An explanation of pressure sensors and their function with a list of pressure manufacturers.
You will learn:
A pressure sensor is a device that senses and measures the amount of force that is applied to a flat surface. They provide real time data regarding the status of equipment and assist in predicting equipment failures. Pressure sensors measure physical changes that are converted into electrical signals that are transmitted as analogue or digital output.
The most common type of pressure sensor is a strain gauge that measures pressure by the deformation or displacement of a material due to applied stress. As with all types of pressure sensors, the readings from the deformation are converted to voltage or electrical signals. The intensity of the signals increases or decreases depending on the applied pressure.
Pressure sensors are classified according to the range of pressure measured, temperature range, or the type of pressure. The different types of pressure are absolute, gauge, sealed gauge and differential. Pressure gauge readings are displayed in accordance with a comparison with a fixed pressure, chamber pressure, atmospheric pressure, or the difference between two pressures.
Pressure sensors that produce the same types of readings are pressure transducers, pressure indicators, and pressure transmitters, which are terms often used for pressure sensors. Each type of pressure sensor serves the same function but differs in their output signals with pressure sensors producing signals relative to the strength of pressure. Pressure transducers convert detected force into voltage while pressure transmitters convert detected force into a current output in milliamps (mA) and pressure indicator display pressure in pounds per square inch (psi).
The purpose of pressure sensors is to notify staff of any risk regarding a process before a process fails, and corrective measures can be taken. There are several types of pressure sensors available for industrial and medical use, which enable users to select a pressure sensor that fits their application.
When examining the many types of pressure sensors, it may be difficult to find one that is ideal for a process or application. Pressure sensor manufacturers work with their customers providing guidance, literature, and expertise to assist in the selection process. The seven most commonly used pressure sensors are aneroid barometer pressure sensors, manometer pressure sensors, bourdon tube pressure sensors, vacuum or Pirani pressure sensors, sealed pressure sensors, piezoelectric pressure sensors, and strain gauge pressure sensors.
Pressure sensors are a vital part of manufacturing operations. They control and monitor gas and fluid flow and speed as well as provide altitude and water level measurements. The many uses of pressure sensors have led to the development of their many different types, which vary by technology, design, performance, durability, and cost.
The monitoring of pressure involves a measurement of the amount of force that is exerted by a gas or liquid against a surface. Pressure sensors are a specialized tool designed to provide data on the level of pressure being applied. They are used to predict when an application may fail using real time data.
In order to understand the different types of pressure sensors, it is important to know the types of pressure that is measured. The categories of pressure measurement modes are absolute, gauge, differential, vacuum, and compound, which are categorized by comparative pressures or reference pressures.
Absolute pressure measurement is made by comparing the target pressure to the known pressure of an absolute vacuum. Using a vacuum makes all measurements larger than the reference. Boyle’s Law states that the pressure of a gas is inversely proportional to its volume, at a constant temperature. Thus, the measurement of an absolute pressure sensor varies in accordance with the temperature.
The measurements of absolute pressure are independent of atmospheric conditions and are not influenced by weather or altitude and are denoted by Pabs. Barometric pressure is an example of absolute pressure. The manufacture of absolute pressure sensors requires sealing a vacuum behind a sensing diaphragm.
Differential pressure sensors use two points in a system to supply readings. They provide comparative measurements, such as before and after a valve when a valve is closed. The purpose of differential pressure sensors is not to provide pressure readings but to determine if there is a malfunction in a valve, pipe, or system. They have two ports that are inserted into a system. The readings from a differential type pressure sensor are unrelated to atmospheric pressure or other factors. The pressures being measured occur on either side of a diaphragm where the deflection of the diaphragm, positive or negative, creates the difference in pressure. They are commonly used to protect filters from failure.
Gauge pressure, or overpressure, is the pressure in a system that is above atmospheric pressure and is zero referenced against the ambient air pressure. Readings from gauge pressure gauges are the pressure from the weight of the atmosphere. Measurements of gauge pressure vary in accordance with their placement above sea level and weather conditions. Absolute pressure, that uses zero pressure as a referent, is equal to atmospheric pressure plus gauge pressure, or gauge pressure is equal to absolute pressure minus atmospheric pressure.
Overpressure takes two forms, which are positive overpressure and negative overpressure. Positive overpressure occurs when absolute pressure is greater than atmospheric pressure. When the opposite occurs where absolute pressure is lower than atmospheric pressure, it's referred to as negative overpressure. Gauge pressure is measured in pounds per square inch.
