Pressure transmitters are also called pressure sensors, pressure probes, pressure gauges or pressure transducers. There is no significant difference between these terms. However, a distinction is made between analogue pressure sensors and intelligent process transmitters.
By definition, a pressure transducer is a pressure measuring device whose principle is to convert the force exerted by the pressure of a fluid on a given surface (deformation) into an electrical signal.
Air pressure, water pressure, vapour pressure, gas pressure, but also flow, level and density can be measured.
Most industrial measurement relates to pressure, especially since pressure sensors can be used to:
The pressure p is expressed in units of force F per unit of area A. p = F / A
The International System of Units (SI) unit of pressure is the pascal, Pa.
It takes its name from Blaise Pascal born in Clermont-Ferrand where the headquarters of the factory of Fuji Electric France SAS, a French manufacturer of pressure sensors, is located.
There are several other units pressure measurement, including :
The Pascal (Pa): this is the International System (SI) base unit for pressure measurement. One Pascal corresponds to a force of 1 Newton per square metre (1 Pa = 1 N/m²).
The Bar: This is a unit of pressure commonly used in industrial applications. One Bar corresponds to a pressure of 100,000 Pa.
The pound per square inch (psi): This is the unit of pressure used in American applications. One psi corresponds to a pressure of 6,894.76 Pa or 0.0689476 bar.
The Torr: This is a unit used in vacuum pressure applications. One Torr corresponds to a pressure of 1/760 atmosphere (1 Torr = 133.322 Pa).
The International Standard Atmosphere (ISA): This is a unit of pressure commonly used to express atmospheric or barometric pressure. One atmosphere corresponds to a pressure of 101,325 Pa.
The pressure of the measured fluid is applied to an internal pressure sensing element through a fitting and then a mechanical interface - a measuring diaphragm made of stainless steel or other noble material. The electronic measuring element converts the pressure into a raw electrical signal.
There are different measurement technologies :
The signal from the measuring element is then filtered, amplified, temperature compensated and then formatted into an analog signal.
The analog output signal is transmitted via an electrical connector.
Pressure transmitters consist of a mechanical connection, a metal or ceramic diaphragm, a capacitive or piezoresistive sensing element, a filling oil integrated in pressure measuring cells, an electronic module for conditioning and amplifying the signal, and an electrical connector.
It is packaged in a plastic or metal housing (aluminium housing, stainless steel housing or stainless steel).
A digital indicator can be added as an option.
The signal from a pressure gauge can be either analogue or digital. The analogue electrical signal is usually a 4-20mA current, 0-10V voltage or 1-5V voltage. The output signal is sent to the controller to influence the manufacturing process.
The 4-20mA signal is commonly used for pressure transmitters because of several advantages. Firstly, it is resistant to signal loss due to the transmission line, which ensures accurate pressure measurement. In addition, it allows the distance between the measuring instrument and the system concerned to be extended. In addition, the absence of current makes it possible to detect line faults, which facilitates troubleshooting. The 4-20mA electrical signal is also less sensitive to electromagnetic interference, which ensures its reliability. Finally, it can be used in a loop to power several devices such as the display, controller and recorder.
A digital signal can be available with different communication protocols - HART - Fieldbus - Profibus - Modbus. These protocols transmit the value of the pressure measured and also allow the configuration of the pressure measuring instruments. This is known as a Smart Electronic Transmitter or SMART.
The instruments can have digital indicators for pressure measurement. The values of the pressure measured are displayed in a unit of your choice.
The industrial pressure transmitter should be selected according to the fluid to be measured, the pressure range and the operating conditions of the application and process.
To measure pressure, research should be done to answer the questions for choosing the industrial pressure transmitter and determine:
Gauge pressure transmitters allow the measurement of the gauge pressure, also called gage pressure, of the process with respect to the local atmospheric pressure. The normal atmospheric pressure is measured using a reference cavity inside the transmitter.
Differential pressure transmitter uses two separate chambers connected by a flexible diaphragm. Measuring differential pressure requires the pressure to be measured on either side of the diaphragm. The differential pressures (dp) refer to the pressure difference between these two different pressure points - a reference pressure on the low pressure side (LP) and a pressure on the high pressure side (HP).
In order to measure absolute pressure, absolute pressure transmitters compare the gauge pressure with the absolute vacuum, also called zero pressure. The absolute pressure is always positive.
The advantage of the absolute pressure gauge is that it is free from the external pressure due to the ambient atmospheric pressure variations thanks to a vacuum reference chamber and thus achieves a better accurate measurement.
