Talk:Relative density

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Major copy edit proposed[edit]

The problem with this article is that it goes in depth into related issues and this is not required. The density of water is not an issue to be discussed in any detail here. I (Paul) am proposing a major re-write which will say everything there is to be said about relative density directly, with links to air and water and density and other articles too. Therefore, the article will be MUCH SHORTER. Paul Beardsell 04:15, 18 November 2006 (UTC)

I agree with many of your points. However, I think the article should be re-named "Specific gravity" and re-written. The vast majority of readers having any knowledge whatsoever of chemistry, engineering or physics are much, much more familiar with the term "specific gravity" ... so why get involved in arcane terminology like "relative density" ... and why introduce those who have no knowledge of chemistry, engineering or physics to an arcane term that is not commonly used? - mbeychok 06:12, 18 January 2007 (UTC)


I suggest that the material relating to the kidney be moved to Kidney or to Kidney function or, if we must, to Specific gravity (kidney). The kidney is a machine (a living machine, an organ) and an important one but there are numerous machines (and organs) for which the specific gravity of a solution or suchlike is important. This article should say what RD / SG is and describe that BUT discussion of machine / organ function and medical treatment details / engineering maintenance instructions should be moved to the page about the organ / machine. Material should not be duplicated at WP - it should be linked to. Move imminent. What say you? Paul Beardsell 23:33, 19 January 2007 (UTC)

Move to a subsection of Kidney function. This is so random an inclusion in this article it is laughable! SauliH 00:00, 20 January 2007 (UTC)

I've cut the material. I'll put it here in case anyone wants to rescue it for inclusion in a more appropriate article. Paul Beardsell 21:22, 22 January 2007 (UTC)

Actually, in the clinic, when we say "specific gravity", we are referring to specific gravity as it relates to kidney function and not "specific gravity" in a general sense. So, it's really not all that surprising (or laughable for that matter) that an editor included the renal discussion in this article, as anyone with a medical background will likely arrive at this article when searching for information on urinary specific gravity. Given that, I'd suggest moving the text in question to Specific gravity (kidney) and highly suggest linking to it from this article or from a disambiguation page. The article on Kidney function covers general issues of kidney physiology and would not be appropriate for a clinical concept like specific gravity. JayLitman 17:48, 1 February 2007 (UTC)

I went ahead and moved the text to Specific gravity (kidney). There are also several articles on WP that link readers to specific gravity for further information on urinary specific gravity. I'll try to fix as many as I can. JayLitman 14:55, 2 February 2007 (UTC)

sink'n' float[edit]

relative densities are not about liquids (only). I will be removing the para that goes on about sinking and floating "in the reference [substance]" as if the reference could only be a liquid, as if the other substance is not itself another liquid, as if the the other substance would not dissolve! Too clever. Paul Beardsell 07:08, 29 January 2007 (UTC)

