How Do You Select a Pressure Transmitter?

The two areas of focus when selecting a pressure transmitter are the operating conditions (environment) and performance requirements. Environmental information allows for the selection of an appropriately rugged pressure transmitter able to withstand the media and operating /ambient temperatures. Knowing the specific performance requirements allows a pressure transmitter to be specified that can handle the system’s pressure at the desired accuracy.

Operating Conditions

Knowing where the transmitter will be operating and choosing accordingly the transmitter case and wetted materials will extend its service life. Each transmitter should be selected partly on the basis of the medium being measured. Ensure that the parts exposed to the medium are compatible with or can withstand its particular characteristics.

Keep in mind that heavy vibration, shock, moisture and electric interference will affect pressure sensing. Since each application is specific to a certain set of requirements, understanding the application’s needs will determine the model required.

Extreme temperature ranges can also produce adverse effects to reading accuracy. Make sure you identify an acceptable temperature effect on span (thermal drift) when selecting your transmitter. These can be as low as 0.2% FSO per 10°K.

If the transmitter is installed for outdoor use, ensure that the appropriate type of enclosure/seal is selected. All of Winters’ pressure transmitters are sealed against water and dust entry, and meet standard NEMA enclosure ratings. Standard housings are 304 or 316 series stainless steel.

Performance Requirements

Is the pressure range being measured in the positive or vacuum scale of measurement? Is each point of measurement stand-alone or are the differences between two points being measured (differential)?

In general, pressure transmitters provide highly accurate pressure readings. It is important, however, to always choose a range that is suitable to your requirements. Consider this scenario: a transmitter that has an operating range of 0 to 200 psi and has an accuracy of ±0.25% is being used to measure pressure on a piece of equipment that only goes to a maximum of 20 psi. An accuracy of ±0.25% over a 200 psi range means that the maximum error you may see is ±0.5 psi. Therefore, for a measurement output of 7 psi, the real number may be between 6.75 to 7.25psi. An error of 0.5 psi means you have selected a transmitter with an error of 7.14% (0.05 divided by 7) at 7 psi. So, select a transmitter that has the closest normal operating range to what you are measuring, and consider an acceptable accuracy percentage.

Also, be sure about the type of output you need. Winters’ standard transmitter outputs are in milliamps. Output in volts is available.

How Transmitters Work

Transmitters can be thought of as electronic pressure gauges. There are two kinds of electronic pressure gauges: pressure transmitters and pressure transducers. The general difference between a transmitter and transducer is the electrical output, where a transmitter outputs signals in milliamps (mA) and a transducer outputs signals in volts (V) or millivolts per volts (mV/V).

Power is supplied to the circuit.

  1. Pressure pushes on a sensor
  2. The sensor flexes and changes the resistance or capacitance of a circuit mounted on the backside of the diaphragm (the change in the flow of electricity across the circuit is in direct proportion to mechanical force/pressure applied to the diaphragm)
  3. Special electronic circuitry conditions amplify the sensor’s signal and converts (transducers) it to a useable signal (i.e. 4-20 mA)
  4. Electrical output signal is sent to an indicator or computer

Note: When selecting a pressure switch, always refer to CAN/CSA-C22.2 NO. 61010-1-12 – Safety requirements for electrical equipment for measurement, control, and laboratory use – Part 1: General requirements (Tri-national standard), with UL 61010-1 and ANSI/ISA-61010-1 (82.02.01)