This page provides you with all the basic information you need to know about pressure transducers.
As you go through the article, you will learn more about:
A pressure transducer is a mechanical device that converts applied pressure, a physical quantity, into a measurable and industry-standard electrical signal which is linearly and proportionally related to the applied pressure.
There are two major components of a pressure transducer, an elastic material and an electrical device. The functions of each component are briefly discussed below.
There are different shapes and sizes of elastic material used in pressure transducers. The configuration depends on the pressure range and sensing principle. The main purpose of the elastic material is it deforms when subjected to pressure so the electrical device can detect it.
The elastic component is usually in the form of a diaphragm. These elements are either made of circular metal discs or flexible materials including rubber, plastic, or leather. On the other hand, diaphragm shapes can be circular, flat, or corrugated. Diaphragm elements are advantageous in highly corrosive environments or with over-pressure systems.
The electrical device detects the deformation of the elastic material and then converts this into an electrical signal. Different operational principles of the electrical device commonly include resistive, capacitive, or inductive.
Correct calibration of pressure transducers ensures output accuracy. Generally, pressure transducers should be carefully used concerning:
To select the most appropriate pressure transducer for your application, the following specifications should be considered:
There are various ways in which pressure can be measured and referenced. To read and report pressure measurements reliably, one should understand that pressure instruments use these references below:
When you hear about pressure instruments, you will encounter the following terms: pressure sensor, pressure transmitter, and pressure transducer. In industrial processes, these terms are interchangeably used. To further understand what a pressure transducer is, one must take note of its difference vs the two other terms.
Below are brief details like functions, output signals, and application, as well as some of the advantages and limitations of each pressure instrument.
A pressure sensor is an element that directly receives the pressure exerted by a fluid. For instance, the capsule of a potentiometric pressure transducer. The pressure from the media stresses the capsule which is connected to the wiper through a linkage rod. This results in changes in the wiper position across the potentiometer.
Pressure sensors can also be considered as a general term for any instrument that measures pressure and gives an output in return. The type of sensor is defined by the output interface properties.
Pressure transducers have voltage as an output. The output can have a magnitude of millivolts or a higher voltage.
As the name suggests, pressure transducers have output signals in millivolts (mV). The power supply is proportionally related to the output signal. For instance, a 0-50mV output on the sensor is generated by a 5V DC supply with a 10mV/V output signal.
MEMS sensors commonly produce an output of 20mV/V while old strain gauge sensors generate a 2-3mV/V output signal.
Pressure transducers with higher output voltage can withstand harsher electrical environments because these transducers are less likely to be influenced by noise. Voltage-output pressure transducers have signal amplification which increases the output voltage up to 5V or 10V.
These pressure transducers can also be used in battery-operated equipment because of their low current consumption. The voltage supply usually ranges from 8-28V DC.
Older models of voltage-output pressure transducers do not provide an output signal when pressure is zero (live zero). Transducers with no live zero are disadvantageous since it is hard to determine whether the pressure is indeed zero or there is just a failed sensor with no output.
Unlike voltage-output pressure transducers, pressure transmitters deliver low-impedance current as an output which is typically designed to be connected to an industrial standard 4-20mA for sensing and control. They are commonly used with a 2-wire or 4-current loop in industries.
Pressure transmitters are advantageous in transmitting signals to far distances because 4-20mA transmitters have good electrical noise immunity. Unregulated supply is used to power transmitters.
In summary, the advantages and disadvantages of millivolt-output pressure transducers, voltage-output pressure transducers, and pressure transmitters are as follows:
Millivolt-output pressure transducerThe three modes in which pressure sensors operate include absolute, gauge, or differential pressure measurement.
An absolute pressure sensor allows the fluid to go in and exert pressure on the sensing element, at one port only. The pressure output is always positive, and its magnitude is proportionally related to the media pressure.
Gauge pressure sensors have two ports so that the fluids at the reference pressure (atmospheric pressure), and at the pressure to be measured are allowed to enter. The measured pressure is relative to the reference pressure.
Like the gauge pressure sensors, differential pressure sensors also have two ports. The ports allow the entry of the fluid from two different points in the system. The differential output can be positive or negative. The pressure between the two points is proportionally related to the magnitude of the change.
The gas or liquid pressure is usually converted into a physical displacement of a pressure sensing element. The physical movement can be translated into an electrically measurable response like resistance or capacitance change that is proportional to the medium pressure.
