Gas-Based Pressure Fixed Points

Abstract

In Chaps. 7 and 8, we discussed the pressure measurements in gaseous media from 100 Pa to 100 MPa, as well as the appropriate primary and secondary pressure standards and the problems connected with their use at the lowest uncertainty level.

Appendix: Properties of Substances not Gaseous at Room Temperature of Interest for Realization of Pressure Fixed Point

Mercury t.p.

• Fixed point on the ITS-90 temperature scale

• t.p. temperature (ITS-90) 234.3156 K

• t.p. pressure 0.063 kPa

See Furukawa et al. (1982)

Water t.p.

• Fixed point of the ITS-90 temperature scale

• t.p. temperature (ITS-90) 273.16 K

• t.p. pressure 0.611657 kPa

• dp/dT liq. 0.0444 kPa K^{− 1}

• temperature reproducibility 0.03 mK

• Experimental t.p. pressure reproducibility 0.005 Pa

• Uncertainty of t.p. pressure 0.01 Pa

See Guildner et al. (1976)

H₂O(I)-H₂O(III)-H₂O(liq) t.p.

• Solid I-solid III-liquid triple point

• t.p. temperature 250.93 K

• t.p. pressure 208829.0 kPa

• Temperature reproducibility 40 mK

• Experimental t.p. pressure reproducibility 3.6 kPa

• Uncertainty of t.p. pressure 25.0 kPa

See Bignell and Bean (1989)

Water c.p.

• c.p. temperature 647.256 K

• c.p. pressure 22120.0 kPa

Mercury Melting Line

The mercury melting line equation is

$$ p/\text{MPa}=19{.}32845( d/\text{K} )+0{.}0018333{{( d/\text{K} )}^{2}}+0{.}000059791{{( d/\text{K} )}^{3}} $$

with d/K = T/K − 234.3156; T is the ITS-90 temperature; and p is the absolute pressure.

The above equation is referred to the ITS-90 and uses 234.3156 K as the mercury triple point. The uncertainties achievable in the application of the above equation, when the temperature stability around the mercury cell is maintained at the 2 mK level, are 0.1, 0.16 and 0.39 MPa, respectively, at the absolute pressures of 227.0, 756.0 and 1200.0 MPa. The total uncertainty is calculated as the sum of pressure and temperature uncertainties, plus three times the residual standard deviation of the polynomial fitting equation reported above (Molinar et al. 1980, 1991).

This equation referred to the IPTS-68 temperature scale is given here, too, because of its large use in pressure metrology:

$$ p/\text{MPa}=19{.}32835( d/\text{K} )+0{.}0017068{{( d/\text{K} )}^{2}}+0{.}000060867{{( d/\text{K} )}^{3}} $$

with d/K = T/K − 234.309; T is the IPTS-68 temperature; and p is the absolute pressure.

Notes

• 1. Other relevant data of vapor-pressure properties are given in Chap. 4, Fig. 4.1 (range of pressure and temperature values) and 4.6 (range of dp/dT and temperature values).

• 2. For the helium-3 melting line, see the relevant data in Chap. 5, Fig. 5.1.