The self-heating effect in platinum resistance thermometers is a concern that requires careful consideration of the measuring current to minimize it. These thermometers have a sensing element at the tip of the sheath, which is ceramic and can vary in length and diameter depending on the application. The sensing element contains a coil(s) of fine platinum wire supported within the body, preventing individual coils from shorting together during heating/cooling.
To measure resistance, a platinum resistance thermometer is connected to a temperature measurement device, which passes a known current (I) through the sensing element and develops a voltage (V) across the sensor. The resistance (Rt) is calculated using ohms law, where tE is the temperature of the thermometer due to the environment being measured and ?tSH is the additional temperature increase due to the self-heating effect.
To measure the self-heating effect of a platinum resistance thermometer, two measurements are made: one with the normal measuring current and one with a current that either halves or doubles the power. The self-heating effect (?RSH) can be calculated along with the zero-power resistance (RZP).
The magnitude of self-heating of a thermometer depends on factors such as the size of the current passed through the sensing element, nominal resistance of the sensing element, construction of the thermometer, and thermal contact with the environment being measured. For high accuracy measurements in fixed point cells, the self-heating effect must be corrected for.