Electronic Pressure Sensor Basics I

There are different technologies that convert fluid pressure into an electronic signal and gives us feedback by using electronic pressure sensing elements called transducers. Different types of sensing elements are:

• Strain gauge
• Capacitive
• Potentiometric
• Resonant wire
• Piezoelectric
• Magnetic (inductive and reluctive type)
• Optical

Strain gauge (Piezoresistive sensors)

Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. The strain gauge is an example of a piezoresistive element. 

A strain gauge must be bonded onto a larger specimen capable of withstanding an applied force. The conductors of the strain gauge are deformed, as the test specimen is stretched or compressed by the applied force. The electrical resistance of any conductor is proportional to the ratio of length over the cross-sectional area (R ∝ l/A).

When we attach a strain gauge to a diaphragm, it results in a device that changes resistance with applied pressure. When the pressure applied to the diaphragm, it will deform and causes the strain gauge to change resistance. We can calculate the amount of pressure applied to the diaphragm by measuring this change in resistance.

 

As the diaphragm bows outward with applied fluid pressure, the strain gauge deforms, causing its resistance to change. There is a voltage (Vout) proportional to the amount of applied pressure because of change in resistance causes imbalances in the bridge circuit. Hence, the strain gauge generally converts an applied pressure/force into a measurable voltage signal which may be amplified and converted into a 4-20 mA loop current signal.

An isolating diaphragm transfers process fluid pressure to the fill fluid, which will transfer pressure to the silicon wafer. As it must transfer fluid pressure from the process fluid to the fill fluid, the isolating diaphragm is designed to be much more flexible than the silicon diaphragm. Without having direct contact with the process fluid, the sensor achieves the same as it would if it were directly exposed to the process fluid.

Capacitance Transducers

When the elastic element moves cause a change in capacitance and detected by the sensors. The materials of elastic elements are Inconel, Ni-Span C, or stainless-steel diaphragm or a metal-coated quartz element exposed to the process pressure on one side and to the reference pressure on the other. The unit can measure absolute, gauge, or differential pressure corresponding to the reference pressure used.

To energize the sensing element a high voltage and high-frequency oscillator is used. A bridge circuit detects the change in capacitance by considering the deflection diaphragm which is deflected by the process pressure. The sensing element’s capacitance is converted and amplified into a DC mA current signal to be used further. These types of transducers were developed for low vacuum researchers. Popular to use particularly on low absolute and low differential pressure applications. 

Advantages are good accuracy, rangeability, linearity, and speed of response. 

Temperature sensitivity is their limitation because of high output impedance, sensitivity to stray capacitance, sensitivity to vibration, low overpressure capability, and corrosion sensitivity.

Potentiometric Transducers

In this, bourdon or bellows are linked to the connecting rod or wiper arms of transducers. When the process pressure changes the connecting rod moves over a precision potentiometer, which converts measuring pressure into electronic resistance. Connecting rod tip is generally made of noble metal.

The advantages of these transducers are low cost, small size, and high output signals. 

Disadvantages are short life and high noise levels due to mechanical wear as the wiper experiences nonlinear mechanical loading, static friction, or loss of intimate contact with the resistive element.

Resonant Wire Transducers

In this, a wire oscillates at its resonant frequency by an oscillator circuit while the wire tension is dependent upon the process pressure. To detect differential pressures resonant wire is used. By the high and low-pressure diaphragm, the process pressure is detected. The fill fluid transmits a corresponding force to the wire, excited by the magnetic field when the pressure difference increases. To prevent overpressure damage, the backup plate supports the diaphragms. The resonant frequency of the wire was modified by a change in the wire tension counted digitally.

The advantages of these transducers are good repeatability, accuracy, stability, low hysteresis, high resolution, strong output signal, and the generation of an inherently digital signal.

Disadvantages are sensitivity to ambient temperature variations, which requires built-in temperature compensation; a nonlinear output signal; and some sensitivity to shock and vibration.

For other electronic pressure sensors or transducers follow the link: Instrumentation Basics: Electronic Pressure Sensor Basics II (instrumentbasics.blogspot.com)

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