Jan 2016 by "Retired PChem Prof" that made the "Physical Mechanism" section gobbledegook for me

Just added the "How the effect arises" section from IP reverting the 2 paragraphs I found so simple and useful — Preceding unsigned comment added by NicholasB54 (talkcontribs) 00:32, 29 August 2016 (UTC)

@NicholasB54: These two paragraphs are entirely unsourced. Please provide reference material. Ariadacapo (talk) 06:21, 29 August 2016 (UTC)
@NicholasB54:It seems that someone other than me has reverted the text to what I had written. The problem with the two paragraphs that you found so simple and useful is that they are wrong. For instance, they refer to "no external work", but the Joule-Thomson effect is due to external work. To the extent that those paragraphs are correct, they apply to the free expansion, where you can still find something similar.
I can see why you might have trouble with the current text, but I don't see how that can be helped. The Joule-Thomson expansion is for a process at constant enthalpy. There is no simple physical explanation for what enthalpy is. As a result, there is (so far as I know) no simple physical explanation for the Joule-Thomson effect. I'd love to find one. Retired Pchem Prof (talk) 18:53, 13 September 2016 (UTC)

"gas must not be expanded through a turbine"[edit]

"No external work is extracted from the gas during the expansion (the gas must not be expanded through a turbine, for example)."

But isnt this exactly how its done today (turbine) and also why they couldnt liquify Helium due to the friction in the valve (unused energy remaining in the gas)?! This needs to be backed up as it contradicts how this process is actually carried out (much more efficiently) today. — Preceding unsigned comment added by (talk) 07:07, 19 October 2017 (UTC)

vapor-compression refrigeration due to Joule–Thomson effect?![edit]

"In vapor-compression refrigeration, as used in common refrigerators and air conditioners, it [Joule–Thomson expansion] produces cooling in the throttling valve."

This technically might be* correct, but sounds like this is how vapor-compression refrigeration actually cools things.

  • Its a liqid beeing pushed through the valve, not a gas. Does the J-T-Effect apply there? I dont think so. Watercuting would be impossible if there is a relevant drop in temperature due to expansion of several 1000 bar - there would be ice shooting out of the nozzle and freezing it.

But even for a pure gas these few bar of pressure difference would only be able to produce a delta-T of ~3K anyway, thus a irrelevant temperaturchange. I remove this somewhat missleading and potentially false sentence. — Preceding unsigned comment added by (talk) 07:19, 19 October 2017 (UTC)