The use of thermocouple psychrometers to measure water potential is based on a sound physical-chemical foundation. A definite, quantitative relation exists between water potential of a sample and the relative vapor pressure above it (Barrs, 1968, p. 281; Rawlins, 1972; Savage and Cass, 1984), as follows:
where y = water potential, R = ideal gas constant, T = absolute temperature (°K), Vw° = molar volume of pure water, e = partial pressure of water vapor in air, e° = saturated vapor pressure, and e/e = relative humidity. Equation 16.1 is called the Kelvin equation (Rawlins, 1972).
Except for T, which is always in °K, units vary according to values used. If y is expressed in bars, then R = 83.2 cm3-bar/mole-degree, V°w = 18.048 cm3/mole at 20°C, e and e° = bars (or millibars). Other values of R are: 0.0821 L-atm/mole-degree; 0.0832 L-bar/mole-degree; and 82.1 cm3-atm/mole-degree. We remember that °K = °C + 273.16. Absolute zero is at -273.16 (or -459.69°F) and it is the temperature at which a gas would show no pressure if the general law for gases would hold for all temperatures (Weast, 1964, p. F-29). Absolute zero is the hypothetical point at which a substance would have no molecular motion and no heat (Webster's New World Dictionary of the American Language, 1959). The Kelvin scale of temperature measured is in degrees centigrade from absolute zero and is named after William Thomson, Baron Kelvin. (For a biography of William Thomson, see the Appendix, Section XI.) In 1967 the 13th General Conference on Weights and Measures adopted the unit kelvin (K) as its standard for temperature, making it one of the seven base units of Le Système International d'Unités (SI system; see Chapter 2, Section II).
A measurement of relative vapor pressure, or of some related property, gives the water potential of the sample directly, provided that the sample and the space in the chamber have first come to equilibrium. Suitable electrical transducers are thermocouple psychrometers (Barrs, 1968, p. 281) or, using the more general term, thermocouple hygrometers. A hygrometer is an instrument for measuring humidity or the moisture in the air. A psy-chrometer is a type of hygrometer in which the humidity is measured with wet and dry bulb thermometers. The initial combining form of the word is psychro-, which comes from the Greek word psychros, meaning "cold" (Webster's New World Dictionary of the American Language, 1959).
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