Generating low-pressure shock waves for calibrating high-frequency pressure sensors
Measurements of hypersonic boundary-layer instabilities have recently been performed in a wide range of wind tunnels with fast-response pressure transducers. In order to achieve accurate amplitude measurements, the calibration and frequency response of the sensors must be understood. Hypersonic instabilities are high-frequency, low-amplitude waves, so relevant calibrations must use similar inputs. This is particularly important for the PCB-132 sensors which have been widely used to measure hypersonic boundary-layer instabilities despite the lack of low-amplitude calibrations or frequency response information. This work demonstrates the creation of extremely low-amplitude shock waves in a shock tube, which can be used to calibrate sensors for instability measurements. The shocks are created using weak diaphragms and low driven pressures on the order of 1 millitorr. A method for automatically measuring the shock arrival time and amplitude was developed. The method uses a rising-edge detector, a minimum peak-width criterion, and a simple low-pass filter to detect small shock waves in traces with low signal-to-noise ratio. Shock amplitudes as low as 0.001 psi are demonstrated. The flow is shown to agree with theoretical expectations within the existing uncertainties. Calibrations of PCB-132 sensors were performed between 0.001 and 1 psi. The sensors were found to have a linear calibration over the entire range within the sensor uncertainty. Step responses were measured for PCB-132 and Kulite sensors. The PCB-132 step responses are complicated and vary between sensors, but they are repeatable. They normalize well with the maximum peak voltage, demonstrating that the sensors are a linear system.
Schneider, Purdue University.
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