ASTM E377-08 (2020) pdf free.Standard Practice for Internal Temperature Measurements in Low-Conductivity Materials.
1.1 General- -The thermocouple junction needs to be accurately located to assure reproducibility of data from specimen to specimen and for accurate use of temperature data in computer programs for determining material thickness requirements, etc.
1.2 Recommendations- -The actual location of each thermocouple should be verified by X ray prior to temperature measurement experiments. Care should be taken to correct X-ray measurements for parallax. Thermocouple junction,lead-wire location, and gas pockets are best checked by two X rays taken in front and side view.
2. Summary of Recommendations (also summarized in Fig.1 and Fig. 2)
2.1 Thermocouple Junction Bead Diameter- -Make beads no larger than 1.5 wire diameters for butt-welded junctions and 2 wire diameters for other types of welds.
2.2 Thermocouple Lead Wire in Isothermal Surface that Includes the Junction Use a length of wire at least 25 wire diameters on both sides of the junction.
2.3 Thermocouple Wire Diameter and Holes for Wires- -Use a maximum of No.36 AWG gage wire and holes as small as possible but no larger than 3 wire diameters.
2.4 Thermal Conductance of Thermocouple AssemblyAvoid the use of relatively conductive insulation around wire in the isothermal surface that includes the junction. Bond thermocouple to the material with same or similar compound.
2.5 Electric Shorting by Char- Use a ceramic coating or tubing around the thermocouple lead wires.
2.6 Thermocouple Sensor Hot Junction Location Accuracy- Locate by X rays taken in front and side view.
4.1 General Ideally, to measure the true internal temperature of a solid body, it would be desirable not to have any foreign substance present that would create a disturbance affecting the natural flow of heat in the body. Since it is physically impossible to exclude the temperature sensor from the internal confines of the body, it is necessary that the thermal disturbance introduced by the sensor be minimized for accurate temperature measurements (See Refs (4-10)).ASTM E377 pdf download.