|WikiProject Measurement||(Rated C-class, Mid-importance)|
temperature and pressure
188.8.131.52 20:42, 13 August 2007 (UTC)The artilce introduces the Torr using this text: It is the atmospheric pressure that supports a column of mercury 1 millimeter high. This wording is ambiguous as atmospheric pressure is ~760 torr, or ~760mmHg. One Torr is the pressure that supports a column of mercury 1 millimeter high. It is most often used to measure gas pressures including atmospheric pressures.
184.108.40.206 20:42, 13 August 2007 (UTC)
Presumably this is 1 mmHg at standard temperature and pressure?
Can we stick to either Engineering Notation or normalized Scientific Notation (or even plain decimal)? For example in the conversion table, in the atm column, it has 1.0197×10−5 in one row and 70.307×10−3 in another. It seems like consistently using normalized Scientific Notation (i.e., changing the latter to 7.0307×10−2) would make it a lot easier to get a quick sense of the different magnitudes.Erikmartin (talk) 18:28, 22 October 2008 (UTC)
I eliminated this:
'The Torr' is also the name given to a Bristol based rock band which formed in 1998 and are still performing and recording today. Noteable members include: Jon Steadman, Neil Berry & James Trotham.
If it is found useful, it should be put in a stub page or in a disambiguation page. SaintCahier 17:00, 6 August 2007 (UTC)
More importantly, 1mmHG at Earth's gravity
Is there an official torr now defined in terms of another measurement system, or do you measure your own?
- This is close to 1 mmHg at 0 °C and standard gravity. That is the temperature that was most often used for metric units based on a column of mercury; for inches of mercury, in earlier times 60 °F was more common but the ones still in use today are most often exactly equal to 24.4 mmHg, so are also essentially equivalent to the 32 °F (0 °C) variety, though mercury is no longer actually used in the precise definitions.
- It's just that defining a standard atmosphere as both 101.325 kPa and 760 mmHg makes a more precise definition of these units based on the units for which standards are currently maintained, namely the SI units. That 101.325 kPa number was chosen because it was, to the nearest pascal, the calculated pressure of 760 mm mercury at a certain conventional density under those conditions. Gene Nygaard 22:22, 20 Dec 2004 (UTC)
At no stage have we heard which authority originally defined the Torr, nor which international authority currently owns / maintains the definition. The arguments between AMA vs NIST are parochial for the international readership, unless one of these turns out to be the internationally-recognised authority for the definition. While millimetre of mercury may be a non-SI unit, the 'millimeter' spelling is sanctioned by the US Government. The SI English spelling is millimetre. Ian Cairns 22:49, 24 Jan 2005 (UTC)
- Where did you get the silly notion that some one body must "own" each unit of measure?
- There is no rule that any international body must act.
- There is no international body with plenary authority in this area.
- There can be and are disagreements among various international standards setting agencies, just as there are differences between the AMA Style Guide and the NIST Style Guide concerning the symbol for millimeters of mercury.
- There is no "SI English spelling." The BIPM chooses to follow use one spelling in their work; they do not state it as a general rule. Do you understand the difference? That's a "house rule", not a prescription.
- — Gene Nygaard 23:41, 24 Jan 2005 (UTC)
- Most relevant to this discussion is that we at Wikipedia have our own "house rules"— and one of them is that you do not arbitrarily mess with an article's consistent use of a particular spelling of litre/liter, color/colour, tyre/tire, and the like. Gene Nygaard 23:50, 24 Jan 2005 (UTC)
- Another point: There are lots of Canadian Tire stores, and not one Canadian Tyre store. We also spell this word tire in the United States, so this is an international English spelling. Yet there are Wikipedia articles which spell this word tyre. International does not mean prescriptive for the whole English-speaking world. Gene Nygaard 00:55, 25 Jan 2005 (UTC)
I reverted this addition by User:Urhixidur:
- Strictly speaking, the millimetre of mercury and the torr are slighly different units. Using the conventional density of mercury  of 13595.1 kg/m3 and the conventional acceleration of gravity, 760 mmHg equals a pressure of exactly 101,325.0144354 Pa, which was rounded down in 1954 by the 10th CGPM to give the conventional value of the atmosphere (101,325 Pa). The torr, defined as 1/760 of the latter value, is thus slightly smaller than the mmHg (by about 0.14 ppm), although both are used interchangeably in practice.
