What is a pressure transmitter?

What are pressure transmitters?

Pressure transmitters are electronic pressure gauges used to measure and monitor pressure. The term pressure transmitter is also often used for pressure transducers or pressure sensors. However, this is not a synonym, as the term pressure sensor in that sense describes only part of the pressure transmitter.

They are available in many designs to suit pressure and electrical connections, measurement procedures, electrical output signals and certifications (CE, EX, rail and marine). Superior technology and precise manufacturing ensure that these pressure transmitters work perfectly. This is especially important in applications where high demands are placed on long-term stability, vibration resistance, electromagnetic compatibility, shock resistance or temperature insensitivity.

What does a pressure transmitter consist of?

A pressure transmitter consists of a sensor that converts the pressure or a pressure change into an electrical signal via a physical principle. In addition, a pressure transmitter contains measurement transducer electronics that processes the sensor signal and converts it into a standardized electrical output signal. The output signal is provided at the electrical connection. Pressure is applied to the sensor through the pressure connection (also called process connection). The sensor and electronics are housed in a housing that protects these two sensitive components from environmental influences on the one hand and connects them to the electrical connection and pressure connection on the other.

What are the selection criteria for a pressure transmitter?

The selection criteria for a pressure transmitter are the appropriate measuring range, the required accuracy and the desired output signal. However, another very important criterion is the suitability of the pressure transmitter for the desired application. The measuring range, accuracy and output signal are parameters that are relatively easy to determine. Determining which type of pressure transmitter is suitable for a specific application, on the other hand, requires some experience and careful consideration of various aspects.

Selection criteria for the ideal pressure transmitter in your application

Print Type: In pressure measurement technology, a distinction is made between absolute pressure, relative pressure and differential pressure. Absolute pressure always refers to the absolute vacuum as zero point. Relative pressure measurement is the measurement of the pressure difference between a medium and the ambient or atmospheric pressure (approx. 1 bar). The following article explains the differences in detail.

Measuring range: This is the pressure range between the minimum pressure (where the output signal is 0%) and the maximum pressure (where the output signal is 100%). The difference between the minimum and maximum values is called the range and serves as the reference for almost all accuracy specifications in pressure measurement technology. As a rule, the measurement range of pressure transmitters is standardized to a specific pressure measurement unit, for example, bar, mbar or psi. In addition to the pure signal range, overpressure and peak pressure limits must also be considered. These are important in applications where - even very short - pressure peaks can occur far above the measuring range.

Accuracy class: In practice, defining the required accuracy class or permissible measurement uncertainty of the manometer has proven to be a major challenge. On the one hand, the accuracy class includes different aspects or parameters of measurement uncertainty, which do not have the same meaning in most applications. On the other hand, even on the application side, it is often difficult to determine how accurate the measurement should actually be. Higher accuracy almost always entails a huge impact on the product cost. Therefore, it is important to carefully weigh what is mandatory when selecting the accuracy class.

Output signal: Three main categories are distinguished for the output signal: Unamplified sensor signal, analog (standard) signals and digital signals. The output of the unamplified sensor signal is very rarely desired for manometers; this is in contrast to temperature gauges, which often provide the signal from the PT100/PT1000 resistor directly and without additional electronics. Now when the unamplified sensor signal is delivered, the pressure measuring instrument is not a transmitter or measuring transducer in the narrower sense. It is then referred to as a measuring cell with housing. These are often also called transducers. Analog signals are still the most commonly used in pressure measurement technology in industry, especially the current signal 4 ... 20 mA. The advantage of analog signals is still the significantly lower cost for pressure transmitters and often also for current downstream evaluation electronics. However, the cost of digital transmitters and evaluation units has dropped significantly in recent years. Moreover, the spread of sensor bus systems such as IO-Link or CANopen in pressure measurement is increasing rapidly. The main advantages of digital signals are the higher safety against errors, diagnosis and parameterization capabilities and the combination of several measurement parameters in one device, for example pressure and temperature. Electronic pressure switches are also included among pressure transmitters with digital signals.

Sensor: The pressure sensor is the core element of the manometer. In pressure transmitters, this pressure sensor is usually an element where a change in pressure leads to a deformation of the diaphragm. This in turn leads to a change in electrical resistance on specially fitted resistive elements. The most commonly used sensor technologies are thin-film-on-steel, thick-film-on-ceramic and piezo-resistive sensors. In thin-film-on-steel sensors, resistors are sputtered onto a stainless steel membrane. The main advantage of these pressure sensors is their excellent long-term stability and high robustness against pressure spikes and temperature influences, as well as pressure measurement over a wide pressure range from about 200 mbar to more than 3,000 bar. Thick film-on-ceramic sensors are based on a ceramic base body to which the resistance bridges are applied and then burned in. The ceramic membrane is considered extremely robust against virtually all corrosive liquids and gases and is preferentially used when aggressive chemicals must be measured. Measuring ranges start at about 100 mbar and increase to about 400 bar. In piezo-resistive pressure transducers, the resistance of silicon semiconductor elements changes with pressure. These semiconductor elements are isolated from the measuring medium by an oil filling and a thin separation membrane. Due to the high sensitivity and low hysteresis of the silicon element, piezo-resistive sensors are particularly suitable for low pressures in the mbar range and when higher accuracy is required.

Pressure connection / process connection: The pressure port connects the pressure gauge to the process where the pressure is to be measured. The pressure transducer in the transmitter must be securely connected to the pressure port (welded or with elastomeric seals). There are numerous different pressure connections on the market, whose geometry and dimensions are specified in standards (for example, the pressure gauge connection in DIN EN 837-1). Besides industry-specific preferences, the decisive factor in the choice of connections is primarily the type of seal: metal seal or with elastomer seals. The metal seals are tapered threads or sealing cones. Each is mechanically deformed by its counterpart, such that a sealing effect is created. For pressures above 1,000 bar, metal sealing solutions are usually chosen.

Elastomeric seals - also called O-rings or profile seals - use cylindrical mounting threads. In a groove, the elastomer seal is compressed during assembly, creating a sealing effect. The elastomer material must be chosen to be compatible with the measured media and seal over the entire temperature range.

Electrical connection: There is less variation in electrical connection because each industry segment uses only a few connector types. In general, it can be said that cable versions directly on the pressure transmitter are less commonly used because the wiring is more complex than versions with connectors.

Main criteria for selecting the appropriate connection are tightness against liquids and dust, vibration resistance and the cost of measuring equipment and cabling including commissioning. A special requirement for pressure transmitters may be proper pressure equalization from the transmitter interior and the environment: In practice, many electrical connections can cause unexpected problems when measuring pressure.

Marktechnical offers a wide range of different models, designs and variants, as well as matching accessories to best meet the requirements in any application area.

Our pressure measurement experts will be happy to explain the differences between our products and advise you which device is best suited to reliably fulfill your demanding measurement task.