Talk:Inch of mercury

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What are the measurements of the column?[edit]

Inches --agr 16:35, 20 October 2005 (UTC)

Requested move - 2006[edit]

The following discussion is closed. Please do not modify it. Subsequent comments should be made in a new section.

InHgInch of mercury – I believe spelled-out names are a part of the naming conventions, but I can't find the chapter and verse right now. Note that currently inches of mercury redirects to inHg, but inch of mercury redirects to pressure. Indefatigable 23:09, 6 January 2006 (UTC)


Add *Support or *Oppose followed by an optional one-sentence explanation, then sign your vote with ~~~~
  • oppose, I think. Titling it "inch of mercury" sounds like it's an article about an inch of mercury; it isn't, but an article about a scientific unit. I can't think of any other scientific unit whose name also has a commonsense meaning (aside from those named for people, which are already necessarily disambiguated because of that). Furthermore, let's make life easy for authors of other articles by minimizing the extent to which they need to use the pipe trick. Doops | talk 23:15, 6 January 2006 (UTC)
    • Sorry, I don't understand. What could "inch of mercury" mean besides a pressure unit? And there would be no need to use the pipe trick if the article is moved. A redirect would take care of it: that's what redirects are for. Indefatigable 17:24, 7 January 2006 (UTC)
Yes, but some people hate redirects and go around changing them all into pipe tricks which makes the original author feel guilty. And "inch of mercury" sounds like it's an article about an inch of mercury. I realize that it's unlikely that such an article would ever exist; so perhaps it doesn't matter that much. Maybe I'm being overly perfectionist. But you see the notion? Doops | talk 20:09, 7 January 2006 (UTC)
  • Oppose as per Doops. The Inch of mercury redirect should point here instead of pressure, obviously. Jokermage "Timor Mentum Occidit" 00:55, 7 January 2006 (UTC)
    Neutral. After thinking about it, I don't really care either way. Jokermage "Timor Mentum Occidit" 12:48, 8 January 2006 (UTC)
    Support. I checked some other articles about units of measure and they all use spelled out names as the main article, with disambs or redirects at the abbreviations. For consistency, we should do the same here. Jokermage "Timor Mentum Occidit" 18:48, 8 January 2006 (UTC)
  • support Foot-pounds are found under foot-pound force, not ft-lb. So here. Septentrionalis 00:20, 8 January 2006 (UTC)
  • support Should be at spelled out name, with symbol as redirect in this case (for many other units needs to go to disamb page, but not here). Gene Nygaard 03:33, 8 January 2006 (UTC)
  • I Remember when CmH2O was the title of another pressure unit. What an ugly title that was, an the uppercase "I" in this title is as ugly as that C in centimeters of water. Gene Nygaard 03:37, 8 January 2006 (UTC)
  • Change my vote to Merge this stub with the long-existing article at torr, another name for this unit, and redirect both "InHg" and "inch of mercury" to that article. Or put the article at the spelled out "inch of mercury" and redirect "torr" to it, but that article is much better than this one. Gene Nygaard 03:40, 8 January 2006 (UTC)
    • Torr is not the same unit, it's a millimetre of mercury. Since inHg is widely used in the U.S, it deserves it's own unit. I don't have a strong view on the renaming of the article as long as all the indirects point to it.--agr 16:44, 8 January 2006 (UTC)
  • Change my vote back to support. Oops, that was rather boneheaded of me, wasn't it. I do know the difference between and inch of mercury and a millimeter of mercury. Yes, it should be at the spelled out name; at least an initial capital letter of the spelled out word doesn't violate the rules for the use of symbols. 17:45, 8 January 2006 (UTC)

[that was me, not logged in Gene Nygaard 00:26, 9 January 2006 (UTC)]

  • Support. Units should be written out. This article is about an inch of mercury, so I fail to see how that could be a problem with the move. — Knowledge Seeker 04:03, 9 January 2006 (UTC)
  • Support: the most simular pages I could find in terms of name are Centimetre of water and Pound-force per square inch which are both spelled out in full thus. Jonathunder 23:40, 10 January 2006 (UTC)
  • Support - LuiKhuntek 02:25, 11 January 2006 (UTC)

Moved. —Nightstallion (?) 07:53, 11 January 2006 (UTC)

The above discussion is closed. Please do not modify it. Subsequent comments should be made in a new section.

