How pressure transmitters measure line and vessel pressure, and the difference between gauge and absolute readings.
A pressure transmitter has a thin diaphragm that flexes slightly under pressure. A sensing element turns that flex into an electrical signal, and the electronics scale it to an output such as 4–20 mA, a switch point, or IO-Link.
- Piezoresistive (silicon) sensors. Strain-sensitive resistors on a silicon chip change resistance as the diaphragm flexes. The chip is usually protected behind a thin stainless steel diaphragm, with oil transferring the pressure. Accurate and sensitive, suited to a wide range of pressures.
- Ceramic sensors. A ceramic diaphragm flexes, and either thick-film resistors on it or the change in capacitance between it and a base plate is measured. Ceramic is very resistant to corrosion and abrasion and has no fill oil, but is less tolerant of sharp overpressure spikes than metal.
- Thin-film metal sensors. Strain gauges deposited directly on a stainless steel diaphragm; robust against overpressure and vibration.
¶ Reference: gauge, absolute, and sealed
| Type |
Measures relative to |
Reads zero when |
Typical use |
| Gauge |
Local atmospheric pressure |
Open to air |
Line and pump pressure — the usual choice |
| Absolute |
A perfect vacuum |
In a vacuum |
Vacuum, barometric, and some level work |
| Sealed gauge |
A fixed sealed reference, near standard atmosphere |
Open to air at sea-level pressure |
Harsh or wet locations where a vent would let water in |
A gauge sensor needs a small vent (in the housing or cable) so it can see atmospheric pressure. A blocked or wet vent gives errors. Absolute sensors read about 1 bar (14.7 psi) higher than gauge sensors at sea level.
| Unit |
Equivalent |
| 1 bar |
100 kPa, about 14.5 psi |
| 1 psi |
about 6.89 kPa, 0.0689 bar |
| 1 MPa |
10 bar, about 145 psi |
| 10 m of water |
about 1 bar (about 33 ft of water ≈ 14.5 psi) |
Readings are labeled g for gauge (barg, psig) or a for absolute (bara, psia).
- Water hammer and pressure spikes. Fast-closing valves, pump starts and stops, and solenoid valves create pressure spikes far above normal operating pressure, lasting milliseconds. They can overrange the sensor, shift its zero, or crack a ceramic diaphragm. The transmitter may never show the spike.
- Pulsation from positive-displacement pumps, including diaphragm metering pumps, makes the reading swing with each stroke.
- Overpressure beyond the rating, including hydrostatic testing of the system with the sensor still in place.
- Temperature. Hot water or steam at the diaphragm shifts the reading and can damage the sensor. Freezing water in the port can burst the diaphragm.
- Blocked or wet vent on a gauge sensor.
- Mounting height. A sensor mounted below the point of interest reads higher by the height of water in between (about 0.1 bar per meter, or 0.43 psi per foot); above it, lower. Trapped air in the impulse line causes its own error.
- Clogged ports from scale or debris slow the response or block it.
- Chemical attack on the diaphragm or seals when the wetted materials do not suit the fluid.
- Electrical noise on long analog cable runs, and loose connections at the connector.
- Isolation valve. Fit an isolation valve (and ideally a bleed or vent) between the process and the sensor, so it can be removed without draining the line. See Isolation.
- Snubber or damper. Fit a snubber (a porous or restricting element) where there is water hammer or pump pulsation, such as on metering pump discharges or near fast-acting valves. Some transmitters have a damping setting that smooths the output; this does not protect the sensor from spikes — a snubber does.
- Pulsation dampener on metering pump lines protects both the sensor and the pipework.
- Mount on the side of a horizontal pipe for liquids, so air collects above and debris below the port. For gas, mount on top.
- Keep it away from turbulence. Avoid mounting right after elbows, valves, or pumps where the flow is disturbed.
- Account for height. Mount at the level of interest, or offset the zero for the height difference.
- Protect from heat and frost. Use a siphon (pigtail) or length of impulse line on hot lines, and insulate or heat-trace outdoor lines that can freeze.
- Tighten on the hex, not the housing, and use the thread sealant or gasket the connection requires.
- Choose a range so normal operating pressure sits in the middle of the span, with the maximum expected spike below the sensor’s overpressure rating.
- Check materials against the fluid — see Material Compatibility.
Never loosen or remove a pressure sensor from a line that is under pressure. Isolate, bleed down to zero, and confirm zero before breaking the connection. See Pressurized Systems.
¶ Maintenance
- Zero check. With the sensor isolated and vented to atmosphere, a gauge sensor should read zero. A zero shift points to overpressure or spike damage.
- Span check. Compare against a calibrated test gauge or pressure calibrator at one or more points in the normal range. There is normally little drift; many sites check annually.
- Clear the port of scale and debris when the sensor is removed. Do not poke the diaphragm with tools — it is thin and easily damaged.
- Check snubbers, which can clog in dirty water and slow the reading.
- Check the vent and connector for water ingress.
- Service life. Transmitters commonly last many years; replacement is usually due to spike damage, chemical attack, or water ingress rather than wear.
Models documented in this wiki: