This method covers the procedure for determining total soil suction force using thermocouple psychrometers. The test results of this method are to be used in conjunction with AASHTO T 258.

The thermocouple psychrometer measures the relative humidity in the soil by a technique called Peltier cooling. By causing a small direct current of approximately four to eight milliamperes to flow through the thermocouple junction for approximately 15 s in the correct direction, this junction will cool and water will condense on it when the dewpoint temperature is reached. Condensation of this water inhibits further cooling of the junction and the voltage difference developed between the thermocouple and the reference junctions can be measured using a microvoltmeter. With proper calibration, the thermocouple psychrometer output in microvolts can be converted directly to soil suction in convenient units of pressure. Typical thermocouple psychrometer output voltages vary from less than one microvolt for relative humidities close to 100 percent or total soil suction less than 95.76 kPa (1 tsf) to about 25 microvolts for relative humidities of about 95 percent or total soil suctions of about 5746 kPa (60 tsf).