Actually, the concept of "sinking and floating" also applies to the relative density of gases, as gases with a realtive density greater than 1.0 will "sink" below the reference gas. For example, if you have a mixture of nitrogen and carbon dioxide at room temperature, the carbon dioxide will "sink" to the bottom of the mixture. I'm not sure that removing the section will necessarily help, but I would suggest substituing the word "float" with "rise." I think "float" is little less descriptive, and possibly misleading for some people. JayLitman 15:09, 2 February 2007 (UTC)
You miss the point. Relative density is not only to do with fluids (which, by the way, includes both gases and liquids) but also with solids. Paul Beardsell 21:36, 2 February 2007 (UTC)
You're right. I don't see your point. I'm well aware that relative density applies to more phases than just liquid, as should be evident from my response above, but I still don't see why the paragraph on "sinking and floating in a reference substance" should be removed. I would rather see it revised. Perhaps an extra sentance or two expanding on relative density as it relates to different phases. JayLitman 22:33, 2 February 2007 (UTC)
Sinking and floating depends upon gravity as well as relative density. In space relative density survives but there is no buoyancy. What you want to write about is well covered at the buoyancy article. That's the place for it. Paul Beardsell 05:41, 8 February 2007 (UTC)
Also: Light salt "sinks" past heavy stones. This sinking and floating is a nonsense and should not be here. Paul Beardsell 05:42, 8 February 2007 (UTC)
Leave as is. It demonstrates a simple concept that most readers will be familiar with; but yes, you need gravity for it to be true, but I really don't think anyone will care since this concept really only is used on Earth. The same "float" "sink" concept applies for solids ... how do you think Plate tectonics works? The term "float" is not a toy concept term ... continents (or crust) really do float on asthenosphere, and none of these solids! +mwtoews 09:36, 8 February 2007 (UTC)
No. I'm removing. It's buoyancy. Paul Beardsell 10:25, 8 February 2007 (UTC)
You really didn't explain why you don't like it, and I'll argue that your "improvement" is in the wrong direction, as there was nothing inaccurate about "sink" and "float" (where there is gravity), and most readers can understand that concept better. Certainly yes, buoyancy is the force coupled with gravity that allows things to actually float, but now you've lost John Q Public as he is trying to figure out his alcohol content in his home-brew! Your initial argument about liquids is stale in this discussion since clearly the "float"/"sink" concept works for solids and fluids; so please elaborate why you made this change. +mwtoews 17:48, 8 February 2007 (UTC)
I did explain why, and well enough. But that is not to say that more explanation is not possible. I note however that you do not address arguments I have already made above. But here goes: (i) What is happening, by bringing in "sink and float" here, is conflation of two separate albeit related concepts as if they were equivalent. This, it goes without saying, is bad. (ii) This is happening in the introductory paras of the article - the last place for it! (iii) Sink and float does NOT work well for solids, see salt'n'rock arg above for just one example. (iv) Sink'n'float requires gravitation (or other acceleration) but this is not mentioned in this article. (v) What is being described is buoyancy but this is not referenced. (vi) The home brew will dissolve in the reference substance and never float nor sink. I could go on and on. If you want to mention sink and float then please do so, later in the article, not in the intro, and clearly referening buoyancy. Thanks. Paul Beardsell 20:43, 8 February 2007 (UTC)
OK, thanks for specifying your exact reasons. I agree that it is a separate/related concept, so should not appear in the intro para, and we'll leave your edits as is. I may add it later (time-wise and article-wise), as an example behaviour, and spell out the conditions in which the related concept holds true. The only reason why I consider this detail important is in the interpretation of the numeric value of relative density, for d<1, d>1 and d=1. (In general, this article needs a bit of work.) Also, in the home brew example above, I was referring to one of these; dissolving does not apply here—its just a relative measure of sinking-ness or floating-ness to determine alcohol content. +mwtoews 21:12, 8 February 2007 (UTC)
Yes, this article needs work. I certainly was not advocating the return to some previous version. As happens so often at WP this article is improved by to and fro argumentdiscussion that happens on the talk pages. Paul Beardsell 03:24, 9 February 2007 (UTC)


If man would possess any sort of "logic" a whole new term would be derived "Gravitational Density" and both pages would be place under that and the whole goofy debate would be put to rest. Good grief. And by the way, Gravitational Density is what keeps each planet their exact distance from the sun. Must see —Preceding unsigned comment added by (talk) 19:43, 19 March 2011 (UTC)

I think this page ought to be moved to Specific Gravity. The term relative density has meanings other than Specific Gravity - i.e. the denominator is not always the density of water. SG, however, is always relative to water. Toiyabe 19:07, 13 March 2006 (UTC)

I agree wholeheartedly. The vast majority of readers having any knowledge whatsoever of chemistry, engineering or physics are much, much more familiar with the term "specific gravity" ... so why get involved in arcane terminology like "relative density" ... and why introduce those who have no knowledge of chemistry, engineering or physics to an arcane term that is not commonly used? - mbeychok 06:07, 18 January 2007 (UTC)
Coming from a gemology perspective, I would much prefer readers linking from gemology articles land on an article about SG not 'relative density'. SG is an article very much in need of writing, as it is used in various fields of study - relative density doesn't cut it. Can someone who knows the subject area take care of this please? SauliH 07:58, 19 January 2007 (UTC)

