A pressure transducer can directly measure the force of gas or liquid, and convert the value into an electrical signal. Pressure transducers typically include a sensing diaphragm capable of responding to changes in pressure. Pressure pushes on this diaphragm, changing its position, and this changes the inductance of sensing coils mounted opposite the sensing diaphragm. The coils are excited with an AC waveform and the resulting change in electrical impedance represents the applied pressure. The electrical output of the coils is converted to a DC signal.
Pressure Transducers – Classifications
Pressure transducers come in a variety of shapes, sizes, and output signal types. In addition to a DC voltage output, current signals are often used for electrically noisy environments commonly present in industrial applications. A 4 to 20 mA signal has been adopted as the industry standard and the current signals can be sent accurately beyond 1,000 feet. All pressure transducers are generally characterized by their pressure measurement range. They are also classified by accuracy, errors due to temperature change, and the amount of static pressure the sensor can tolerate. Temperature always affects transducer accuracy and most transducers have a scheme to correct for ambient temperature changes Resolution is another characteristic used to evaluate a pressure transducer, and this is defined as the smallest amount of pressure change that can be detected – typically a function of the signal-to-noise ratio of the output.
Electromagnetic interference (EMI), can also affect transducers. Some units are protected against EMI effects, but only up to certain intensities. Materials used to make the sensors vary, and include plastic, silicon, stainless steel, or epoxies. Epoxies can be adversely affected by certain fluids under pressure.
Some pressure sensors are mounted to a circuit board with contacts to secure a solid connection. Others are designed for industrial environments and sturdily constructed with weatherproof enclosures. If the device is for general use, it is likely to have a standard design that allows it to easily connect to commonly used receiving devices such as computers, programmable controllers and panel meters. The costlier transducers are known for their high-accuracy readings and low rates of error as a percentage of full scale range. At Validyne, we offer many different configurations to meet your exact requirements.
Types of Transducers
At Validyne, we serve several major markets and carry a variety of transducers, which include differential pressure sensors, gage pressure transducers, USB pressure transducers, electronic pressure manometers, low pressure transducers, OEM pressure sensors, and more.
Transducers perform a critical job in every industry, especially in automation and control. Transducers used in aircraft or healthcare applications have lives depending on reliable and accurate performance.
Digital display specifications can be confusing. Here is a short application note that describes the terminology and capabilities of a digital display..
Digital displays are specified as being either 3-1/2 digits or 4-1/2 digits. In a 3-1/2 digit display the three rightmost digits may read any value between 1 and 9. The left-most digit is limited to 1 or a blank. It is often easier to think in terms of counts: a 3-1/2 digit display can read up to 1999 counts, so it is suitable for displaying readings that have a maximum value of 20, 200 or 2000. Note that the decimal point may be placed within the right most three digits: 19.99, 199.9 or 1999.
Similarly a 4-1/2 digit display may read 19.999, 199.99, 1999.9 or 19999.
To specify the correct number of digits in a display, keep the following in mind: The resolution of the display should be five to twenty times better than the accuracy.
For example, if a 20 psi transducer with 0.25% FS accuracy is used, a 3-1/2 digit display configured to read 19.99 psi will provide a resolution of 0.01 psi when the accuracy of the transducer is 0.05 psi, and this is a good balance between resolution of the display and accuracy of the sensor.
A 4-1/2 digit display specified for the same transducer would read 19.999 psig at full scale, and the last digit would be unusable since it represents 50 times more resolution than the transducer accuracy.
However, if the transducer accuracy is 0.1% FS, then the 4-1/2 digit display would be appropriate because the accuracy of the transducer is now 0.02 psi and the 4-1/2 digit display would have a resolution 20 times greater than the accuracy. A 3-1/2 digit display, by contrast, would only read to 0.01 psi – just twice the accuracy of the sensor.
The diagram below shows how the 3-1/2 and 4-1/2 digital displays are used.
Check out Validyne products with a Digital Display
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.
Variable reluctance pressure transducers may be disassembled and the range changed by installing a new sensing diaphragm. When changing diaphragms, the correct body bolt torque is extremely important to performance.
The sequence for torquing the body bolts is just like putting the wheel on a car. Try to apply the torque in stages. Do not torque one bolt fully while the others are still loose.
Here is a listing of the bolt part numbers and correct body bolt torque settings, by transducer model number:
Here are the tools needed: