Lead-resistance effects in a constant voltage anemometer

Abstract

Two effects of the lead resistances connecting the hot wire to a constant voltage anemometer (CVA) were analyzed and tested: one concerns the change in the sensitivity coefficient relating the anemometer output to velocity or temperature fluctuations, and the other the time constant of the hot wire determined by an in situ square-wave test technique. Small perturbations were assumed in both cases. The CVA output sensitivity was found to be reduced and the time constant increased with the lead resistance. Explicit formulas which involve the lead resistance, the cold wire resistance, and the wire overheat, as well as some characteristics of the CVA circuit, were established to take into account these effects. In the ranges tested, each effect can individually introduce as much as 10% error. Product of the two governs the overall response for the CVA. However, because the two effects change in opposite directions, interestingly, variation in the net response from their product is minimized. This feature may be very useful for many engineering applications of the CVA. Results of experiments conducted with the CVA in a subsonic jet are presented. They confirm the analysis and also establish that accurate measurements can be performed even with a large ratio of lead resistance to hot-wire resistance by applying the correction formulas developed with the analysis. Results from earlier experiments in a supersonic boundary layer also are presented.