## Torr and mmHg ??

General discussion about calculators, Swiss Micros or otherwise
Walter
Posts: 1570
Joined: Tue May 02, 2017 11:13 am
Location: Close to FRA, Germany

### Torr and mmHg ??

There are several outdated units for pressure measurement. In particular, you may find torr and mmHg on old instruments (and even on modern medical instruments). There is, however, some uncertainty or contradicting information about these units. Some claim these are equivalent (e.g. https://de.wikipedia.org/wiki/Torr, referring to two publications of 2011 and 2015; https://fr.wikipedia.org/wiki/Torr), some others claim there are subtle differences (e.g. https://en.wikipedia.org/wiki/Torr and https://en.wikipedia.org/wiki/Millimetre_of_mercury, referring to a publication of 1974). Unfortunately, I cannot find a definite statement of BIPM about this.

I'm aware of this being a pretty academic problem. Nevertheless, can anyone help?

For sake of clarity: I'd like to see some younger (>2015) reference for the second claim.
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Jaymos
Posts: 614
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Location: Cape Town

### Re: Torr and mmHg ??

Walter wrote:
Mon Jun 22, 2020 4:29 pm
...referring to a publication of 1974). Unfortunately, I cannot find a definite statement of BIPM about this.

I'm aware of this being a pretty academic problem. Nevertheless, can anyone help?

For sake of clarity: I'd like to see some younger (>2015) reference for the second claim.
The 1974 doc is superseded. "BS 350-1:1974 Conversion factors and tables. Basis of tables. Conversion factors. Status : Revised, Withdrawn Published : March 1974 Replaced By : BS 350:2004".

BS 350:2004 p50, p51 under 33.2 Liquid columns:
"The following pressure units are based upon conventional density and gravity conditions:
the conventional millimetre of mercury (symbol mmHg)...
1 mmHg = 13.595 1 mmH2O = 13.5951×9.80665Pa = 133.322 Pa (approx.) 24) [ed: = 133.322 387 415]
24): 24) For detailed information on barometer conventions see BS 2520."

I do not have BS 2520.

Another pressure unit in common use, known as the torr, is equal, within one part in 7 million, to the conventional millimetre of mercury (mmHg). It is, however, precisely defined in terms of the pascal as follows: 1 Torr = 101325 / 760 Pa.

More (or less) info from the US:

Taylor, 2008, NIST 811, Guide for the Use of the International System of Units (SI)
(US National Institute of Standards and Technology)

https://physics.nist.gov/cuu/pdf/sp811.pdf

Code: Select all

p11
1 Torr = (101 325/760) Pa

p50
inch of mercury (32 deg F)                    3.386 38  E+03 Pa
inch of mercury (60 deg F)                    3.376 85  E+03 Pa
inch of mercury, conventional (inHg)          3.386 389 E+03 Pa

p51
millimeter of mercury, conventional (mmHg)    1.333 224 E+02 Pa


p47
Footnote 12: Conversion factors for mercury manometer pressure units are calculated using the standard value for the acceleration of gravity and the density of mercury at the stated temperature. Additional digits are not justified because the definitions of the units do not take into account the compressibility of mercury or the change in density caused by the revised practical temperature scale, ITS-90 ... based on Ref. [4: ISO 80000-4].
Jaco Mostert
Elec Eng, South Africa
WP34C, HP42S, DM42 for complex math; 35S, 28C, 32Sii, WP34S, EL-506P, EL-W506, PB700; owned FX702P & 11C; used 67 & 85. iOS: 42s (Byron), Free42, WP31S/34S, HCalc.
WP43C running on DM42 sn. 03818 .

Jaymos
Posts: 614
Joined: Sun Nov 04, 2018 7:03 pm
Location: Cape Town

### Re: Torr and mmHg ??

Further info from NPL, 1998:

https://www.scribd.com/document/3368356 ... vacuum-pdf
Guide to the measurement of pressure and vacuum
(NPL: This Guide to the Measurement of Pressure and Vacuum has been prepared by the National Physical Laboratory and the Institute of Measurement & Control, supported by the National Measurement System Policy Unit of the Department of Trade and Industry. London.)

footnote on p13:

(ii) millimetres and inches of mercury The conventional millimetre of mercury (mmHg) and the conventional inch of mercury (inHg) are defined in terms of the pressure generated by a mercury column of unit length and of assigned density 13 595.1 kg/m3 at 0 deg C under standard gravity of 9.806 65 m/s2. (See note (iv) below and [ 5 ] BS 2520: 1983 Barometer conventions, their application and use).

(iv) inch of water The conventional inch of water (inH2O) is defined in terms of the pressure generated by a water column of unit length and of assigned density 1 000 kg/m3 whilst subject to standard gravity of 9.806 65 m/s2. As with other ‘manometric’ unit definitions (see note (iii) above), this definition inherently limits knowledge of its relationship with the pascal. Furthermore, there are contradictory definitions in use which lead to pressure values differing by up to 0.2%. This can cause serious errors and continued use of the unit is firmly discouraged. The range of values given in Table 4-2 reflects the problem.

(v) torr The torr is defined as exactly 101 325/760 Pa E the ‘760’ coming from the original and arbitrary definition of standard atmosphere. Its value differs from the conventional millimetre of mercury by about 1 part in 7 million. (See [ 5 ] BS 2520: 1983 Barometer conventions, their application and use.)
Jaco Mostert
Elec Eng, South Africa
WP34C, HP42S, DM42 for complex math; 35S, 28C, 32Sii, WP34S, EL-506P, EL-W506, PB700; owned FX702P & 11C; used 67 & 85. iOS: 42s (Byron), Free42, WP31S/34S, HCalc.
WP43C running on DM42 sn. 03818 .

