Pressure is defined as exertion of force per unit area. There are many different instruments used for pressure measurement, including pressure gauges, pressure sensors, pressure transducers, as well as pressure transmitters. There are also specialized devices for measuring pressure extremes, such as vacuum pressure.
Pressure sensors operate by sensing pressure and converting it into an electrical quantity. Piezoresistive and capacitive are the two most commonly seen types in industrial applications with piezoresistive being the most commonly used.
The electrical resistance of piezoresistive materials, measured in Ohms (Ω), will change when they are strained or compressed. A piezoresistive pressure sensor consists of a micro-machined silicon diaphragm that has a piezoresistive strain gauge diffused into it. The diaphragm is fused to a silicon or glass backplate. The sensor contains resistors that are typically arranged in the form of a Wheatstone Bridge Circuit. As pressure increases on the piezoresistive material, it is more resistant to the electrical current passing through it. This results in the output of the Wheatstone Bridge, measured in millivolts, being directly proportional to pressure.
Capacitive pressure sensors make use of a thin diaphragm that serves as one plate of a capacitor. This diaphragm is usually a metal or metal-coated quartz component. The diaphragm is exposed to a reference pressure on one side and to the process pressure on the other. Changes in pressure will cause a slight deformation of the pressure sensing plate, which will in turn cause changes in the electrical capacitance. These changes in capacitance are directly proportional to the pressure applied to the plate that is exposed to the process.
Diaphragm seal principle of operation
A transducer converts a quantifiable physical parameter, such as pressure, load, force, or others, and converts the measured value into a low-level electrical signal. A transmitter includes all of the elements of the transducer and adds signal conditioning and amplification that allows the electrical signal to be transmitted over a longer distance.
Comparing Pressure Transducers and Transmitters
Pressure transducers usually consist of a thin-film or piezo-resistive pressure sensor mounted to an industry standard process connection, such as NPT threads or an ANSI flange. The transducer converts pressure into an analog electronic signal, which is typically as a millivolt per unit pressure output. These signals are not linearized or temperature compensated. Pressure transmitters have additional circuitry that linearizes, compensates, and amplifies the signal from a transducer. The different signal types are typically industry standard voltage signals (e.g., 0 – 5 or 0 -10 VDC), milliamp (e.g., 4 – 20 or 0 – 20 mA), or digital protocols such as HART or Modbus. Pressure transmitters can transmit the signal over large distances to a remote receiver and can also provide calibration features that can improve the accuracy and measuring range of the instrument, such turndown and zero/span adjustment. Smartmeasurement offers transmitters that are calibrated, tested, and can be reset remotely using MODBUS, Hart or other digital communication protocols.
Choosing Pressure Transducers versus Pressure Transmitters
The choice between the pressure transducer and pressure transmitter boils down to the requirements of the particular application where the instrument will be used, such as Accuracy, pressure range, working temperature, fluid media, and the ambient environment where it will be installed. As for the output signal, here are some factors to take into account:
- Pressure transducers typically has an mV outputs do not have temperature compensation.
- Pressure transmitters make use of the industry standard 4-20mA current output signal which is more immune to EMI / RFI interference and noise than a voltage signal.
- The 4-20mA signal also travels a much farther without degradation than the millivolt output from a typical transducer.
- Pressure transmitters have the ability to collect more information on other variables other than just pressure, have the ability to communicate via digital signals such as Modbus and HART, and offer the ability to manipulate and adjust the transmitter remotely.
- The wider variety of outputs provided by the pressure transmitters allows it to communicate with a wider variety of receiving devices such as PLCs, panel mount controllers, data acquisition cards and strip chart recorders.
The application engineers at SmartmeasurementTM are able to help navigate these variables to help you select the ideal instrument for your application.
Pressure gauges are relatively inexpensive mechanical devices that are read visually, do not require and electrical power, and do not provide any feedback signal. One of the most well-known types is called the Bourdon gauge, which contains a thin-walled metal tube that is typically threaded into the compartment where pressure is being measured. As pressure increases in the tube, the tube begins to straighten. On the exit side of the tube is a lever system that contains a pointer. As the tube straightens, the pointer moves. The pressure is read by observing the position of the pointer against graduated scale, in much the same was that an analog speedometer or tachometer in a car would be read. Common tube shapes include curved or C-shaped, spiral, and helical. This is a mechanical device that is manually read. Another type of mechanical gauge that operates in a similar fashion and also contains a pointer is called a diaphragm gauge.
Traditional gauges such as the Bourdon and diaphragm gauges are sensitive to vibration and condensation. Another type is called a “filled” pressure gauge, and it is filled with a viscous oil, typically glycerin. This design has fewer moving parts than the traditional pressure gauges, and is more reliable. This design dampens pointer vibration and is not susceptible to condensation.