Page 165 - Mechanical Engineers' Handbook (Volume 2)

154 Temperature and Flow Transducers

Figure 17 Cutaway views of two laboratory-grade probes. 34,35

to stabilize at a temperature higher than its surroundings. The amount of self-heating depends
on three factors: (1) the power dissipated in the element, (2) the probe’s internal thermal
resistance, and (3) the external thermal resistance between the surface of the probe and the
surrounding material whose temperature is to be measured. The self-heating temperature rise
is given by

T sens T spec W(R int R ) (5)
ext
A typical probe exposed to still air will display self-heating errors on the order of 0.1–
1.0 C/mW (commercial probes of 1.5–5 mm in diameter). At 1 m/s air velocity, the self-
heating error may only be 0.03–0.3 C, while in water (at 1 m/s velocity), the self-heating
effect would be reduced by a factor of 4 or 5, depending on the relative importance of the
internal and the external thermal resistances, compared to the values in moving air.
Self-heating error can be kept small by using probes with low internal thermal resistance,
by locating the probes in regions of high fluid velocity (or placing the probes in tight-fitting
holes in structures), or by operating at very low power dissipation. Pulse interrogation can
also be used; it will reduce self-heating regardless of the internal and external resistances.
The temperature rise of an element subjected to a pulse of current depends on the duration