Fig. 4.16 The dependency of total suction with RH using Kelvin's equation

The commonly used in-direct measuring techniques for total suction are

(i)  Thermocouple psychrometers

(ii)  Chilled-mirror hygrometers

(iii) Polymer resistance/capacitance sensors

(iv) Contact filter paper technique

(v)  Isopiestic (constant pressure) Humidity Control

(vi) Two-Pressure Humidity Control

The first four techniques are based on measuring the RH by controlling the moisture content of the soil samples. Other two techniques, humidity control techniques, control the ambient RH in a controlled environment while monitoring the changes in moisture contents. The applicability of humidity control techniques is generally in the dry side of water retention curve. Thus, these techniques are used in combination with other methods of total suction measurement on wet side of retention curve. The advantage of the later techniques is that a continuous distribution of total suction vs. gravimetric water content can be generated in a single test. The former techniques require manual measurement of RH values at given moisture contents for suction vs. water content distribution. The following sections describe some of the commonly used measurement techniques.