OPTIMIZING THE UNCERTAINTY DUE TO THE SELF-HEAT OF PLATINUM RESISTANCE THERMOMETERS IN PRACTICAL USE

Abstract − Self-heat of platinum resistance thermometers (PRTs) is a well-known phenomenon that occurs when measurement current additionally heats up a PRT sensor. This temperature increase depends on measurement current, PRT design, operating temperature and surrounding medium. Self-heat temperature increase can be corrected with some residual uncertainty, but this applies mainly to calibration of PRTs, while in practical temperature measurements self-heat measurement or estimation is difficult due to poor temperature stability and/or short measurement time that does not allow temperature transient effects to fade away. If not handled properly, self-heat uncertainty can be one of the largest, but often neglected uncertainty contributions in temperature measurement. A study of uncertainty optimization is presented for a measurement system composed of up to twenty PRTs that are connected to the ohmmeter via a scanner and sequentially measured. The optimal measurement procedure is discussed and the uncertainty analysis is given.