I've written a short description of SG. SG is specific in that it intrinsically refers to the standard substance as water. To speak of just relative density, leaves the question of which standard substance used unanswered. The article on Relative density also seems to cover topics which to me seem better explained and covered elsewhere. Pae_nor Oslo, Norway, 18:48, 14 February 2007

The content from specific gravity has been merged into this article, and some distinctions have been made to try and point out the subtle differences between the terms. That is that SG has a reference density as water, while relative density is more general. Please feel free to expand on this. +mwtoews 20:59, 14 February 2007 (UTC)

Inconsistent article[edit]

This article is in need of some cleanup. The article is inconsistent, inaccurate and somewhat verbose. E.g it states that relative density is both similar and the same as Specific gravity. Also, it is inaccurate to state that "quantify the buoyancy between two materials": Bouyancy is, simply stated, an upward force as experienced by an object wholy or partly immersed in a surrounding fluid (gas or liquid), i.e substances incapable of sustaining shear forces. Calculation of SG can be done irrespective of units used, as long as the units used are consistent, i.e. don't divide slugs/ft3 with kg/m3... When it comes to Specific gravity (SG) i still feel that the article I wrote is more accurate w.r.t SG and should not be redirected. Pae_nor 09:52, 15 February 2007 (UTC)

Yes, I'll agree that the article needs a good revision, and some of the content is questionable and needs references. However, splitting this article into Relative density and Specific gravity articles is not desirable, as these are very similar, as the only difference that I am aware of is:
G=\frac{\rho_\mathrm{object}}{\rho_\mathrm{reference}}\quad \mathrm{and}\quad SG=\frac{\rho_\mathrm{object}}{\rho_\mathrm{H_2O}}
Am I missing something? (Also, I'm unsure of what a more proper notation for relative density should be ... ideas?) +mwtoews 18:05, 15 February 2007 (UTC)

Water density (Feb 2007)[edit]

since density of water is (approximately) - I am no physics expert but I had been taught that the metric system was designed around the physical characteristics of wtaer - meaning the density of water would be precisely as stated and not approximate. Am I wrong? SauliH 18:29, 27 February 2007 (UTC)

The metric system was only sort-of based on 1kg=1L of water, and for many calculations, this assumption works just fine. However, as it turns out the density of water is not constant, as it is temperature dependent (everything is—not just water). That's why lakes stratify in the winter (4°C at the bottom, since it is densest). The metric system is now based on the wavelength by an atom of cesium-133 and the such.+mwtoews 01:23, 28 February 2007 (UTC)
Ok. You learn something ne... Cheers!SauliH 01:49, 28 February 2007 (UTC)

Overhaul of the page[edit]

I did a recent semi-overhaul of the page. But I'm not an expert, I'm just an engineering college graduate who got A's in his physics classes and math classes. I know the power of using natural units and I appreciate using unitless dimensions. Hence the large section for the unitless properties of RD. Also, my physics proff. wanted us to demonstrate the hydrometer problem shown. I figured it was important enough to be encyclopedic. I've got a second proof, showing that a hyperbolic cross sectional area would create a linear relationship between displacement and change in RD, but decided it was not encyclopedic enough to warrant placing it on here. Since it required the use of calculus, I doubt most readers would understand it as well. (Also, I'm not quite an expert, as I've said before.)