The absolute pressure can be expressed from the gauge pressure by adding 1.013 bar, i.e. absolute pressure (bar abs.) = gauge pressure (bar) + 1.013. This device for measuring atmospheric pressure is used in meteorology, for example.
A diaphragm seals pressure transmitter separates the process fluid being measured from the pressure cell. The diaphragm and the contact parts are made of a material resistant to the fluid being measured and are welded to the base of the pressure cell. A capillary tube or connecting sleeve connects the diaphragm and the pressure sensing cell. This space must be degassed under vacuum, then filled with a filling oil and sealed. The measured pressure exerts a force on the outer surface of the diaphragm. As the diaphragm flexes inwards, it attempts to compress the filling fluid inside the instrument. This filling fluid is designed to resist compression, so the applied pressure is channelled directly to the pressure measurement cell. The entire operation of a diaphragm pressure transducer is based on the Blaise Pascal principle.
Diaphragm pressure transducers are made from various materials such as stainless steel, titanium, Inconel, Hastelloy, Monel, Tantalum and Nickel. The materials used depend on the type of application and temperature for which the industrial pressure measurement device is designed.
Pressure transducers with separators are used to measure the pressure produced of fluids at high temperatures. Applications for high-temperature pressure transmitters with isolating diaphragms include differential pressure measurement of liquid flow, gas flow or vapour flow, level measurement with a flush diaphragm sensor of a fluid in a tank, density measurement of a fluid or gauge pressure measurement.
Multivariable pressure transmitters combine a differential pressure measurement, an absolute pressure measurement and a temperature measurement in the same sensor.
They are used, for example, for mass flow measurement.
Hydrostatic level transmitter are measuring devices for determining the filling level of a liquid in a tank or reservoir.
The measuring principle is based on the hydrostatic pressure, which is the pressure exerted by a liquid as a function of the filling height.
These hydrostatic pressure probes can be immersed in a liquid and are used to measure the level of a tank or reservoir. They are usually fitted with a stainless steel diaphragm.
Industrial pressure transmitters require periodic calibration to ensure accurate metrology throughout their life cycle and to avoid factors that influence accuracy. The calibration period is defined by the pressure transducer manufacturers. The zero and full scale of the sensor should be calibrated.
In the factory, in order to check the linearity of the output signal, the accuracy is checked on several points of the pressure range.
Calibration consists of applying a defined reference pressure to the mechanical interface of the sensor, checking the output signal and then applying compensation. The sensor can be calibrated using an external adjustment screw, a local digital indicator, a programming interface or programming software.
In order to carry out the various manipulations, it may be necessary to have a shut-off valve or manifold mounted on the pressure transmitter to isolate it from the process.
For your annual calibrations, you can call on a company specialising in pressure transmitter calibration.
Regular calibration will ensure that the pressure measurement is accurate to ensure consistent results.
The pressure transducer can be fixed by means of a mechanical connection to the measuring device or to the pipe where the pressure is to be measured.
Special precautions must be taken depending on the pressure and temperature conditions of the process. The output of the sensor can be connected to a display system (digital panel meter, recorder or supervision) or to a PLC (programmable logic controller) in order to handle a pressure control system and regulate the pressure of a process.
If you need help, it is recommended that a professional company is used to install and commission your measuring instrument.
A pressure transducer can be tested by applying a defined known pressure to the mechanical interface of the pressure measurement device and checking the measured analogue output signal or the value displayed on the indicator.
If your instrument is faulty you can have your instrument repaired by a specialist or one of the manufacturers of industrial sensors.
There are different ways to set up pressure sensors.
Pressure sensors can have a local digital indicator that can be adjusted to adjust the parameters.
They can also be configured remotely with a programming interface (hand held communicator) or a configuration software via the digital signal and HART protocol.
Pressure transmitters are used in many applications in various industries. Measuring instruments for industrial pressure measurement can measure pressures ranging up to several thousand millibars and are therefore essential in a wide range of industries. The special features of the various versions as well as the pressure range and ambient conditions are decisive in the selection of a suitable sensor to solve a problem
Depending on your industrial pressure measurement, the most suitable pressure measuring device should be selected according to the application and the requirements of the industrial environment:
To ensure that your processes operate as expected and to avoid errors, the experts at Fuji Electric, a French manufacturer of industrial measurement instruments, offer guidance and pressure transducers designed for the most demanding applications and processes.
Fuji Electric instruments for pressure measurements are known for their accuracy, wide pressure range, long term stability, quality of manufacture, reliability, durability, technical support and improved delivery service.