There are four common pressure sensing elements used in the industry today. Each is discussed below:
As previously discussed in Chapter 1, the most usual form of an elastic element in a pressure transducer is a diaphragm. This type of pressure sensing element is usually subjected to the pressure media on one side only. The opposite side may be a sealed chamber or vented depending on what type of pressure sensor it is. An absolute pressure sensor has a sealed chamber on the other side of the diaphragm while a gauge or differential pressure sensor has a diaphragm vented on one side.
The pressure of the media causes the deflection of the diaphragm. The physical displacement, which is proportional to the magnitude of the pressure, causes changes in the resistance or capacitance value of an electrical component. Pressure sensing diaphragms can be made from:
Pressure sensing diaphragms are common in various industries such as food and pharmaceutical manufacturing. These sensors are very simple to design and construct even in small sizes.
Pressure sensing capsules are composed of two diaphragms that are welded at the edge. With this configuration, both sides are exposed to pressure media simultaneously. Compared with pressure sensing diaphragms, capsules display twice the deflection, relative to the pressure applied.
There are three types of pressure sensing capsules including single capsule, stacked capsule, and profiled capsule.
These pressure sensors are used in low-pressure gas systems. Since they do not have the ability to self-drain, capsules are not compatible with liquid media.
The main advantages of capsules are:
An expanding bellow is another type of pressure sensing element. The common materials for expanding bellows are:
It responds to the applied pressure by expanding or contracting. Typically, a bellow is connected to a pointer that is then attached to a spring. The expansion and contraction of the bellow cause movement of the pointer. The mechanical properties of the bellow and spring both contribute to deflection characteristics. The changing applied pressure is proportionally related to the pointer deflection. An electrical analogue of the applied pressure can also be obtained by attaching the movement to a potentiometer instead.
The advantages of expanding bellows are:
On the other hand, the disadvantages include:
Equipment manufacturers have an impressive array of options for commercially available pressure transducers today. There are two major classifications of pressure transducers:
Below is the list of different types of pressure transducers and their corresponding working principles:
This type of pressure transducer utilizes a foil or silicon strain gauge (arranged as a Wheatstone bridge) attached to the surface of the diaphragm, on the opposite side of the media. When a change in pressure of the media occurs, this will result in deformation of the elastic material thereby also changing the resistance of the strain gauge. This resistance change is converted into an electrical signal which is then amplified and conditioned to provide transducer-voltage or transmitter-current output.
Strain gauge transducers have many classifications including gauged diaphragm pressure transducers, cantilever type transducers, embedded strain gauge transducers, and unbounded strain gauge pressure transducers.
A capacitive pressure transducer consists of two capacitive plates: diaphragm and electrode, parallel to each other. The latter is bonded to an unpressurized surface and is placed at a set distance from the diaphragm plate (starting capacitance) therefore creating a space between the two plates. When a change in pressure is encountered, the gap either narrows or widens which changes the capacitance (ΔC). A usable signal is then derived from this capacitance change.
There are two types of inductive pressure transducer:
A simple inductance pressure transducer is composed of a coil, a movable ferromagnetic core, and a diaphragm (or any pressure sensing element). The diaphragm and the ferromagnetic core are attached. As the diaphragm is deflected due to pressure change of the medium, the ferromagnetic core also moves. Meanwhile, the coil is powered by an AC voltage. If the core moves, the inductance of this coil, as well as the equivalent output, changes.
A two-coil mutual inductance pressure transducer is composed of a ferromagnetic core coupled to the diaphragm and has a primary coil and two secondary windings. The AC powered primary coil generates induced current on the secondary coil pick-up coils. When the core is centered, the equal voltage will be induced to the two secondary coils. A change in media pressure deflects the diaphragm and causes the movement of the ferromagnetic core. This affects the voltage ratio between the two secondary coils. The voltage change is proportional to the pressure change.
A typical resonant wire pressure transducer consists of different components such as:
In this type of pressure transducer, a wire is held by a static member at one side, and by a diaphragm at the other end. The wire oscillates while in a magnetic field. The oscillator circuit results in the wire oscillating at its resonant frequency. The high-pressure and low-pressure diaphragms are sensing the changing process pressure on the right and left of the unit. The changes in pressure influence the tension in the wire thereby affecting also the wire resonant frequency. The shift is detected by a digital counter circuit.
Resonant wire pressure transducers can be used to measure absolute and gauge pressures. They are also advantageous in low differential pressure systems.
Piezoelectric Pressure Transducer Unlike any pressure transducers such as capacitive and piezoresistive transducers, piezoelectric pressure transducers do not make use of external sources (voltage or current) in their working principle.Certain materials create a charge, called piezoelectricity, across them when subjected to mechanical stress. Quartz and tourmaline are commonly used materials for piezoelectric pressure transducers. When pressure is applied to their surface, a charge is generated. The magnitude of the charge is proportionally related to the applied pressure.