If anyone has grammar issues, spelling issues, etc... change up the page. I'm not a grammar expert, and I don't know how to program this math code for beans. If anyone thinks the math isn't important enough to warrant placing here, let me know and I'll just put it on my talk page. I don't have any source for the proof I listed concerning the hydrometers, but I did cite other wiki pages which had sources and used basic algebra for my proof. Let me know if you think this info is usefull or not. --Markozeta 01:35, 13 March 2007 (UTC)

"Peer Review" March 2007[edit]

Please see comments at Scientific peer review/Relative density (March 2007) Could this and other highly technical articles have a Basic, or Simple, paragraph, or a few, near the top? Because all I was looking for was to compare how dense water is to iron [as was discussed on a program about the universe]. It seems related to the issue that reality is very thin, it's practically a ghost world. (talk) 06:15, 15 February 2013 (UTC)

Standard International Units.[edit]

While I understand the insignificance of units in the solution to a RD calculation, I think it would be more appropriate for the article to be consistent with the unit's it uses to describe RD. Presently, the movement between the SI and Imperial systems (namely in the 'Specific Gravity' section) may be confusing for those not familiar with the topic, and it may also create unnecessary difficulties for those who are attempting to make such calculations. It may also be wise to do so purely in the interest of maintaining a set standard, the SI. Psydexzerity 13:50, 25 June 2007 (UTC)

Request #1,282 for a Separate 'Specific Gravity' Article[edit]

Can we just have a plain specific gravity article for those who need it? Just an explanation of what specific gravity is and how to measure it? At a pinch, where the measurement could be useful? This article is full of overly technical bollocks and is hence fairly useless. And it has no mention of pycnometer measurement of sg. Last time someone tried to do the right thing and separate the two they were merged again. I am happy to assist in creating an sg article (I am a chemist and know little about its application outside chemistry so would not volunteer to write it by myself).-- 04:52, 12 July 2007 (UTC)

Relative density v Specific gravity[edit]

I think the use of these terms has changed over the years. The latest Macquarie Dictionary in Australia for relative density redirects to specific gravity. The definition of specific gravity states that it is the density relative to that of a standard, with the standard being water for liquids and solids and hydrogen or air for gases. We need to address this in detail looking at different definitions in different places at different times and have everything referenced. Does anyone have a number of different science or physics dictionaries to hand? --Bduke 23:10, 30 July 2007 (UTC)

Cleanup -lengthy mathematical derivations (Mar 2008)[edit]

I am tempted to remove some of the lengthy mathematical derivations in this article and just retain the key steps, concepts and results. At the moment it looks more like a textbook than an encyclopedia article. Does anyone object to this? (talk) 02:34, 6 March 2008 (UTC)

Since there have been no comments I have started by condensing the hydrometer explanation. (talk) 23:56, 11 March 2008 (UTC)
I ended up reorganising most of the article. I've cleaned it up as best I can and removed the cleanup tag. I left the "expert attention" tag because I am not an expert and I haven't actually changed the information content. (talk) 15:10, 12 March 2008 (UTC)
Hey, I added that textbook jargon you saw before - and I want to say - thanks for the summary, I think it's better this way. --Markozeta (talk) 00:06, 1 May 2008 (UTC)

Equation image error?[edit]

Under the section Measurement, the second image showing an equation seems to introduce specific numeric values that come out of nowhere. I think these should be trimmed off, leaving the first bit, but wanted to ask if I was missing something. Thoughts? Edgehawk (talk) 15:19, 4 September 2010 (UTC)

Merge discussion[edit]

Specific gravity and relative density cover almost exactly the same material. The either need to be made more distinct or merged. Not that long ago it was tried to merge it under relative density, but that failed. I am hoping that the reason it failed was because there wasn't enough discussion before hand. So lets form a consensus and get this thing done ;) .TStein (talk) 17:57, 22 October 2010 (UTC)

  • merge to specific gravity: My experience is that specific gravity (while easily confused with specific weight is the more common. And it is noted that the term 'specific gravity' is now often used more loosely to refer to using other fluids like gas. Unless there is some technique used for other fluids and not water I see no reason to have relative density hanging around. TStein (talk) 17:57, 22 October 2010 (UTC)
  • Merge to specific gravity, far more common term in my experience (geology/mineralogy); and check the what links here for each. Also note the relative density article uses specific gravity quite a bit -- perhaps a remnant of bungled merge? Way too much overlap in the two articles. Vsmith (talk) 00:25, 23 October 2010 (UTC)
Note re my comment about "what links here", many of those are mineral articles with the link in the mineral infobox. My brain just kicked in after posting that. Vsmith (talk) 00:45, 23 October 2010 (UTC)