Walter
Posts: 1570
Joined: Tue May 02, 2017 11:13 am
Location: Close to FRA, Germany

### Re: Torr and mmHg ??

Jaymos wrote:
Mon Jun 22, 2020 6:50 pm
The 1974 doc is superseded. "BS 350-1:1974 Conversion factors and tables. Basis of tables. Conversion factors. Status : Revised, Withdrawn Published : March 1974 Replaced By : BS 350:2004".

BS 350:2004 p50, p51 under 33.2 Liquid columns:
"The following pressure units are based upon conventional density and gravity conditions:
the conventional millimetre of mercury (symbol mmHg)...
1 mmHg = 13.595 1 mmH2O = 13.5951×9.80665Pa = 133.322 Pa (approx.) 24)
24): 24) For detailed information on barometer conventions see BS 2520."

I do not have BS 2520.

Another pressure unit in common use, known as the torr, is equal, within one part in 7 million, to the conventional millimetre of mercury (mmHg). It is, however, precisely defined in terms of the pascal as follows: 1 Torr = 101325 / 760 Pa.

More (or less) info from the US:

Taylor, 2008, NIST 811, Guide for the Use of the International System of Units (SI)
(US National Institute of Standards and Technology)

https://physics.nist.gov/cuu/pdf/sp811.pdf

Code: Select all

p11
1 Torr = (101 325/760) Pa

p50
inch of mercury (32 deg F)                    3.386 38  E+03 Pa
inch of mercury (60 deg F)                    3.376 85  E+03 Pa
inch of mercury, conventional (inHg)          3.386 389 E+03 Pa

p51
millimeter of mercury, conventional (mmHg)    1.333 224 E+02 Pa


p47
Footnote 12: Conversion factors for mercury manometer pressure units are calculated using the standard value for the acceleration of gravity and the density of mercury at the stated temperature. Additional digits are not justified because the definitions of the units do not take into account the compressibility of mercury or the change in density caused by the revised practical temperature scale, ITS-90 ... based on Ref. [4: ISO 80000-4].
Thanks! The latter is something I was looking for. If I understand it correctly, the NIST says 1 mmHg = 133.3224 Pa and more digits are not justified here. OTOH, 1 Torr = 101325/760 Pa = 133.3223684210526315789473684210526 Pa (34 digits).

And a publication of the NIST in 2008 supersedes older stuff, IMO.
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Jaymos
Posts: 614
Joined: Sun Nov 04, 2018 7:03 pm
Location: Cape Town

### Re: Torr and mmHg ??

NIST (US) certainly does not "supersede" BS or NPL (UK), as would BS (UK) not supersede DIN (DE), regardless of the year of publication.

I prefer the reference, called conventional, to the BS sources, stating the conditions for the conventional assumptions to get to 1 mmHg = 13.5951×9.80665Pa. Most physical quantities are measured based on reference conditions, which differ if you move away from it, and so is this.
Jaco Mostert
Elec Eng, South Africa
WP34C, HP42S, DM42 for complex math; 35S, 28C, 32Sii, WP34S, EL-506P, EL-W506, PB700; owned FX702P & 11C; used 67 & 85. iOS: 42s (Byron), Free42, WP31S/34S, HCalc.
WP43C running on DM42 sn. 03818 .

Walter
Posts: 1570
Joined: Tue May 02, 2017 11:13 am
Location: Close to FRA, Germany

### Re: Torr and mmHg ??

Maybe 'supersede' wasn't the proper term in the English language. But when you have a publication of a national standard institute (e.g. NIST) of 2008 and another publication of another such institute of 1998 (or even 1974), then I dare to say that the younger is more relevant than the older for "canonical" values. Different editions of e.g. the CODATA set of constants carried progress in experimental methods as well as some new definitions for various reasons; so I tend to treat old publications with some reluctance in such matters. No offense against the previous authors - they simply couldn't know better then.
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chr yoko
Posts: 22
Joined: Tue May 05, 2020 11:23 pm
Location: France

### Re: Torr and mmHg ??

NIST should be using US Inch measure based on the mm (25,4mm), while the others may vary quite some depending on the country of origin....

Hereby a simple chart that may help you set the WP43S Unit menues for length conversions :

Gambate Kudasai !
As they say in Japan

Note: It is similarly complex for weight and volumes
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ctrclckws
Posts: 31
Joined: Sun Feb 17, 2019 4:30 pm

### Re: Torr and mmHg ??

That graph is a lovely example of why the metric system should be the standard.
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akaTB
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### Re: Torr and mmHg ??

ctrclckws wrote:
Mon Jun 22, 2020 11:40 pm
That graph is a lovely example of why the metric system should be the standard.
It already is. Then someone prefers to go otherwise...
Greetings,
Massimo
ajcaton
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Walter
Posts: 1570
Joined: Tue May 02, 2017 11:13 am
Location: Close to FRA, Germany

### Re: Torr and mmHg ??

akaTB wrote:
Tue Jun 23, 2020 12:03 am
ctrclckws wrote:
Mon Jun 22, 2020 11:40 pm
That graph is a lovely example of why the metric system should be the standard.
It already is. Then someone prefers to go otherwise...
Yeah, yeah, you-know-who again ...