These are merely successive definitons of the same unit. Nobody redefining this unit has ever intended for there to coexist two slightly different variations of the unit. A particular name may be deprecated, but I'm not sure that that is the case either. Gene Nygaard 15:30, 26 Jun 2005 (UTC)
- I wish Gene were right, but he isn't. As this source and this other source show, the mmHg used to be defined (by the World Meteorological Organization, according to this) as I described in the excerpt above. We're agreed that there was never an intent to have the torr actually different from the mmHg, but in practice they were (and still are) because the definitions disagreed. This point is worth making, if only for the historical insight into metrology it provides.
- Urhixidur 2005 June 28 12:48 (UTC)
- Your first source does not say when WMO defined the mmHg this way, and is not an indication that this is not just a successive definitions problem. Whether or not the WMO has ever formally adopted an updated version is irrelevant to the question, especially when the difference is about four decimal orders of magnitude removed from making a difference in any of these measurements as made in anything in the province of the WMO. And especially since the WMO never uses them any more, except perhaps dealings with the U.S. NWS which uses them yet in public and aviation weather reports, though not in the more "scientific" contexts. Your second source may be an indication of some remaining unresolved inconsistency in British Standards (and once again, likely mostly irrelevant for the purposes for which those definitions are used), still not anything to disprove a successive refinement of the definition of a unit of measure quite legitimately and properly known by both names under both definitions (even if very specific legalistic applications might require one form or the other in some particular context).
- The thing is, as even your arguments point out so well, nobody has plenary authority in this regard. And nobody is required to make these definitions, or to update them immmediately even if a de facto standard becomes established which varies slightly from them. There is nothing improper about one group of users deciding to use the name "millimetre of mercury" and with another group using the name "torr" to refer to the unit defined by saying that a standard atmosphere is equal to both 760 of that unit and to 101.325 kPa, and there is nothing improper about one group of users deciding to use "millimetre of mercury" and with another group using "torr" to refer to the unit defined by saying that this refers to the pressure exerted by mercury at a certain temperature and at a certain acceleration of gravity, and there is nothing improper about one group of users deciding to use "millimetre of mercury" and with another group using "torr" to refer to the unit defined by making that definition less ambiguous by specifying a specific density and a specific acceleration of free fall.
But what we do know or at least can determine is the historical progression of those definitions: The middle of those three first, then the last one, then the first one. There is nothing improper with various groups of people choosing to use "millimeter of mercury" and "torr" as exact synonyms under any of these definitions. Gene Nygaard 28 June 2005 15:03 (UTC)
- Not to reopen the debate on differences in the size or derivation of one mmHg vs one Torr, but this page suggests that Torr are an absolute value, starting with zero Torr at a perfect vacuum, while mmHg indicate a gauge pressure. Admittedly, this is from a manufacturer, not a scientific body, but it still seems worth discussing here, if not on the actual page. Anyone? --Badger151 03:58, 26 July 2006 (UTC)
The following table of conversion factors appears in many Wikipedia articles:
|Pascal||Bar||Technical atmosphere||Standard atmosphere||Torr||Pounds per square inch|
|1 Pa||≡ 1 N/m2||10−5||1.0197×10−5||9.8692×10−6||7.5006×10−3||1.450377×10−4|
|1 bar||105||≡ 106 dyn/cm2||1.0197||0.98692||750.06||14.50377|
|1 at||0.980665 ×105||0.980665||≡ 1 kp/cm2||0.9678411||735.5592||14.22334|
|1 atm||1.01325 ×105||1.01325||1.0332||≡ p0||≡ 760||14.69595|
|1 Torr||133.3224||1.333224×10−3||1.359551×10−3||1.315789×10−3||≈ 1 mmHg||1.933678×10−2|
|1 psi||6.8948×103||6.8948×10−2||7.03069×10−2||6.8046×10−2||51.71493||≡ 1 lbF/in2|
The table was removed from the present article because it flatly contradicts the article itself. The table says that 1 Torr = 1 mmHg. One of the main points of the article is that 1 Torr is not equal to1 mmHg. While it is true that 1 Torr almost equals 1 mmHg, the two units are not identical.
Among the many articles in Wikipedia that deal with pressure, some contain copies of this table while others do not. An example of the former is Pressure. An example of the latter is Conversion of units. Since the table is, at best, misleading (and frankly wrong from a technical viewpoint), I think it should be entirely removed from Wikipedia (not just from the present article). Also, as a matter of principle, “millimeter of mercury” and “mmHg” should not be redirected to Torr.