Specific gravity is the more widely recognized term. However, relative density is more appropriate.I agree the topics should be merged under Specific Gravity. ISAnerd (talk) 20:06, 2 November 2010 (UTC)ISANerd

I support the merging to specific gravity as well. I see little difference between the two, and in my studies I've almost always heard it referred to as specific gravity. While I agree "relative density" makes more sense in terms of words, we should use the most common term per WP:COMMONNAME. (talk) 00:30, 28 December 2010 (UTC)

I am happy with the situation as is appears on the "relative density" page. The term "relative density" is usually used in the resources industry. 17:44, 19 March 2011 (Perth) Johnhvisser — Preceding unsigned comment added by Johnhvisser (talkcontribs)

The term "Relative density" is used in European Pharmacopoeia and all nationals pharmacopoeias. The text explains also methods as they are mentioned in European Pharmacopoeia. I did not meet the term specific gravity in pharmaceutical sciences. — Preceding unsigned comment added by (talk) 15:35, 3 August 2011 (UTC)

make coherent set of density, relative density, etc
Even in outer space, where there is no gravity, there is still mass, meaning that a heavier object needs more force to be accelerated ; So term 'specific gravity' is a bit misleading, what is meant is 'specific density of mass'.
Word 'specific' means that it is a property of a material, but is in practice used as if it means 'relative to density of water'.
A material that has specific density 1 has absolute density 1000 kg/m3, so these terms should not be mixed up.
I think it would be best to have

  • one main article called 'density' (already exists)
  • one main article called 'relative density',

which explains that this is relative to density of water (which is 1000 kg/m3), and links to 'density' and 'water' pages.

  • one small article called 'specific density',

that mentions that this term is in common use, and is also known as 'specific gravity' and 'specific weight' but really means 'relative (mass) density', and links to 'relative density' page.

  • a redirect from 'specific gravity' to 'specific density'
  • a redirect from 'specific weight' to 'specific density'

Thusly organizing these articles seems not difficult, more work would be needed for removal of off-topic content, such as, in 'specific gravity', description of a flask that is said to be used for measuring specific gravity, while it really only allows to measure off a precise volume. Ofcourse this flask can be weighed, but it is still a tool for volume measurement ; to measure density directly one would need to measure buoyancy.Siwardio (talk) 11:00, 27 August 2011 (UTC)

For the reasons you mentioned in the first lines, I also think relative density is the physically more precise term and (at least for me as a chemist) is pretty much self explanatory opposed to specific gravity (.."specific gravity" made me even wonder about things like Three-body problems and Einsteins Special Theory of Relativity .. ;) ). --Saimondo (talk) 18:26, 10 January 2013 (UTC)

Temperature Dependence[edit]

Typo or in need of greater explanation:

I do not see the validity of a simple division used as was referenced:

"Taking into account different sample and reference temperatures we note that while SGH2O = 1.000000 (20°C/20°C) it is also the case that SGH2O = 0.998203/0.998840 = 0.998363 (20°C/4°C)."

Followed by:

"Here temperature is being specified using the current ITS-90 scale and the densities[4] used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale the densities at 20 °C and 4 °C are, respectively, 0.9982071 and 0.9999720 resulting in an SG (20°C/4°C) value for water of 0.9982343."

Again the numbers do not agree. 0.999 362 260

0.998 207 1 / 0.999 972 0 = 0.998 235 050 The average is 0.999 089 55

Was there an interpolation done based on some criteria, a formula used, different numbers used from some other reference, or are these typographic errors?

As for an error in significant digits used, I do not care if they are just typo. But if another process is involved, I would like to know what process was used to generate the value for water of 0.998 234 3

Jamesbdunn (talk) 14:36, 30 December 2012 (UTC)