Vacuum pressure is lower than atmospheric pressure and is defined as space that is devoid of anything. Measuring vacuum pressure involves an examination of the force exerted by a vacuum where the absence of air or gas creates low pressure conditions. Pressure gauges that measure vacuum pressure measure the difference between the pressure on a sensor and the atmosphere. Regardless of their accuracy and usefulness, vacuum pressure sensors are difficult to manufacture due to the problems related to creating a vacuum.
Measurements of vacuum pressure are negative and lower than atmospheric pressure readings or barometric pressure readings. The initial calibration of vacuum pressure sensors is to atmospheric pressure. Units used to measure vacuum pressure are torr, Pascal (Pa), Bar, and atmosphere (atm). Vacuum pressure sensors measure force in inches.
Compound pressure measures positive and negative or vacuum pressures. Gauges that measure compound pressure are a combination of gauge pressure sensors and vacuum pressure sensors. The dials of compound pressure sensors have numerical values that are positive and negative, with one set to the left and another set to the right with atmospheric pressure indicated by zero. Using specific pressure sensors for compound applications will permanently damage the sensor. Compound pressure sensors combine psi and inches into a single pressure gauge.
Experts, engineers, physicists, and pressure sensor designers have comprehensive and complete knowledge of pressure and how important the control of pressure is for the protection of equipment and personnel. The general definition of pressure is force per unit of measure that is applied to an area by gases, liquids, vapors, and various other types of medium and is measured as absolute, gauge, or differential.
Prior to the invention of the plethora of pressure sensing devices, pressure was measured using liquids, such as mercury or water, enclosed in liquid filled manometers. In physics, the symbol for pressure is the letter p, and the Système international (SI) unit used to measure pressure is the pascal (Pa), with one pascal equaling one newton per square meter. Other units of measure are pounds per square inch (psi), torr, and bar.
In most cases, clients who are selecting a pressure sensor have experts that understand the various types of pressure measuring units. It is essential, when working with pressure sensor manufacturers, to have a basic knowledge of pressure units so that they can offer the correct pressure measuring device for an application.
Companies in the United States use psi, bar, or millibar (mbar) measurements. International Standard pressure measurement units are pascal or newton per square meter (N/m²). Other accepted forms of pressure measurement are atmospheres (atm), inches of mercury (in. Hg), millimeters of mercury, and torr. The pressure sensor unit that is chosen depends on the type of pressure instrument, application, the industry, and the typical units used by a culture or country.
| Pressure Measuring Units for Industrial and OEM Applications | ||
|---|---|---|
| System of Units | ||
| SI | MKSA | English |
| bar | kg/cm² | lbf/in² = psi |
| mbar | m H₂0 | psia |
| Pa | cm H₂0 | psid |
| kPa | mm Hg | psis |
| Mpa | torr | in H₂0 |
| N/m² | ft H₂0 | |
| in Hg | ||
The sensing principle of a pressure sensor influences its accuracy, efficiency, reliability, measurement range, and compatibility with its working environment. Mechanical displacement within a pressure sensor supplies pressure readings to an external device through the conversion of electrical signals. The types of pressure sensors are differentiated by their measuring principles, which are resistive, piezoelectric, capacitive, inductive, Hall element, and MEMS.
There is a very long list of the different forms, shapes, types, designs, and kinds of pressure sensors with some versions being developed to serve a specific industry or application. Manufacturers work closely with their clients to find a pressure sensor that perfectly meets a client’s needs. This is especially true in critical situations where accurate monitoring of pressure determines the quality of a product or the success of an application.
All pressure sensors rely on the same basic working principle for measuring variations in pressure. Once a pressure sensor measures any physical change, it converts the data into an electrical signal that is displayed as numerical information.
Pressure sensors are one of three types of pressure sensing devices. Pressure transducers and pressure transmitters are the other two types, which vary in how they transmit the data and their method of presenting the data. Each of the three types are essential to industrial operations as regards monitoring and reporting pressure readings.
The categories of the types of pressure sensors are divided by the pressure they measure and their sensing mechanism. A key factor that is part of the process of selecting the type of pressure sensor is the type of media to be monitored. Pressure sensors for chemical processes need to be capable of withstanding the effects of exposure to corrosive media, such as acids or alkalis. All forms of pressure sensors can be adapted to meet the requirements of dealing with corrosive substances.
Bourdon tube pressure sensors are a form of mechanical pressure sensor that uses a C- shaped spiral tube as a sensing element. The physical movement of the tube moves a pointer along a scale that is attached to the closed end of the tube. Numerical values indicated by the pointer are the level of pressure being applied. Pressure to be measured enters the tube at its open end. As the amount of pressure increases, the tube uncoils and begins to straighten until the pressure reaches the tube's level of elastic resistance.
