Tag Archives: carrier demodulator

Car Door Seal Testing

Car Door SealWhen automobile doors are closed it is expected that the sealing surfaces around the edges of the door will contact the frame properly so that the passenger cabin is weatherproof and the inside protected from rain and water. With all the different styles of doors, frames and gasket materials each new model must be tested to verify that the sealing system is effective.

One way to test the seal is to measure the pressure rise inside the passenger cabin when the door is slammed shut. If the seal is effective there will be a brief rise in pressure. We have probably all experienced this – Volkswagen Beetles were notorious for the ear discomfort on door closings as they were deigned to be waterproof. So a balance between sealing effectiveness and comfort is desired and testing the pressure rise is one way to verify that the right combination of sealing materials is being used.

The measurement of a pressure rise in the passenger cabin requires a pressure transducer with sensitivity to low pressures and fast dynamic response. One automobile manufacturer uses the Validyne DP45 to measure pressure spikes on the order of 400 Pa having a duration of 10 mSec. The DP45 is available in full scale ranges as low as 220 Pa and has a flat dynamic response on the order of 60 Hz and can thus capture a transient whose rise time is 4 mSec.

The system is comprised of the following Validyne Parts:

The system cal is convenient because of the low pressures involved – we calibrate the system here prior to shipment. The customer attaches DC power (9 to 55 Vdc) to the connector and also the 0 to +5 Vdc signal wires to a high speed data acquisition system. The transducer has 1/8” female NPT ports and these are fitted with adapters by the customer to plastic tubing that is run to the inside of the automobile passenger cabin. The door is slammed several times at various velocities and the resultant pressure rises recorded. On this basis the gasket seal and firmness can be evaluated.

Stand-Alone Pressure Transducer or Sensor + Electronics?


Validyne pressure transducers break down into two general categories:

Type 1 – A complete transducer with integral electronics

pressure transducer






Type 2 – A variable reluctance sensor and supporting carrier demodulator electronics.

pressure transducer







Type 1 category products include models P55, P61, P66, the P895 family and the DR800 and P532 process transmitters.

Type 2 category products include models DP15, DP360/363, DP103 with carrier demodulator models CD15, CD23/223, CD280 and CD17.

The transducers in both categories measure the same pressure ranges – so why would you choose one type over another?

Cost Effective DC Power and DC Signal:

Type 1 category transducers are generally more cost effective per point than are the sensor + electronics (Type 2) category. The Type 1 products come ready for DC power and produce a high-level DC signal, +-/5 Vdc or 4-20 mA. The Type 1 transducers include temperature compensation and are also available with higher accuracy because we can program corrections to sensor errors into the microprocessors in these products.

Type 1 products are generally ‘plug and play’ devices and are ideal for permanent installations.

Type 1 products, however, do not lend themselves to the changing of pressure ranges easily. It is possible to disassemble the sensor on a P55, for example, and replace a damaged diaphragm or install a diaphragm with a new range – but the correction factors and temperature compensation in the microprocessor will not be matched to the new assembly. Validyne can do this – and include new temperature compensation and error correction factors – but this takes time and has a cost.

Easy Range Changing:

The biggest reason to use Type 2 products is for convenient range changing. A DP15, for example, will be easier to disassemble and easier to replace a diaphragm than the Type 1 units. The sensor will be easier to calibrate with the zero and span adjustment ranges built into the external carrier demodulators. If fast frequency is important, the smaller variable reluctance sensors can be more conveniently close-coupled to piping than the larger Type 1 units and the electronics supporting Type 2 sensors have a higher low pass filter frequency available – up to 1 Khz.

Type 2 products are best suited to laboratory settings where pressure ranges are frequently changing, where a digital display is needed and where installation flexibility is important.

Type 2 products, however, do not have built-in temperature compensation, must be calibrated by the user with an appropriate pressure standard and are generally more expensive per measurement point.

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

Selecting the Best Validyne Carrier Demodulator

Carrier Demodulator

A carrier demodulator is the electronics that supports a variable reluctance pressure transducer. A carrier demodulator provides power to the transducer, provides calibration adjustments, receives the AC sensor signal and converts it to a useable output. What is the best carrier demodulator to use with your variable reluctance pressure transducer? The answer to that question depends on a number of factors.


Do you need a digital display in units of pressure? The CD23 provides an LED display which can be read from several feet away. The CD379 has a liquid crystal display. If a digital display is not needed, then the basic CD15 is probably sufficient.


Is 110 VAC available? Most Validyne carrier demodulators have a version that will run on standard 110 VAC power. For portable use, the CD379 is battery operated.


Looking for the lowest cost? The CD101 is a carrier demodulator circuit board. Add your own
enclosure and power supply for a custom solution.

High Gain:

The CD12 provides selectable gain settings up to 20 times higher than the standard demodulator.  Combined with the zero suppression feature, the CD12 allows observation of small pressure variations that are riding on high static pressures.


If you have several transducers, then a multi-channel carrier demodulator such as the CD280 or MC1 system may be the most cost-effective carrier demodulation.

Computer Interface:

Do you want the signal to go into a PC? The CD17 is a USB interface for a single variable reluctance sensor.  Up to 16 variable reluctance transducers can be supported by the UPC2100 PCI plug-in card or the USB2250 for USB interface to laptops. These card include complete carrier demodulation and A/D conversion as well as scale and offset factors for conversion into engineering units.

carrier demodulatorFind data sheets and instruction manuals on the products listed above www.validyne.com/carrier demodulator