How was the calculation made?[edit]

Can someone detail the calculation? USA todaysays 29.92 inches of mercury is "standard" atmospheric pressure at sea level ("Q: What is the formula for converting pressure in millibars of pressure to inches of mercury?"). Torr page says 1 atmosphere is 101,325 pascals. Python says:

$ python -c "print 101325/29.92"

which differs in the 5th digit (and onwards) from the 3,386.389 on the page.

What's up? dfrankow 18:06, 20 February 2007 (UTC)

I think it's just round-off error. 1 atm is defined as 101,325 Pa, and 1 Torr is defined as 1 atm/760. If we take 1 Torr = 1 mmHg, which it is for practical purposes, we get 1 inHg = 25.4 mmHg × (1 Torr/mmHg) × (1 atm / 760 Torr) × (101,325 Pa/atm) = 3386.388158 Pa. If we find the conversion of atmospheres to inches of mercury with more precision, it all works out: 1 atm = 760 mmHg × (1 inHg / 25.4 mmHg) = 29.92125984 inHg.
101,325 Pa / (29.92125984 inHg) = 3386.388158 Pa/inHg.
In other words, since 29.92 inHg/atm is accurate only to 4 digits, any calculation you make with it is going to be accurate only to about 4 digits. Indefatigable 04:15, 23 February 2007 (UTC)
No. The conventional value isn't a measured quantity, but a definition.
  • The now-conventional inch of mercury is defined to as 25.4 mmHg, the same as the relationship between an inch and a millimeter (this isn't necessarily true when comparisions include either unit of pressure in an older version, and the relationship between the units of length has also varied over time and place).
  • The now-conventional millimeter of mercury is defined by setting the standard atmosphere to be simultaneously equal to 101.325 kPa and 760 mmHg. In other words, 1 mmHg = 101325/760 Pa.
  • For the standard atmosphere, 1 atm = (760 mmHg)/(25.4 inHg/mmHg) = 29117127 inHg ≈ 29.9212598... inHg
Of course, the same relationship doesn't hold for the old 60 °F inches of mercury (which was a measured, not defined, quantity). But that, too, was known to about six significant digits, and comparing it to the exact, by definition, value for the standard atmosphere of 101.325 kPa will still leave you with six significant digits in that relationship. You can do the calculations yourself;
  • 1 atm = 30.0058 inHg60 °F.
Gene Nygaard 14:13, 21 October 2007 (UTC)


The article says "It is defined as the pressure exerted by a circular column of mercury of 1 inch in height at 32 °F (0 °C) at the standard acceleration of gravity." This is a bit vague since the density of the mercury isn't specified (sure, we can measure the density at 0 °C but this will still be a measured value rather than a defined one).

Gene above, on the other hand gives a different definition (albeit essentially the same since the millimetre of mercury is based on the density at 0° C just standardised to an exact value). According to Gene, the inch of mercury is defined as 25.4 mmHg, which makes much more sense.

So which is it? Jimp 09:34, 24 February 2014 (UTC)

I suspect formal definitions of this conventional unit may vary from place to place. The UK'S standard "Conversion factors and tables" BS 350:Part 1: 1974 says (showing precise factors in bold)
"1 mmHg = 13.5951 mmH2O = 13.5951 x 9.80665 Pa = 133.322 Pa (approx)
"1 inHg = 25.4 mmHg = 9.80665 x 13.5951 x 25.4 Pa = 3386.39 Pa (approx)"
but does not reference any legal definitions. BS 1042:1:1964 notes that the "conventional pressure of 30 inHg is based on the conventional density of mercury (13.5951 g/cm2) at 0oC" but also lists a reference pressure of 30 inHg that was "the pressure due to a 30 inch column of mercury at 60oF under an acceleration of free fall of 32.1740 ft/s2" adding "This reference unit of pressure has been commonly used in the gas industry where, however, the acceleration of free fall is not specified but may be taken as the value mentioned above to within 1 part in 750 in any part of the U.K. and within 1 part in 1500 in or near London."
I think it might be reasonable to say that as conventional definitions have converged, the inHg is now exactly equal to 25.4 mmHg. NebY (talk) 15:34, 25 February 2014 (UTC)