= Saturation-pressure equilibrium measurement
A concrete experiment turns the <phase coexistence curve> into pairs of <temperature> and <saturation pressure>. A 1969 paper reports <water> measurements made in 1942 and 1947–1949 using a boiler and precision <mercury (element)> <manometer>. <Platinum resistance thermometers> read the boiler <temperature> through a <resistance bridge>. The <pressure>-transfer line passed through an ice-water condenser and a dry-ice trap; <helium> filled connecting sections. Corrected <mercury (element)> column height gave the <pressure> at the thermometer coils, including <hydrostatic pressure> contributions from <water> vapor, <helium>, <mercury (element)>, and <mercury (element)> vapor.<Some Precise Measurements of the Vapor Pressure of Water in the Range From 25 to 100 °C by H. F. Stimson>[Stimson, §§1–2, pp. 493–495: boiler, bridge thermometry, and fluid-head corrections]{ref}
Controls included normal/reversed-lead readings, readings at two thermometer <electric currents> extrapolated to zero current to remove self-heating, repeated bridge calibration, and daily <triple point> <thermometer calibration> checks. Cellar <temperature> measurements corrected fluid-column and gauge-block <thermal expansion>. One measurement series was excluded because a trapped gas bubble was suspected of corrupting the <mercury (element)> head. The paper adjusted measurements to round <temperatures> using previously tabulated slopes; those adjusted values therefore are not independent measurements of those slopes.<Some Precise Measurements of the Vapor Pressure of Water in the Range From 25 to 100 °C by H. F. Stimson>[Stimson, §2, pp. 494–495, and §3, p. 495: corrections and suspected gas-bubble bias]{ref}
The thermodynamic inference requires <liquid> and <gas> in <phase coexistence>, a well-defined common <temperature>, and a <pressure> corrected to that same location. Leaks, contamination, or a <temperature gradient> can spoil that interpretation. A separate <calorimetry> measurement of <latent heat>, together with the two <specific volumes>, allows a test of the full <Clapeyron equation>. Converting a measured <phase coexistence curve> slope directly into <latent heat> instead assumes that equation; it is not an independent test.
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