- The table you pasted here says that 1 torr ≈ 1 mm Hg; that's "approximately equal". Maybe your browser doesn't support unicode properly? --Slashme 09:16, 28 October 2007 (UTC)
Conversion of SI pressure units
The SI unit of pressure is the pascal (Pa). Other units in common use are derived from SI units according to the following four definitions (see article for references):
- The atmosphere (symbol: atm) is defined as exactly 101,325 Pa.
- The bar (symbol: bar) is defined as exactly 100,000 Pa.
- The torr (symbol: Torr) is defined as exactly 1/760 atm.
- The millimeter of mercury (symbol: mmHg) is defined as the product of (1) the “standard’ density of mercury at 0 °C (13.5951 g/cm3) and (2) the “standard” acceleration of gravity gn (9.80665 m/s2).
From these four definitions, the entire conversion table can be constructed. In the tables shown below, I omit conversions involving the pound-force per square inch (a non-SI unit) and the technical atmosphere (an obsolete unit involving the avoirdupois kilogram-force). There is nothing wrong with having these units in a conversion table; they are omitted here only because we are discussing SI units.
The first table shows exact conversion factors. This format emphasizes the underlying simplicity of the SI, and shows clearly why the conversion factors for Torr and mmHg are different:
The second table gives approximate conversion factors (to six significant figures). This is the format that is used in practice. The torr and the millimeter of mercury are so similar that their differences do not appear at six significant figures. Note the pairs of identical values scattered throughout the table:
The differences become apparent when additional significant figures are included:
1 Pa = 0.0075006168271 Torr 1 bar = 750.06168271 Torr = 0.0075006157585 mmHg = 750.06157585 mmHg
1 atm = 760.00000000 Torr 1 Torr = 0.99999985753 mmHg = 759.99989173 mmHg 1 mmHg = 1.00000014247 Torr
1 Torr = 0.00133322368421 bar 1 Torr = 133.322368421 Pa 1 mmHg = 0.00133322387415 bar (exactly ) 1 mmHg = 133.322387415 Pa (exactly)
With two exceptions (noted above) these decimal conversion factors are infinitely long. The two exceptions are exact conversion factors in decimal form. Of course, all of the fractions in the first table are exact conversion factors.
Clearly, these distinctions are so small as to be negligible for most purposes. However, scientific articles in Wikipedia should not tolerate ambiguities, especially when thy are misleading. Btarski 23:12, 3 October 2007 (UTC)
The cgs system
The torr was used in conjunction with both the cgs and the mks systems. It was usually taken to mean “millimeter of mercury,” but it was also used to mean 1/760 of a standard atmosphere. As discussed in the article, these two definitions are incompatible.
When the standard atmosphere was defined as exactly 101325 Pa  the torr was defined to make 1 atm = 760 torr. The millimeter of mercury was never a part of the cgs, mks or SI systems.
The cgs system is now obsolete. Any discussion of the torr in relation to the cgs system is of historical interest only.
Unlike the mmHg, the torr (with its modern definition of 1/760 1 atm = 101325/760 Pa) is a perfectly legitimate unit of pressure, derived from SI units. Its use in general is discouraged, since the SI unit of pressure is the pascal.
Various Wikipedia articles continue to confuse the matter by offering different interpretations of both the torr and the mmHg. The articles on pressure, bar (unit) and pascal (unit) all contain the incorrect conversion table discussed above (under “Conversion tables”). This table (which treats the torr and the mmHg as identical) was correct until 1954. It has been obsolete ever since then.
- The Wikipedia article on the cgs system shows the torr and the barye as synonymous, which is incorrect.
- In 1954; see article for reference. Note that the standard atmosphere was originally defined in cgs units (1 atm = 1013250 barye = 1013250 newton-meters per square centimeter). The definition of the standard atmosphere (1954) preceded the adoption of the SI (1960).
Btarski 19:01, 9 October 2007 (UTC)
- You forgot to remove it from the category as well. I'll take care of that. -- Gene Nygaard (talk) 17:07, 16 November 2007 (UTC)
Is Torr absolute?
