Anemometer hot wire6/10/2023 ![]() Both power dissipation and heat leak are proportional to length, so it should wash out. Note that I think only wire diameter and resistivity matters here, not the length. He used Tungsten, of diameter 5 microns and resistance of 4.82 Ohms. That matters! Can't put it in the upper leg of the Wheatstone bridge!īoth Tungsten and Platinum has positive temperature coefficient of resistance. Note the resistive element is wired to ground. constant resistance, seems the normal configuration: Using an op amp circuit to maintain constant temperature, i.e. There is an optimum bias voltage to obtain maximum sensitivity to airflow. Kielbasa ( 10267-Volume59_Issue2-13_paper.pdf) gives a good description of how to run one of these devices. Operating at constant voltage avoids this, since P=V^2/R as temperature goes up the resistance increases and power dissipation goes down. If we ran at constant current, since P=I^2R we'd get thermal runaway. Yes, the resistance increases with temperature. Temperature coefficient of resistance at 20C is 0.39% per degree. Resistance at 20C is 2.69 Ohms and lead resistance is 1.89 Ohms. So it's a Platinum wire of diameter 0.4 mils and length of about 0.125 inches (8/64 = 1/8 inch). This is effectively the use of a thermistor in a regime where self-heating is high, and the equilibration temperature is determined mainly by convective heat loss into the surrounding flowing air. Hot wire anemometers are a traditional way to measure turbulence and have good frequency response. dynamic pressure, but as a quasistatic quantity. ![]() It's easier to measure wind speed than it is to measure pressure. Our problem arises from variations in the dynamic pressure at frequencies above the bandwidth of the active mirror support systems. If the flow field u(x,y,z) was totally time-independent, then the dynamic pressure is steady in time and the mirror support system would compensate. For compressible gases P_d=(1/2)M^2 * gamma * P where M is the flow's dimensionless Mach number M=v/c with c being speed of sound, gamma =1.4 for air is the ratio of specific heats, and P is the static pressure in Pascals. This dynamic pressure exerts a force F=P_d*A on the projected surface A. The total pressure on a surface is a combination of PV=nRT type motions due to thermal energy content in a parcel of gas that is not moving, plus a dynamic term P_d=(1/2) rho * v^2 which is the kinetic energy per unit volume of the material. The terminology used here is to define 'dynamic pressure'. Vibrations of the optical system are driven by wind loading.
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