Overview of pressure measurement

Pressure Transmitters

Pressure is one of the most common properties to detect. We frequently sense differential pressure to determine fluid flow, liquid level, and equipment performance. Pressure transmitters can be designed to measure absolute pressure or gauge pressure. All sensors will sense pressure above atmospheric. Most, but not all, will measure pressure below atmospheric (a partial vacuum). Most sensors may be used to measure static pressures as well as dynamic pressures. Pressure sensing and signal transmission is accomplished in a number of ways. A basic knowledge of the types of sensors available will allow one to properly select a pressure sensor for a given application.Table lists the type of pressure sensors available and a typical application rage.in this session we are gonna discuss about an overview of pressure measurement

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The Sensing Element

The sensing element of a pressure transmitter will either be some form of a Bourdon tube or a diaphragm. The Bourdon tube is commonly used in mechanical pressure gauges. The typical Bourdon tube is a hollow tube, usually of a roughly oval cross section, with a “C” shape. The end of the tube is attached, through a linkage mechanism, to a pointer. When pressure is applied, the tube deflects. The pointer registers the pressure on a calibrated dial. The Bourdon tube may also take on a spiral or helical shape to make it better suited for different pressure ranges. Figure  depicts different forms of the Bourdon tube. Although used primarily for mechanical indication, it may also be used for transmission of an electronic signal How does a Bourdon tube gauge work?

Another common pressure sensor is the diaphragm. This consists of a membrane clamped in a sealed housing. One side of the membrane is subjected to process pressure while the other is subject to a reference pressure. If that reference pressure is atmospheric, the sensor detects a gauge pressure. In such a case, if the process pressure is above atmospheric, it is connected to the ‘high’ port. Conversely, if the process pressure is below atmospheric, it is connected to a low port. On the other hand, if the low port is connected to the low pressure of a process, and the high port is connected to the high pressure of a process, the sensor senses a differential pressure. If the reference
side of the diaphragm is subjected to a sealed vacuum, the sensor will detect absolute pressure. A link is mechanically boded to the diaphragm so as to transmit the movement of the sensor into an electronic signal. There
are a variety of electronic transducer elements used to perform this function.

The Transducer Element

One such transducer element is the strain gauge. Pressure transducers designed on the basis of a strain-gauge are widely used measure the movement of the diaphragm of the sensor. They are frequently used for narrow-span pressure and differential pressure measurements. One type of strain gauge consists of a stationary frame and a moveable armature to which is attached several wire elements. The strain-gauge works upon the principle that the resistance of a wire elements change as they are stretched. Since this resistance change is very small, the electronics used must be sufficiently sensitive to register such changes. The most common method of detecting this resistance change is through the use of a Wheatstone bridge. The Wheatstone bridge is especially appropriate since it is easy to compensate for temperature variations through a trim resistor (Rm). Figure shows a schematic of a strain-gauge instrument.

Capacitance-type pressure transmitter

Capacitance-type pressure devices were designed for sensing low absolute pressures. They can be designed as a single-plate capacitor or as a double-plate capacitor. In each, a high frequency oscillator is used to charge the sensing electrodes. In the two-plate design, the movement of the diaphragm causes a change in the capacitance of the unit. This change in capacitance is directly related to pressure. The single-plate design uses a single plate on
one side of the diaphragm. As the diaphragm moves with applied pressure, the capacitance changes. In both designs, some form of bridge circuit detects this change in capacitance. The transducer element is designed to output a standard process signal.

Potentiometric pressure transducer

The potentiometric pressure transducer is a device comprised of a high quality variable resistor mechanically bonded
to the sensing element. As the position of the sensing element moves in response to an applied process pressure, the resistance varies proportionally. The variable resistance is usually part of a Wheatstone bridge, which transforms the measured resistance to a calibrated control signal. Although a very simple and inexpensive device, it does suffer from temperature errors and mechanical wear.

Vibrating wire pressure transducer 

Another form of transducer is the vibrating wire. This transducer works on the same principle as a string instrument; a wire will vibrate at some natural frequency when placed in tension. In this type of transducer, the sensing element is attached to a wire and places it in tension. An oscillator circuit provides the external excitation to force the wire to vibrate at its natural frequency. A pickup coil senses this frequency that correlates to the applied process pressure. This transducer tends to be highly repeatable and highly accurate. As such, it is an excellent transducer for differential pressure measurement. It’s disadvantages clude susceptibility to external vibration and to temperature variation. It also exhibits a nonlinear output. These disadvantages are often countered by processing the signal through a microprocessor, making this an excellent choice for accurate and precise measurement.

 

Piezoelectric pressure transducer

The piezoelectric pressure transducer is used to measure dynamic pressures such as those resulting from shock or vibration. Its primary element is a quartz crystal. When this crystal is subjected to a pressure, it generates an
electrical signal proportional to that pressure. However, this signal decays rather rapidly, thus the reason it cannot be used for static pressures. Since they generate their own current, they do not need an external power source. They are quite rugged and respond very quickly, but do require special care in installation.

Linear Variable Displacement Transformer (LVDT)

Another class of pressure transducers is based upon the use of electrical inductance, magnetic reluctance, or eddy currents. Once the most common is the Linear Variable Displacement Transformer (LVDT). This unit indicates pressure by sensing the position of a rod within the windings of an insulated coil. As shown in Figure , three coils are wired
within an insulated tube. An AC voltage is applied to the primary winding. An iron core is positioned within the primary winding by the movement of the pressure sensor element inducing a voltage the secondary windings.
The position of the rod results in different voltages in each of the secondary windings. It is this differential voltage that can be related to the applied pressure. The sensitivity and resolution of the device varies with the number of windings and the ratio of the number of windings on the primary coil to the number of windings on the secondary coil.

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