Here's a question that I've struggled with when I first started dealing with vacuum measurements. Without bringing pascals into this, torr is often stated as being equal to mmHg. On most gauges and conversion tables, mmHg is not an absolute measurement, that is, 0 mmHg = air pressure and 760 mmHg = total vacuum. I've been told by more than a few flashlamp manufacturers that Torr is equal to mmHg(absolute), that is, 0 torr = total vacuum and 760 torr = air pressure. To quote from correspondence I had with one expert "what? 730 is more [pressure] than 100 or 500. Is there a typo here?" If torr is an absolute measurement and mmHg is not, (like the difference between Celsius and Kelvin), I think this distinction should be made clearer in the article. Zaereth (talk) 00:39, 26 November 2008 (UTC) Is there anyone here who can answr this question?Zaereth (talk) 20:05, 12 January 2009 (UTC)
- Sure. The answer is no. The "torr" and the "millimeter of mercury" are totally interchangeable, in any of their definitions over time, and whether used to measure gauge pressure or absolute pressure. Gene Nygaard (talk) 17:00, 6 February 2010 (UTC)
- Do you have a source for that. Here's why I ask. When building flashtubes, the pressure for xenon is usually around 500 Torr, whereas the pressure for krypton is usually around 760 Torr, although pressures can be as high as 3000 Torr or as low as 50. On guages they call it mmHg Absolute, but no "absolute" is listed after Torr. Zaereth (talk) 17:34, 6 February 2010 (UTC)
- There's no accounting for everyone's ideosyncracies. Rarely does anybody use both torr and millimeters of mercury; most people will use one name or the other for this unit, and will use the same name whether they are talking about absolute pressures or gauge pressures.
- Just go look at Special:WhatLinksHere/Torr and tally up how many times each name of this unit (mmHg and various other possibilities are on that list through their redirects) is used for an absolute pressure, and how many times that name of the unit is used for gauge pressure. Or search the internet with your favorite search engine, and do the same.
- Find any Wikipedia articles (other than the ones about units of measure or systems of units) which use both "Torr" or "torrs" on the one hand, and "mmHg" or "mm Hg" or a spelled out version with either -er or -re spellings. Tell us how each is used there, and whether or not both were added by the same editor. Gene Nygaard (talk) 14:47, 7 February 2010 (UTC)
- By the way, you can see a whole lot of nonsense on various gauges. For example, see this one at
- It is centered on one atmosphere of pressure, and it measures gauge pressure in either direction (no absolute pressures)
- It measures negative gauge pressures in "CM/VAC" (i.e., cmHg) and it measures positive gauge pressures in "KG/CM²" (kgf/cm²)
- It measures negative gauge pressures in "IN/VAC" (inHg) and it measures positive gauge pressures in "PSI" (lbf/in²)
- By your logic, should we say that "psi" are only used for gauge pressure? That "cmHg" are only used for gauge pressure? I don't think so. Gene Nygaard (talk) 15:30, 7 February 2010 (UTC)
- It is centered on one atmosphere of pressure, and it measures gauge pressure in either direction (no absolute pressures)
- By the way, you can see a whole lot of nonsense on various gauges. For example, see this one at
- One additional point: we know that a pressure of a few micrometers of mercury or especially less than 1 µmHg are absolute pressure, and we know that pressures of a few millitorrs or less are absolute pressure. We don't know that because of the units used, however. We know that because that little difference from a pressure of about one atmosphere is unmeasurable. Our gauges simply cannot show such a small difference as a gauge pressure. Gene Nygaard (talk) 15:42, 7 February 2010 (UTC)
On the first row, third column of the table, it says 1 Pa = 1x10^5, whereas in the comment underneath explaining how to read the table it says 1Pa=1x10^5. Which is correct?
"The torr, a more convenient unit for low pressures," it is claimed "is used in high-vacuum physics and engineering."
- Where's the citation?
- Who says it's "more convenient" (than what)?
- I agree on the first sentence. That's an opinon, because what's convenient for one person may be a hassle for another. (Especially if you don't realize that in high vacuum physics, like plasma physics, the term "Torr" usually means "Torr (Absolute).) However, the fact that it is used quite often in things like plasma physics is pretty well known. I'll see if I can find a source for that. Zaereth (talk) 20:57, 23 August 2011 (UTC)
See MKS Instruments vacuum gauges or Granville Phillips ion gauges (now owned by Helix Technology). I have personally gone to Granville Phillips location in Boulder, CO to pick up ion gauges I bought for high vacuum use. I've also maintained ultra high vacuum systems for physics research and all the gauges read out in Torr. Working as an engineer in the thin-film magnetic recording and MEMS industries the process specifications, production logs, and tooling all used Torr. --NimbleThink (talk) 22:43, 30 January 2012 (UTC)
- Yeah, I started out as a hobbiest, making my own flashtubes for lasers. My first vacuum gauges read in mmHg, none of which were "absolute readings, (ie: 0 mmHg was equal to air pressure and 760 is total vacuum). It was a bit confusing as to what "500 Torr fill pressure" actually meant, until I began talking to flashtube manufacturers and experts, like Advanced Radiation and Don Klipstein. My first gauges that read in Torr came from Cole-Parmer, and those were all "absolute" readings, (ie: 0 Torr equals total vacuum and 760 is air pressure), whereas the mmHg guages they sold were not. I can see where, in the scientific community, "Torr" might be a convenient way to differentiate between the two, similar to the way "Kelvin" is a convenient way to differentiate between Celsius and Celsius (Absolute). However, I have never found a source that explains it in those terms. In the scentific community, this knowledge seems to be taken for granted. Zaereth (talk) 23:51, 30 January 2012 (UTC)
When was word torr introduced?
