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The 1974 doc is superseded.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 Status : Revised, Withdrawn Published : March 1974 Replaced By : BS 350:2004".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].