IEEE Std 442-2017 pdf download – IEEE Guide for Thermal Resistivity Measurements of Soils and Backfill Materials.
4.1 Equipment required for field measurements 4.1.1 Field thermal probe The field thermal probe is fabricated from a stainless steel tubing that can be made of various diameters and lengths. The tubing contains a heater element occupying the length of the seamless stainless steel tubing. A number of temperature sensors, electrically insulated from the heater element, and the probe body are shown in Amnex A. Thermal probes of long lengths (more than 1 m) may contain multiple temperature sensors, positioned at intervals of 300 mm to 450 mm from the probe tip, with suitable means at the top of the probe for making electrical connections. In order to eliminate moisture infiltration in the probe and to reduce the initial thermal transient, the probe may be flled with an epoxy resin. 4.1.2 Power supply/power monitor An adjustable, regulated clectric power supply should be used in the constant current mode. The unit should be capable of providing a stable and constant power ranging from T W up to 250 W to allow fexibility to test with small lab probes and or large field probes. The field probes require higher power to heat the soil to the required temperatures and thus allow the determination of the soil’s thermal resistivity. If such a power source is not available in the field, a portable generator or power inverter may be used to energize the heater in the probe. In such an application, accurate control can be exercised by using an ammeter and variable resistors to adjust the heater current. This configuration shall be able to adjust the currents between 0.1 A to 5 A and thus the heater power. Alternatively, power meter (digital or analog) may also be used to accurately measure the input power being applied to the heater in the probe.