It's the first time I hear about torr. Everyone around me is using mmHg. Is it some kind of new beurocratical initiative like with mho (1/ohm)? When was word torr suggested? Should I write 120/80 torr instead of 120/80 mmHg? — Preceding unsigned comment added by Comecra (talk • contribs) 12:37, 30 May 2012 (UTC)
- Torr was named after Evangelista Torricelli, after he invented mmHg, in the early 1600s. For nearly all intents and purposes, torr is the same as mmHg. However, in most scientific journals, torr usually refers to mmHg (Absolute), (ie: 0 torr = total vacuum), whereas mmHg refers to non-absolute readings, (ie: 0 mmHg = air pressure = 760 torr. Likewise, total vacuum = -760 mmHg). So, 120/80 mmHg will usually equal 880/840 torr . I hope that helps. Zaereth (talk) 16:19, 30 May 2012 (UTC)
- If it helps, here is a definition from the book Exercise Physiology: Nutrition, Energy, and Human Performance, which helps explain the difference between absolute and relative (barometric) pressures:
The torr -- named after the Italian physicist and mathematician Evangelista Torricelli (1608--1647) who invented the barometer in 1644-- is not an SI unit but an expression of gas pressure. One torr equals the pressure necessary to raise a 1 mm column of mercury 1 mm high at 0°C against the standard acceleration of gravity at 45° north latitude (980.6 cm * s -2). One standard atmosphere equals 760 torr. The barometric reading varies with changing weather conditions and becomes lower with increasing altitude.
Zaereth, thank you for your reply. Sometimes an established term is renamed to commemorate inventor. For example term rpm (revolutions per minure) was renamed into Herz in 20-th century (The name was established by the International Electrotechnical Commission (IEC) in 1930). I have never seen anyone referencing mmHg as torrs. When was the word torr suggested? Comecra (talk) 07:10, 31 May 2012 (UTC)
- For that I'm not sure. Perhaps a little more research will reveal the answer, but I'll need to do more research. Torr is related to, but not exactly the same as mmHg, just as Kelvin is related to, but not exactly the same as Celsius. Kelvin equals Celsius (Absolute). Hertz is similar to RPM, but also not the same. Hertz is cycles per second, which usually refers to wave forms, whereas RPM is the revolutions per minute of a rotating object. In example, hertz usually refers to the cycle times of an electrical signal, say 60 hertz AC line voltage in the U.S., but 50 hertz in Europe. RPM, on the other hand, is commonly used in engineering, like automotive mechanics, in which it usually refers to the speed at which the crankshaft is rotating. The two cannot usually be used interchangeably without some confusion on the part of the readers. Personally, I think torr is similar. For instance, it's better to knw the absolute pressure when dealing with lighting construction, like making neon signs or flashtubes, because the relative pressure of the atmosphere is inconsequential. Torr is almost always used in the field of lighting. However, when dealing with blood pressure, the absolute system pressure is not nearly as important as the difference between blood pressure and air pressure, which is a better indication of burst pressure. MmHg is almost always used for blood pressure measurements. I'll try to do more research when I have the time, and maybe I'll run across the information about when torr was first used. In the meantime, it's late here, and I need to get myself home. I'll look some more in the morning. Zaereth (talk) 08:24, 31 May 2012 (UTC)
- I only had a few minutes to look this up, and there are some conflicting sources. The Oxford English Dictionary says torr originated in the 1940s. I find this to be doubtful, because torr is defined in a 1913 version of Websters Dictionary, which says the term first came into popular use "sometime before 1848." The book Vacuum Science and Technology: Pioneers of the 20th Century says the term torr was being commonly used by the late 1800s and early 1900s by scientists like Edison and Dewar, who, by 1910, were routinely drawing vacuums lower than 10-5 torr. This sounds more reasonable, as this era is when high vacuum technology was in its infancy. In 1954, torr was redefined to equal 1/760 Atmosphere, which this article already details. Zaereth (talk) 22:24, 31 May 2012 (UTC)