The types of pressure that bourdon tubes measure are gauge pressure and differential pressure. Bourdon tube pressure sensors are rugged, tough, inexpensive, and durable. They are capable of providing accurate readings for high pressure applications. The downside of bourdon tube pressure sensors is their susceptibility to shock and vibrations due their simple mechanical mechanism. This aspect limits their use and the precision of their measurements for low pressure applications.
Pirani pressure sensors are used for the measurement of low vacuum pressure ranges. Also known as thermal heat transfer gauges or sensors, Pirani pressure sensors are a type of thermal conductive sensor that measures the rate of heat transfer as a function of the number of gas molecules in a vacuum system. They have a heated wire or thin film membrane that has its temperature raised by a bridge circuit. As the temperature rises, the molecular density of gas in the sensor changes, affecting the heating element. The amount of heat or energy lost by the heating element is dependent on vacuum pressure, which can be converted to a pressure value.
The precision of the bridge circuit to maintain control of the sensor’s temperature ensures the accuracy of the pressure measurement. The most common types of bridges used with Pirani pressure sensors are Wheatstone bridge circuits. Pirani pressure sensors are reliable and trouble free. They can offer many years of precision measurements due to their structure, which does not involve any moving parts. Output displays include analog and digital readings that are provided quickly and very responsive to changes in vacuum system pressure changes.
Piezoelectric pressure sensors use the piezoelectric effect to measure changes in pressure, acceleration, temperature, strain, and force. Piezoelectricity is created when mechanical stress is applied to specific materials. As pressure sensors, piezoelectric pressure sensors measure the voltage generated on a piezoelectric element when pressure is applied to it.
The measuring element of a piezoelectric pressure sensor is a crystal that, when experiencing the piezoelectric effect, produces an electrical charge that is proportional to the applied pressure. The produced charge sends a voltage signal that is processed by some form of data system.
The electronics for piezoelectric pressure sensors vary in regard to how they are connected to the sensor. Charges applied by an external device or charge amplifier are referred to as charge output piezoelectric pressure sensors. When electronic circuits are built into the sensor housing, they are referred to as voltage output piezoelectric pressure sensors.
Piezoelectric pressure sensors are used for measuring dynamic pressure in applications that require a fast response and ruggedness. Since piezoelectric pressure sensors are very sensitive, they are ideal for medical applications that require sensitivity and accuracy. The durability of piezoelectric pressure sensors enables them to be used in harsh and demanding environments.
Strain gauge pressure sensors are one of the most common forms of pressure measuring devices. They provide pressure data using the expansion or contraction of a spring that is placed on resistive foil. As force is applied, the spring element deforms. When there are changes to the pressure, resistance fluctuates producing voltage readings that are recorded as electrical signals, which are converted to pressure readings.
The basic principle behind a strain gauge is based on the definition of strain, a response to the application of force. When a stationary material is subjected to force, the result is stress and strain, with stress being an internal resisting force while strain is displacement and deformation. A unique aspect of strain gauges is their ability to pick up expansion and contraction, which is one of the reasons for their general use.
Electrical foil strain gauge pressure sensors are the most common and referred to as conventional. Although foil strain gauge pressure sensors are widely used, there are several other types of strain gauges. All forms use a Wheatstone bridge to calculate any changes in resistance. Other types of strain gauge pressure sensors are quarter bridge, strain gauge rosettes, and piezoresistor, each of which has a unique strain measuring method.
Manometer pressure sensors operate on the principle of liquid displacement using a U-shaped tube. As pressure is applied to one end of the tube, liquid in the other end of the tube moves. The difference between the levels in the two ends is a measurement of the applied pressure. Manometer pressure sensors are one of the oldest forms of pressure meters. They are a simplistic form of pressure sensor and provide moderately accurate readings, which are influenced by temperature fluctuations, the readability of the scale, and the type of liquid in the tube. Manometer pressure sensors are used in low to medium pressure applications.
Although manometer pressure sensors are durable and reliable, they are susceptible to the effects of harsh weather and other environmental conditions. The simplicity of manometers makes them less expensive and low maintenance. Manometers require regular recalibration, which extends their useful life. The measurements of a manometer pressure sensor is a function of gravity and liquid density.
Aneroid barometer pressure sensors use the deformation of a metal partial vacuum chamber to measure pressure. When atmospheric pressure is applied to the vacuum chamber, it compresses. As atmospheric pressure degrees, the vacuum chamber expands. The vacuum chamber is connected to levers and a spring mechanism that is attached to a needle.
