How to specify a pressure transmitter

Start with the measurement

First decide what pressure is being measured. Gauge pressure is referenced to atmosphere; absolute pressure is referenced to a vacuum; differential pressure is the difference between two process points, used for filter monitoring, level and flow. Sealed-gauge types are sometimes used where atmospheric venting is undesirable. Then define the normal operating pressure, the minimum and maximum, and the range you want the output to cover.

Choose a range so that normal operation sits comfortably within it, not at the extremes. Also state any pressure spikes, water hammer or vacuum the instrument may see, so that overpressure limits can be checked.

Process conditions and wetted materials

List the medium, its temperature range and anything that affects materials: chlorides, acids, abrasive solids, viscosity, crystallising or sticky fluids. The diaphragm and process connection must be compatible with the medium, and seals must suit both medium and temperature. Viscous, dirty or hygienic duties may call for a flush diaphragm or a diaphragm seal. Steam service normally needs a siphon or impulse line to keep the sensor away from high temperature, and pulsating pumps may need a snubber.

Accuracy, and what it actually means

Accuracy figures are not all calculated the same way. Some are stated as a percentage of span, some of full scale, and some include only part of the error budget. Ask what the figure includes — non-linearity, hysteresis and repeatability — and look separately at temperature effects and long-term stability, which often dominate in real installations. Specify the accuracy you need for the application rather than the best available, because tighter accuracy usually costs more and needs more careful installation and calibration.

Output, power and communication

State the signal the control system expects. A two-wire 4–20 mA loop is the common industrial choice; voltage outputs and digital protocols such as HART or Modbus are also used. Give the supply voltage available, load or cable length if known, and whether local display or configuration is needed.

Installation and environment

Give the process connection (thread type and size, or flange), mounting position, electrical connection, ambient temperature range, and the ingress protection required for the location. If the transmitter will sit in a hazardous area, state the area classification so that only suitably certified instruments are considered. Note also whether isolation and vent valves or a manifold are needed; a gauge root valve or manifold makes removal and calibration safer.

Documents and checking

Say which documents you need, such as a calibration certificate or material certificates for wetted parts. Plan how the instrument will be checked in service, and whether a calibrator or reference gauge is available on site. A complete specification avoids the most common problems: wrong range, incompatible materials and a signal that does not match the control system.

Common mistakes to avoid

The most frequent problems are a range chosen far wider than the process needs, which wastes resolution; wetted materials chosen by habit rather than checked against the medium; transmitters on steam lines mounted without a siphon or impulse line; and outputs that do not match the input card in the control system. Checking each of these before ordering is quicker than correcting them after installation.