Plasma anemometer and pressure sensor design and characteristics

The characteristics and design of a high-bandwidth flow sensor that uses an AC glow discharge (plasma) as the sensing element is presented. The plasma forms in the air gap between two protruding low profile electrodes attached to a probe body. The output from the anemometer is an amplitude modulated version of the AC voltage input that contains information about the mean and uctuating velocity components. The anemometer circuitry includes resistance and capacitance elements that simulate a dielectric-barrier to maintain a diffuse plasma, and a constant-current feedback control that maintains operation within the desired glow discharge regime over an extended range of air velocities. Mean velocity calibrations are demonstrated over a range from 0 to 140 m/s. Over this velocity range, the mean output voltage varied linearly with air velocity, providing a constant static sensitivity. The effect of the electrode gap and input AC carrier frequency on the anemometer static sensitivity and dynamic response are investigated. Experiments are performed to compare measurements obtained with a plasma sensor operating at two AC carrier frequencies against that of a constant-temperature hot-wire. All three sensors were calibrated against the same known velocity reference. An uncertainty based on the standard deviation of the velocity calibration fit was applied to the mean and fluctuating velocity measurements of the three sensors. The motivation is not to replace hot-wires as a general measurement tool, but rather as an alternative to hot-wires in harsh environments or at high Mach numbers where they either have di&;The characteristics and design of a high-bandwidth pressure sensor that uses an AC glow discharge (plasma) as the sensing element is also presented. A dielectric barrier was utilized between two electrodes with plasma forming at its surface. As with the plasma anemometer, the output from the plasma pressure sensor is an am- plitude modulated version of the AC voltage input that contains information about the mean and uctuating pressure components. Static pressure calibration was performed to approximately 4 bar. The static calibration followed a power-law relation. AC carrier frequency and electrode length are investigated with respect to their effect on static sensitivity. The static sensitivity increased with increasing electrode length which produced a larger volume of plasma. The power-law exponent increased with increasing AC frequency, thereby, becoming more nonlinear. Shock tube exper- iments were conducted to determine the factors involved in frequency response of the plasma sensor. The dielectric thickness, electrode length, and AC carrier frequency were varied. The frequency response was maximized with the thinnest dielectric layer, moderate electrode (plasma) length, and the highest AC carrier frequency. Under these conditions the plasma pressure sensor performed as well as a high frequency response commercial pressure transducer.