As the vacuum chamber expands or contracts, it moves the lever that turns a spring attached to the needle to register the pressure reading. The accuracy of the readings provided are dependent on the size of the chamber with larger chambers providing more accurate readings.
The original barometers used liquids to record changes in pressure. Aneroid barometer pressure sensors, which do not use liquid, were introduced in 1844 and are widely used for modern applications. The appearance and construction of aneroid barometer pressure sensors is similar to the older versions but include microelectromechanical systems (MEMS) technology. Modern aneroid barometer pressure sensors are smaller, compact, and can be integrated into any system.
Sealed pressure sensors measure pressure using atmospheric pressure as a referent. They combine relative pressure with sealed sensor elements. The reference pressure for a sealed pressure sensor is atmospheric pressure trapped behind the sensor’s diaphragm, which is at 14.7 psi or sea level pressure. Sealed pressure sensors are used with applications that require a high-pressure range and consistent temperature where venting is not available.
Certain applications for pressure sensors are unable to provide a vent path, such as submersibles. Conditions without a vent path rely on sealed pressure sensors to provide accurate readings. Although sealed pressure sensors can be used in a variety of applications, they are mostly used for high pressure ranges using a sealed gauge reference. Their use for lower ranges is restricted to short periods of time where there are minimal pressure changes.
With the advancement of technology, pressure sensors have found wider use in a variety of industries. Used as monitoring devices, safety measures, and process oversight, pressure sensors provide reliable and accurate information regarding the characteristics of different materials. The type of pressure sensor used for an application is partially dependent on the material to be monitored.
Pressurized air has multiple purposes from helping maintain tires to the tanks on compressors. Air pressure sensors assist in helping with the application of air to different processes. They indicate the level of air that has been applied. Knowledge of air pressure assists with air flow rates for tools, breathing assistance, sandblasting, and valves. The different types of air pressure sensors are absolute, gauge, and differential.
Gas pressure sensors are designed to measure rapid pressure changes using electrical current to provide feedback. The types of gas pressure sensors include the full spectrum of pressure sensors from gauge pressure sensors and sealed pressure sensors to vacuum pressure sensors and differential pressure sensors. The difference between the many types of gas sensors is their suitability for an application, their cost, technological advancements, physical size, fittings, and types of process connectors.
The working range of gas pressure sensors is defined in kilopascals, atmospheres, or millimeters of mercury. In addition, they have different response times and operate at different temperatures.
Water pressure sensors are used to measure the level of water in tanks and the rate of change in the level of water. They are placed on an open-ended tube that is submerged in a tank. As the water level changes, air in the tube is compressed, which increases the pressure on the sensor. Analogue converters convert the signal from the sensor to a digital value. Water pressure sensors are also used to monitor the pressure in pipes for water distribution systems.
Absolute, gauge, and differential pressure sensors are used for measuring water pressure. Most water sensors have a diaphragm made of silicon and use a strain gauge to measure electrical resistance from the applied force of the water. The key features of water pressure sensors are their resistance to vibrations and moisture.
Most pressure sensors are capable of measuring a wide range of liquids, gases, water and air. Viscous liquids require a special type of sensor that can measure melted plastics, paper pulp, bitumen, rubber, asphalt, crude oil, and sludge. The two ways that viscous pressure is measured are absolute and gauge. Absolute pressure sensors are used due to their measuring pressure against a set value. Gauge pressure sensors use the surrounding atmosphere as a referent.
Most pressure sensors have a narrow vent that allows a liquid to enter and press against the diaphragm. Such a structure is impractical with viscous liquids due to their lower flow rates and tendency to solidify. To overcome this difficulty, viscous pressure sensors have a flatter open design with a flush diaphragm to allow liquids to flow freely. In most cases, viscous pressure sensors have an open design, which makes it easier to clean them.
Like pressure sensors for viscous liquids, pressure sensors for corrosive materials have to be specially designed to withstand the effects of the liquids. Corrosive substances are capable of destroying other substances, metals, and organic compounds. As with viscous liquids, pressure sensors for corrosive liquids are absolute and gauge pressure sensors. diaphragms for corrosive liquids are made of elastic ceramic material, which acts as a semiconductor distortion gauge. When the corrosive material changes the shape of the ceramic diaphragm, the piezoelectric crystalline structure is distorted due to the electrical resistance.
