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 specialized device that converts the applied pressure—a tangible measurement—into an electrical signal that can be standardized and accurately measured for industrial applications. This electrical signal is directly proportional to the pressure experienced by the transducer.
There are two essential parts of a pressure transducer: an elastic material and an electrical mechanism. Below is a brief overview of the function of each.
The elasticity employed in pressure transducers can vary in form and dimension, selected based on the pressure category and the sensing technique. The main function of this material is to flex under pressure, enabling the attached electrical device to gauge the deformation.
Typically, the elastic section of a pressure transducer is designed as a diaphragm, constructed from materials like circular metal discs, rubber, plastic, or leather. Diaphragms can be circular, flat, or corrugated in shape. These elements are particularly advantageous in harsh corrosive environments or systems where overpressure conditions frequently occur.
The electrical mechanism in a pressure transducer detects deformation in the elastic material and translates this into an electrical signal. This process might employ several principles, such as resistive, capacitive, or inductive methods.
Proper calibration is crucial to achieving precise output from pressure transducers. These devices should be used judiciously concerning:
When selecting the most suitable pressure transducer for your application, consider these specifications:
Pressure can be measured and referenced in various ways. To ensure reliable pressure readings and reporting, it is important to understand the different reference types used by pressure instruments:
Absolute pressure is measured relative to a perfect vacuum (0 Pa). This type of pressure reference includes both atmospheric pressure and the gauge pressure of the media. Absolute pressure provides a consistent measurement since it eliminates the influence of changing atmospheric pressure caused by elevation or location. Consequently, it relies on a fixed pressure range for its reference.
Gauge pressure measures pressure relative to atmospheric pressure, which serves as its zero point. Pressure indicators using gauge pressure typically display units such as PSIG, BARG, or kPaG. These sensors are equipped with vents to reference atmospheric pressure. As a result, gauge pressure measurements depend on the sensor's installation location.
Differential pressure measures the difference in pressure between two distinct points in a system. For example, it can be used to monitor the pressure drop across a filter in a water pipeline.
Sealed pressure uses a predetermined reference point that is not necessarily a vacuum. Similar to gauge pressure, it allows for pressure measurements at different locations without being affected by changes in atmospheric pressure. However, unlike gauge pressure sensors, sealed pressure sensors typically need to be installed with a vent.
When discussing pressure instruments, you may come across terms such as pressure sensor, pressure transmitter, and pressure transducer. In industrial processes, these terms are often used interchangeably. However, to fully grasp what a pressure transducer is, it is crucial to understand the distinctions between a transducer, a sensor, and a transmitter.
The distinction between sensors and transducers is straightforward: sensors detect changes in the environment, while transducers convert one form of energy into another. A transducer includes a sensor that identifies environmental changes and generates a non-electrical signal. The process by which a transducer performs this conversion is known as transduction.
Below are brief details covering the functions, output signals, applications, advantages, and limitations of each pressure instrument.
A pressure sensor is a component that directly measures the pressure exerted by a fluid. For example, in a potentiometric pressure transducer, the pressure from the media deforms the capsule, which is connected to the wiper via a linkage rod. This deformation causes changes in the wiper's position along the potentiometer.
Pressure sensors can be broadly defined as any instrument that measures pressure and provides an output. The specific type of sensor is determined by the characteristics of the output interface.
Pressure transducers typically provide an output in the form of voltage. This voltage can range from millivolts to higher levels, depending on the specific transducer.
As the name implies, pressure transducers often have output signals measured in millivolts (mV). The output signal is proportional to the power supply. For example, a sensor with a 0-50 mV output may be powered by a 5V DC supply, producing a 10 mV/V output signal.
MEMS sensors typically produce an output of 20 mV/V, whereas older strain gauge sensors generate an output signal of 2-3 mV/V.
MEMS sensors typically produce an output of 20 mV/V, whereas older strain gauge sensors generate an output signal of 2-3 mV/V.
These pressure transducers are also suitable for battery-operated equipment due to their low current consumption. They typically operate with a voltage supply ranging from 8 to 28V DC.
Older models of voltage-output pressure transducers do not provide an output signal at zero pressure (live zero). Without a live zero, it can be challenging to distinguish between an actual zero pressure condition and a failed sensor that is not producing an output.
Unlike voltage-output pressure transducers, pressure transmitters provide a low-impedance current output, typically designed for connection to an industrial standard 4-20 mA loop for sensing and control. They are commonly used in industries with 2-wire or 4-wire current loops.
Pressure transmitters are advantageous for transmitting signals over long distances because 4-20 mA transmitters offer excellent electrical noise immunity. These transmitters are powered using an unregulated supply.
In summary, the advantages and disadvantages of millivolt-output pressure transducers, voltage-output pressure transducers, and pressure transmitters are outlined below:
Pressure sensors operate in three modes: absolute, gauge, or differential pressure measurement.
An absolute pressure sensor measures pressure by allowing the fluid to exert force on the sensing element through a single port. The pressure output is always positive and is directly proportional to the media pressure.