Piezoelectric pressure transducers are used in systems with fast-changing dynamic pressures. The major drawback of this transducer is that they are sensitive to vibrations and shocks.
A piezoresistive pressure transducer is composed of semiconductor material and a diaphragm. It uses the resistance change of the semiconductor, when stretched due to diaphragm deflection, to measure the applied pressure. This pressure transducer is advantageous to systems that involve very minimal pressure changes. Piezoresistive pressure transducers are simple, robust, and can be used to measure absolute, gauge, relative and differential pressure changes.
Pressure transducers can also be categorized based on the type of pressure measurement:
After the sensing element has detected the applied pressure, pressure transducers convert this data into electrical signals optimized for transmission. Electrical pressure transducers have three modulation modes:
The analog output is a DC signal which is proportional to the input signal.
The output signal is an AC signal in which its amplitude is a function of the measured quantity. On the other hand, the frequency remains constant.
The output signal is an AC signal in which its frequency is a function of the measured quantity. On the other hand, the amplitude remains constant.
Pressure transducers are used in almost all, if not all industries that involve pressure monitoring of liquid or gas media inside a vessel, pipe, storage tank, etc. Some of the industries that use pressure transducers are:
In a more specific view, pressure transducers are used in the following applications:
The liquid level inside a tank is one of the most common parameters monitored in every industrial process. The liquid level is directly related to its pressure at the bottom of the tank. While a sight glass directly measures a fluid level, pressure transducers use the applied pressure of the column of liquid and eventually relates this to the liquid level.
There are three different methods in which the fluid level in a tank can be measured:
A correction factor must be applied to accurately measure the level inside the tank if any of these cases apply:
The pressure transducer is placed way below the tank bottom – for correction on the fluid head in the tubing connected to the sensor and port.
The tank contains a liquid other than water – for correction on the specific gravity since pressure transducers are normally calibrated in inches H2O.
The location of a water leak can be detected using pressure transducers. A large, unusual pressure drop across a pipeline might indicate a leak. If there are two pressure transducers placed consecutively in a pipeline and the difference between the pressure measurement is too large even though there are no other obstructions that can cause the pressure drop, then water leakage could be a probable root cause.
Aside from liquid media, pressure transducers are also used for gases like non-combustible, combustible, corrosive, or non-corrosive. Depending on the type of gas, the pressure transducer should be carefully selected.
There are pressure transducers that can specifically handle corrosive gases such as ammonia, hydrogen chloride, and methylamine. For combustible gas applications, the pressure transducer should be explosion-proof or has a certification that it is safe to use for this purpose.
The suction pressure and discharge pressure are important pump parameters. For instance, these parameters can be used to calculate the total dynamic head of the pump. In industrial processes, the suction pressure and discharge pressure are normally measured and monitored. A micro pressure transducer is commonly used for very small pumps while typical pressure transducers can handle larger pumps. For systems with larger pumps, the pressure transducers should be installed a little farther downstream of the pump discharge to prevent the transducer from being damaged due to water hammering.
Some systems involve extremely hot gases or liquids and require pressure measurement. To avoid the high temperature damaging the pressure transducer, a siphon effect should be applied.
A siphon is a simple device, usually a metal tube, that allows heat dissipation of the media before it comes into contact with the pressure transducer. With the help of the siphon effect, a pressure transducer with a low-temperature range can now be possibly used in high-temperature systems.
There are various configurations of siphons, these include:
A typical siphon would be made from iron, brass, steel, stainless steel or carbon steel.
Orifice plates and venturi tubes are simple devices used to measure flow. These devices provide restriction along the pipe that results in pressure drop which is related to flow rate in the pipe. Differential pressure transducers are used to infer the fluid flow rate by measuring the pressure drop.
Differential pressure transducers are also used to measure the pressure drop across a filter. A large pressure drop indicates that the filter is loaded with contaminants therefore it already needs to be replaced. The pressure drop increases as the filter gets dirty because less fluid can pass through the filter, therefore, resulting in a relatively lower pressure downstream of it.
Some pressure transducers can withstand temperatures above 1000oC but only within short intervals to allow a cooling-off of the sensors. These pressure transducers have a piezoelectric core for pressure measurement.
Normal-pressure transducers are not suitable for this application because they can be damaged due to the expansion of metal parts. This can also lead to output errors.
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