measure all six samples by rotating the sample changer after each measurement. This 

 technique has the advantage of being simple to use, easy to calibrate, and the same 

 thermocouple is used to make all measurements. Also, one or more of the six sample 

 chambers may be used as calibration chambers by saturating a filter paper lining inside 

 the Teflon cup with standard KCf solutions. 



Measurements of water ■potential in situ: The adaptability of psychrometers for use 

 under field conditions is the most important and significant improvement resulting from 

 research in psychrometric technology. These recent contributions have opened a whole 

 new frontier in soil-plant water relations and for the first time enable the researcher 

 to gain a quantitative and true measure of the water status in natural systems. Because 

 thermocouple psychrometers have such a high degree of sensitivity to temperature fluc- 

 tuations, their use in natural systems, which are subject to large ambient temperature 

 changes, has evolved only recently. Rawlins and Dalton (1967) explained that there are 

 at least four ways in which changes in ambient temperature can cause changes in thermo- 

 couple output. First, the relationship between water potential and vapor pressure is 

 temperature dependent, as shown in equation (2). Second, the relationship between wet- 

 bulb depression and internal vapor pressure in the chamber is temperature dependent. 

 Changes in psychrometer sensitivity due to temperature shifts can be explained by the 

 temperature dependence of thermal conductivity of the chamber air; water vapor diffu- 

 sivity of the chamber air; and the heat of vaporization of water. Third, changes in 

 ambient temperature produce slight temperature gradients between the sensing junction 

 and reference junctions in the thermocouple (figure 1). Fourth, an increasing tempera- 

 ture will result in an increased water-vapor-holding capacity within the sample chamber, 

 and the relative humidity of the air will decrease until vapor equilibrium is again 

 achieved. If the sample chamber was sealed, the resulting error would be about 1 bar 

 per 0.01°C. at 25°C. ; but if the chamber was open or porous to free vapor exchange, the 

 error would be considerably less. 



In an attempt to meet the requirements of ambient temperature fluctuations and the 

 resistance to vapor flow, Rawlins and Dalton (1967) constructed a psychrometer designed 

 for use directly in the soil mass (figure 3) . Surrounding the thermocouple was a ceram- 

 ic bulb through which vapor exchange must occur between the soil and the internal atmos- 

 phere. They concluded that this instrument estimates the soil water potential within a 

 few tenths of a bar. Rawlins and others (1968) used this same psychrometer for in situ 

 measurements of soil water potential in a greenhouse study on water relations of a 

 pepper plant. 



In a somewhat unique approach to soil-plant water relations, Wiebe and others 

 (1970) used a variation of the Rawlins and Dalton pyschrometer (figure 4) consisting 

 of a small ceramic cup surrounding the thermocouple; this system measured the water 

 potential gradients in trees from the soil, up through the trunk, and through the 

 branches. These instruments were buried in the soil at various depths, and were 

 installed about 1 cm. under the cambium in the tree trunk. Diurnal fluctuations in 

 the water potential gradient were recorded for periods of up to 6 weeks. The psy- 

 chrometers had to be moved periodically in some species because of wood decay and resin 

 exudation into the cavity. 



Hoffman and Splinter (1968a, 1968b) used a psychrometer design similar to that of 

 the Rawlins and Dalton model but without the ceramic cup. The thermocouple was centered 

 in a small Teflon cup open at the bottom end, and was then buried in the soil or mounted 

 on a tobacco leaf surface with an adhesive. This design provides a psychrometer which 

 has a geometry that is identical for both calibration and measurement of soil and leaf 

 water potential. However, there is the danger of the exposed thermocouple becoming 

 damaged or bent after placing the psychrometer in the soil. Lang (1968) used a Spanner 



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