Gauge pressure sensors have two ports, allowing fluid at atmospheric pressure (reference pressure) and the fluid to be measured to enter. The measured pressure is relative to this reference pressure.
Differential pressure sensors, like gauge pressure sensors, have two ports for fluid entry from two different points in the system. The differential output can be either positive or negative, and it is proportionally related to the magnitude of the pressure change between the two points.
Gas or liquid pressure is typically converted into physical displacement of a pressure sensing element. This movement is then translated into an electrically measurable response, such as changes in resistance or capacitance, that is proportional to the medium pressure.
There are four commonly used pressure sensing elements in the industry today, each of which is discussed below:
As discussed in Chapter 1, the most common type of elastic element in a pressure transducer is a diaphragm. This pressure sensing element is typically exposed to pressure media on one side only. The opposite side may either be a sealed chamber or vented, depending on the type of pressure sensor. An absolute pressure sensor features a sealed chamber on the other side of the diaphragm, while a gauge or differential pressure sensor has a diaphragm that is vented on one side.
The pressure of the media causes the diaphragm to deflect. This physical displacement, proportional to the pressure magnitude, leads to changes in the resistance or capacitance of an electrical component.
Pressure sensing diaphragms can be made from:
Pressure sensing diaphragms are commonly used in industries like food and pharmaceutical manufacturing. They are simple to design and construct, even in small sizes.
Pressure sensing capsules consist of two diaphragms welded at the edges, allowing both sides to be exposed to pressure media simultaneously. Compared to pressure sensing diaphragms, capsules exhibit twice the deflection relative to the applied pressure.
There are three types of pressure sensing capsules: single capsule, stacked capsule, and profiled capsule.
These pressure sensors are used in low-pressure gas systems. Because they lack the ability to self-drain, capsules are not suitable for use with liquid media.
The main advantages of capsules include:
Another type of pressure sensing element is the expanding bellow. The typical materials used for expanding bellows include:
Bellows respond to applied pressure by expanding or contracting. Typically, a bellow is connected to a pointer, which is linked to a spring. As the bellow expands and contracts, it moves the pointer. Both the mechanical properties of the bellow and the spring influence the deflection characteristics. The deflection of the pointer is proportional to the applied pressure. Alternatively, an electrical analogue of the applied pressure can be obtained by connecting the movement to a potentiometer.
The advantages of expanding bellows are:
However, the drawbacks include:
Equipment manufacturers now have a wide range of commercially available pressure transducers. These transducers are primarily classified into two major categories:
Below is a list of different types of pressure transducers and their respective working principles:
This type of pressure transducer uses a foil or silicon strain gauge, arranged in a Wheatstone bridge, attached to the diaphragm's surface opposite the media. When the pressure changes, it deforms the elastic material, altering the strain gauge's resistance. This change in resistance is converted into an electrical signal, which is then amplified and conditioned to produce a transducer voltage or transmitter current output.
Strain gauge transducers are classified into several types, including gauged diaphragm pressure transducers, cantilever-type transducers, embedded strain gauge transducers, and unbonded strain gauge pressure transducers.
A capacitive pressure transducer comprises two parallel capacitive plates: a diaphragm and an electrode. The electrode is fixed to an unpressurized surface and is positioned at a set distance from the diaphragm plate, creating an initial capacitance. When pressure changes, the gap between the plates either narrows or widens, altering the capacitance (ΔC). This change in capacitance is then used to derive a usable signal.
There are two types of inductive pressure transducers:
A basic inductance pressure transducer consists of a coil, a movable ferromagnetic core, and a diaphragm (or another pressure-sensing element). The diaphragm and the ferromagnetic core are connected. When the diaphragm deflects due to pressure changes in the medium, the ferromagnetic core moves accordingly. The coil, powered by an AC voltage, experiences a change in inductance as the core moves, resulting in a corresponding change in the output signal.
A two-coil mutual inductance pressure transducer consists of a ferromagnetic core attached to the diaphragm, with a primary coil and two secondary windings. The AC-powered primary coil generates an induced current in the secondary pick-up coils. When the core is centered, the voltage induced in the two secondary coils is equal. As pressure changes deflect the diaphragm and move the ferromagnetic core, the voltage ratio between the two secondary coils shifts. This voltage change is proportional to the pressure change.
A potentiometric pressure transducer consists of three main components: a capsule, a sliding contact wiper, and resistance wire winding. The capsule is linked to the wiper via a linkage rod. When pressure is applied to the capsule, it moves the wiper across the potentiometer, altering the resistance between the wiper and the potentiometer. Consequently, the mechanical deflection is translated into a resistance measurement.
A typical resonant wire pressure transducer includes various components such as:
In this type of pressure transducer, a wire is anchored at one end by a static member and at the other end by a diaphragm. The wire oscillates within a magnetic field, and the oscillator circuit makes it vibrate at its resonant frequency. The diaphragms on either side of the unit sense changes in pressure. Variations in pressure affect the wire's tension, which in turn alters its resonant frequency. This frequency shift is detected by a digital counter circuit.
