Talk:Torr

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199.168.7.243 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.

199.168.7.243 20:42, 13 August 2007 (UTC)

Presumably this is 1 mmHg at standard temperature and pressure?

Contents

[edit] Band?

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)

[edit] 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)

  1. Where did you get the silly notion that some one body must "own" each unit of measure?
  2. There is no rule that any international body must act.
  3. There is no international body with plenary authority in this area.
  4. 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.
  5. 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)

[edit] Nonexistent distinction

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 [1] 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)

[edit] Conversion tables

The following table of conversion factors appears in many Wikipedia articles:

Pressure Units
 
pascal
(Pa)
bar
(bar)		 technical atmosphere
(at)
atmosphere
(atm)
torr
(Torr)		 pound-force per
square inch
(psi)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 145.04×10−6
1 bar 100,000 ≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.504
1 at 98,066.5 0.980665 ≡ 1 kgf/cm2 0.96784 735.56 14.223
1 atm 101,325 1.01325 1.0332 ≡ 1 atm 760 14.696
1 torr 133.322 1.3332×10−3 1.3595×10−3 1.3158×10−3 ≡ 1 Torr; ≈ 1 mmHg 19.337×10−3
1 psi 6,894.76 68.948×10−3 70.307×10−3 68.046×10−3 51.715 ≡ 1 lbf/in2

Example reading:  1 Pa = 1 N/m2  = 10−5 bar  = 10.197×10−6 at  = 9.8692×10−6 atm, etc.
Note:  mmHg is an abbreviation for millimetres of mercury.


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)
It should be "equals". See the section above this. Just like feet or meters or whatever, these units have had various definitions over time. That's all that is involved here. -- Gene Nygaard (talk) 17:16, 16 November 2007 (UTC)

[edit] 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)

[edit] The cgs system

The statement that the torr “was used in the cgs system of units” is incorrect. The cgs unit of pressure was the dyne per square centimeter. It was called the barye, equal to 0.1 Pa.[1]

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 [2] 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, [3]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.

  1. ^ The Wikipedia article on the cgs system shows the torr and the barye as synonymous, which is incorrect.
  2. ^ 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).
  3. ^ http://physics.nist.gov/Pubs/SP811/sec05.html

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)