Validyne variable reluctance sensors such as the DP15 Pressure Sensor, DP103 Low Pressure Sensor, DP360 High Pressure Sensor, etc all have signal output(s) specified in ‘millivolts per volt’ or mV/V. What does this mean and how can you use this to determine the best electronics to use with these sensors.
A variable reluctance pressure sensor is a passive device – it will not produce a signal unless it receives an external power called excitation. Pressure sensors such as model DP15 Pressure Sensor require a special excitation: an AC voltage of 5 Vrms at a frequency of 3 or 5 Khz. Variable reluctance pressure sensors cannot be powered with DC voltages – a variable reluctance pressure sensor is an inductive device and requires AC excitation.
When differential pressure is applied to the pressure sensor the sensing diaphragm causes the inductance in the sensor coils to change. This causes a small AC signal to be developed that is proportional to the applied pressure. The amount of signal developed is expressed as a ratio of the excitation voltage. So ‘millivolts per volt’ is the amount of signal a pressure sensor will develop when full scale pressure is applied.
Validyne pressure sensors have a minimum signal of 20 mV/V, or at least 20 millivolts for each volt of applied excitation at full scale pressure. If the applied excitation is 5 Vrms @ 3 Khz, then the expected output of a sensor at its full scale pressure is 100 mVrms @ 3 Khz. The output of the pressure sensor may be much greater, however – as high as 35 mV/V – and in such a case the signal level would be 175 mVrms.
Carrier demodulator electronics rectify and amplify this small AC signal to provide a standard DC output signal of +/-5 or +/-10 Vdc. Note that the gain of a Validyne carrier demodulator will always be higher than the minimum sensor output at full scale. This allows the user to calibrate for a full scale DC output at the next lower diaphragm range using the span adjustment.
Note also that DC transducers like the P55 General Purpose Pressure Transducer, P895 Test and Measurement Pressure Transducer, P365 High Line Pressure Transducer, etc already have the carrier demodulator electronics integral to the pressure transducer enclosure and are calibrated at the factory for the full scale pressure specified on the order.