Resonant wire pressure transducers can measure both absolute and gauge pressures and are particularly advantageous for low differential pressure systems.
Unlike capacitive and piezoresistive pressure transducers, piezoelectric pressure transducers do not require external voltage or current sources for their operation.
Certain materials generate a charge, known as piezoelectricity, when subjected to mechanical stress. Quartz and tourmaline are commonly used in piezoelectric pressure transducers. When pressure is applied to these materials, they produce a charge whose magnitude is directly proportional to the applied pressure.
Piezoelectric pressure transducers are ideal for systems with rapidly changing dynamic pressures. However, a major drawback is their sensitivity to vibrations and shocks.
A piezoresistive pressure transducer consists of a semiconductor material and a diaphragm. It measures applied pressure by detecting changes in the semiconductor's resistance as it stretches due to diaphragm deflection. This type of transducer is well-suited for systems with minimal pressure changes. Piezoresistive pressure transducers are simple, robust, and capable of measuring absolute, gauge, relative, and differential pressure changes.
Pressure transducers can also be categorized according to the type of pressure measurement they provide:
A digital output pressure transducer features a backlit LCD screen that displays the pressure readings.
readings in real time and is powered by DC. It is small, lightweight, and compact.
pressure transducer with anti-vibration and anti-shock features. Like all such devices, it ensures durability and reliability under various conditions.
pressure transducers, a digital output pressure transducer can measure the pressure with high precision and display it in real time.
pressure of gases, air, steam, hydraulics, and high-temperature liquids.
One benefit of a digital output pressure transducer is that it avoids signal loss or interference. Since it interfaces directly with a computer via a digital connection, there is no need to convert an analog reading to digital. The microprocessor inside the transducer digitally represents the pressure measurement, thereby eliminating linearity errors.
Digital output pressure transducers deliver the most accurate signals but require stable sensing technology, as repeatability and hysteresis can be unpredictable. For optimal performance, these transducers should be used in applications with low hysteresis and high repeatability.
After the sensing element detects the applied pressure, pressure transducers convert this data into electrical signals suitable for transmission. Electrical pressure transducers operate in three modulation modes:
The analog output is a DC signal that is proportional to the input signal.
The output signal is an AC signal where the amplitude varies according to the measured quantity, while the frequency remains constant.
The output signal is an AC signal where the frequency varies with the measured quantity, while the amplitude remains constant.
Pressure transducers are used in almost, 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:
More specifically, pressure transducers are used in the following applications:
Monitoring the liquid level inside a tank is a common parameter in industrial processes. This level is directly related to the pressure at the bottom of the tank. While a sight glass provides a direct measurement of the fluid level, pressure transducers measure the pressure exerted by the liquid column and correlate this pressure to the liquid level.
There are three different methods for measuring the fluid level in a tank:
If any of the following conditions apply, a correction factor must be applied to accurately measure the level inside the tank:
Pressure transducers can detect the location of a water leak by identifying significant pressure drops across a pipeline. If two pressure transducers are placed consecutively and a large difference in their measurements is observed, with no other obstructions to explain the drop, water leakage may be the likely cause.
In addition to liquid media, pressure transducers are used for various gases, including non-combustible, combustible, corrosive, and non-corrosive gases. The choice of pressure transducer should be carefully matched to the type of gas being measured.
Some pressure transducers are specifically designed to handle corrosive gases like ammonia, hydrogen chloride, and methylamine. For applications involving combustible gases, it is essential to use pressure transducers that are explosion-proof or certified for safe use in such environments.
Suction pressure and discharge pressure are crucial parameters for pumps. They can be used to calculate the total dynamic head of the pump. In industrial processes, these pressures are typically measured and monitored. Micro pressure transducers are used for very small pumps, while standard pressure transducers are suitable for larger pumps. For systems with larger pumps, pressure transducers should be installed slightly downstream of the pump discharge to avoid damage from water hammering.
In systems with extremely hot gases or liquids, a siphon effect can be used to protect the pressure transducer from high temperatures. This method helps prevent the hot media from directly damaging the transducer.
A siphon is a simple device, typically a metal tube, designed to dissipate heat from the media before it reaches the pressure transducer. By utilizing the siphon effect, a pressure transducer with a low-temperature rating can be used effectively in high-temperature systems.
Various configurations of siphons include:
A typical siphon is made from materials such as 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 used to measure the pressure drop across a filter. A significant pressure drop indicates that the filter is becoming clogged with contaminants and needs replacement. As the filter accumulates dirt, the pressure drop increases because less fluid can pass through, resulting in a relatively lower pressure downstream.
Some pressure transducers, equipped with a piezoelectric core for pressure measurement, can withstand temperatures up to 1832 °F (1000 °C) but only for short intervals to allow the sensors to cool down.
Normal-pressure transducers are unsuitable for this application because the expansion of metal parts can damage them and lead to output errors.
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