ABSTRACT
The same apparatus used for constant-current hot-wire anemometry* can also be used, with proper adjustments, for measuring rapidly fl uctuating fl uid temperatures. When we analyzed (Doebelin, 2003, p. 599) the hot-wire anemometer as a fl uid-velocity sensing device, we assumed for simplicity that the fl uid temperature was steady. In some hot-wire applications, the fl uid temperature does change signifi cantly, causing the instrument output signal to be related to both temperature and velocity, and thus complicating the measurement of velocity. When this is the case, a separate measurement of fl uid temperature is needed to correct the hot-wire reading so that it represents only velocity. If the temperature fl uctuations are large but slow, this correction can be accomplished with relatively simple (slow) temperature sensors. When the fl uid temperature variations are fast, then a fast thermometer is required. The instrument usually used here is called a cold-wire, and uses the same basic sensing element (a very fi ne wire) as does the velocity-measuring instrument, but operates it at such a small temperature difference (relative to the fl uid temperature) that its sensitivity to fl ow velocity is almost zero, while its sensitivity to fl uid temperature is good. Such cold-wire thermometers are also useful, not just for correcting hot-wire velocity measurements, but for general-purpose fast temperature sensing in fl uids.
