Monthly Archives: December 2013

What is the Meaning of Signal Output in mV/V?

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.

Signal Output

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.

Basics of Validyne Pressure Sensor Calibration

A pressure sensor is a device that changes pressure into an electrical signal. That electrical signal is only useful if it accurately represents the pressure applied to the sensor. Calibration is the process by which the sensor electrical signal is adjusted so that it has a known relationship to the applied pressure. After calibration the electrical signal can be measured and that can be used to determine the pressure at the sensor.

The electronics that support Validyne pressure sensors are called carrier demodulators. These connect to the transducer and – once calibrated – provide a DC signal that will be proportional to the pressure at the transducer. Typical Validyne pressure sensors are models DP15 Range Changeable Pressure Sensor, DP45 Low Pressure Differential Pressure Sensor, DP103 Very Low Pressure Sensor or DP360 High Line Pressure Sensor. Typical carrier demodulators for these sensors are CD15 Carrier Demodulator, CD17 USB Carrier Demodulator, CD23 Signal Conditioning with LED Display, CD280 Multi-Channel Carrier Demodulator or CD379 Portable Carrier Demodulator.

Note that the calibration is done as a system – transducer, cable and electronics together. We are often asked, but cannot provide, calibrated sensors. The calibration must be done with the transducer and electronics connected together.

Calibration adjustments for Validyne sensors involve two parameters: zero and span. These adjustments are located on the carrier demodulator. For the CD15 carrier demodulator, for example, the zero adjustment sets the output signal to 0 Vdc when the pressure applied to the sensor is 0. The span adjustment is used to set the output signal to +10 Vdc when the full scale pressure is applied to the sensor. The 10 VDC output signal is read with a voltmeter connected to the front-panel binding posts.

Here is a typical sequence:

  1.  Connect Voltmeter to the output terminals of the carrier demodulator
  2.  With zero pressure applied to the transducer, turn the Zero adjustment until the voltmeter reads 0 Vdc
  3.  Apply full scale pressure to the sensor.
  4. Turn the Span adjustment until the output at the voltmeter reads +10 Vdc.
  5.  Bleed out the pressure and check the zero pressure reading again – adjust to 0 Vdc as needed.

Tolerances on the voltmeter readings are typically +/-0.005 Vdc.

The pressure applied to the transducer should be known to an accuracy of 0.05%, if possible.  Pressure standards for calibration of transducers can be a slant manometer, dead-weight tester or more sophisticated devices. The important thing is to be able to apply a known pressure.

Here is a typical calibration setup:pressure sensor