Housings for corrosive liquid pressure sensors are made of stainless steel or highly durable plastics such as PVC or PPS. Many pressure sensors for corrosive liquids are temperature compensated to prevent their readings from being affected by the temperature of the media.
A common and necessary use of pressure sensors is for the monitoring of pneumatic and hydraulic fluids, which are essential to the operation of production equipment. Pressure sensors for hydraulic and pneumatic fluids are differential or absolute pressure sensors. The diaphragms for pneumatic and hydraulic pressure sensors are made of silicon and are attached to a strain gauge.
The terms pressure sensor, pressure transducer, and pressure transmitter are often used interchangeably. Although they have similar functions, they have slight differences. The factor that distinguishes the three types is their signal output. All three types measure detected force and convert their recorded signals into readable data.
Pressure transducers are electromechanical devices that measure applied pressure and output an electrical signal using mechanical and electrical components. They transform the detected force into a continuous voltage output. Three types of output from a pressure transducer are millivolt, amplified voltage, and 4-20mA with the most common pressure transducer output being voltage and milliamps.
Output from a pressure transducer has a specific relationship to the input quantity. They have a physical reaction inside the sensing element. Pressure transducers are able to handle signal conditioning, which allows them to send data over great distance.
Pressure transmitters operate in a similar manner to that of pressure transducers. The difference between them is their types of outputs. Pressure transducers output voltage readings while pressure transmitters output current signals in milliamps across a low impedance load. They are commonly used for large scale processing applications such as chemical plants, food and beverage manufacturing, and power generation. Pressure transmitters are the preferred choice for sending signals over long distances.
In many cases, pressure sensor is used as a general term to describe pressure transducers and pressure transmitters. The process used by pressure transducers and pressure transmitters converts detected force into different voltages. Pressure sensors sense the force and produce a millivolt output signal that corresponds to the strength of the force being exerted. They produce a physical reaction to pressure using a sensor module.
Pressure sensors have a millivolt output and can be used ten to twenty feet away from electronics without signal loss. Millivolt signals give designers more flexibility.
The best way to select a pressure sensor is to speak to a pressure sensor professional who can present the various types of pressure sensors and their features. In many cases, the selection process includes a conversation as to the media, application, and environment where a pressure sensor will be used. Manufacturer representatives use the collected data to select the right pressure sensor for an application and help in avoiding making incorrect and unnecessary purchases.
Of the questions that are explored during a pressure sensor discussion, the most important of them is the application for which a pressure sensor will be used. A full understanding of the application determines the types of demands that will be placed on a pressure sensor. The main consideration is how specialized an application is, such as clean rooms and extreme sanitary conditions.
Each type of media requires a certain type of pressure sensor. In the food and beverage industry, cleanliness and sanitary conditions are mandatory. Viscous and corrosive liquids are media that can clog a conventional pressure sensor. Pressure sensor manufacturers provide a variety of pressure sensor solutions that can meet the requirements of any type of media. Working with manufacturers and identifying the type of media ensures the selection of the right pressure sensor for a process.
An obvious factor regarding the selection of a pressure sensor is accuracy, which would seem to have substantial significance for an industrial device. Although accuracy is important, it is not important for all applications. Different applications require different levels of accuracy. Refrigeration and HVAC systems do not require the same level of accuracy as test benches, calibration, and laboratories. Sensitivity is related to accuracy as regards the ability to detect small incremental changes, a factor that is important in medical pressure sensors.
The hostility of an industrial environment can play a major role in the reliability of a pressure sensor. The temperature of the working environment affects pressure sensor accuracy. Challenging environments require that a pressure sensor be chemically compatible and capable of enduring temperature changes. Pressure sensors that are required to endure extreme environments of temperature, vibrations, impact, and media are specially designed and expensive. Conditions that affect a pressure sensor are humid and acidic, explosive, dust and dirt, and electromagnetic fields.
In any business or industry, the first factor that is considered when purchasing new equipment is cost. Although this is just as important as regards pressure sensors, the above factors tend to take precedence due to the importance of proper pressure readings. Pressure sensors for specific and harsh applications are more expensive than traditional pressure sensors. As technological advancements are added to a pressure sensor, its cost increases as its durability and accuracy improve.
The five factors listed above are some of the foundational considerations clients need to examine when purchasing a pressure sensor. Different manufacturers and experts have their own concepts they believe need to be considered, such as pressure range, portability, configurability, stability, structure of a pressure sensor and its materials, and pressure reference. Of the many factors available to weigh when choosing a pressure sensor, the best choice is to seek the guidance of an experienced pressure sensor manufacturer who can use their expertise to provide the right pressure sensor at the right cost.
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