The short answer
Pressure Transmitters
A pressure transmitter senses pressure on a diaphragm, converts the deflection to an electrical signal, and outputs 4 to 20 mA scaled to its calibrated range. Gauge transmitters measure against atmosphere, absolute against vacuum, and differential between two connections. Selecting one means matching the reference, the range with margin for overpressure, the accuracy the loop needs, and the wetted materials to the fluid. Installing one means a connection that carries pressure without carrying pulsation, solids, or a column of liquid the calibration did not expect.
Key points
- Gauge, absolute, or differential: the reference decides the instrument, and using the wrong one is an offset that no calibration removes.
- Range with turndown lets one transmitter be calibrated across a wide span, but accuracy specifications are usually a percent of span, and a narrow span on a wide-range unit is less accurate than it looks.
- Overpressure and burst ratings are separate from range; a water hammer that exceeds them ends the transmitter.
- A snubber tames pulsation, an isolation valve permits calibration, and a diaphragm seal keeps the process out of the sensor.
- A column of liquid between the tap and the transmitter is a pressure the transmitter reads; zero it out or account for it.
Three references
- Gauge
- Measures relative to the atmosphere, through a vent in the housing. Reads zero with the process open to air. The right instrument for discharge pressure, system pressure, and most pump work.
- Absolute
- Measures relative to a sealed vacuum. Reads atmospheric pressure, about 14.7 psi at sea level, with the process open to air. Used for vacuum service, for barometric compensation, and where atmospheric changes would matter.
- Differential
- Measures the difference between a high side and a low side connection. Used for flow across an orifice or a venturi, for level in a closed tank, for filter loading, and for any measurement where the difference matters and the absolute value does not. It has a static pressure rating, the pressure both sides can carry together, which is separate from its differential range.
Sensing
The process pressure deflects an isolating diaphragm, the deflection is transferred through a fill fluid to a sensing element, and the element, piezoresistive or capacitance on most current designs, produces a signal the electronics linearize, compensate for temperature, and convert to 4 to 20 mA, usually with HART on the loop. The fill fluid matters in two places: it must not react with the process if the diaphragm fails, and its expansion with temperature is part of the temperature error on a diaphragm seal with a long capillary.
Range, span, and turndown
A transmitter has an upper range limit, the most it can measure, and a calibrated span, the part of that range mapped onto 4 to 20 mA. Turndown is the ratio of the two: a 300 psi transmitter calibrated for 0 to 30 psi has a turndown of ten. Modern transmitters offer turndowns of a hundred or more, which lets one stocked unit serve many applications. The catch is in the accuracy specification.
Overpressure
Separate from the range are the overpressure limit, the pressure the transmitter can see without permanent damage, and the burst pressure, beyond which the housing fails. Water hammer from a valve slamming, a pump starting against a closed check, or a surge on a long force main produces short spikes well above operating pressure. A transmitter at the discharge of a pump station sees them regularly. Select the range so that the overpressure limit covers the spike, or fit a snubber to spread it, or both.
The process connection
- Isolation valve
- Between the tap and the transmitter, so that the transmitter can be removed or calibrated with the process live. A manifold with isolation, equalizing, and vent valves does the same for a differential unit.
- Snubber
- A restriction in the connection that damps pulsation from a positive displacement pump or a surge. It slows the response, which is the point, and it can plug.
- Diaphragm seal
- A second diaphragm at the process with a capillary to the transmitter, for fluids that would plug, corrode, or freeze in the connection: sludge, slurries, chemicals, hot fluids. Adds temperature error from the capillary fill and must be specified as a matched assembly.
- Bleed or drain
- To purge air from a liquid connection or condensate from a gas one before zeroing. A connection with trapped air reads wrong and reads differently as the air compresses.
- Pressure gauge
- A local gauge on the same connection gives a technician a reference without a calibrator, and shows the process pressure when the loop is dead.
Installation errors that look like calibration errors
- Elevation. A transmitter mounted below its tap on a liquid line reads the column of liquid in the connection as extra pressure, about 0.433 psi per foot of water. Mount at the tap elevation or zero the offset out after filling, and record that it was done.
- Trapped air in a liquid connection, or trapped liquid in a gas connection. Bleed before zeroing.
- A tap in turbulent flow, at an elbow, or in the pump discharge before the check valve. Reads a pressure that includes velocity effects and pulsation.
- A blocked snubber or a plugged connection. The transmitter reads a pressure that no longer follows the process, often plausibly.
- Temperature. The transmitter and its fill are rated for a range; a unit on a hot line or in direct sun outside it drifts.
- Vibration. A transmitter mounted on a vibrating pump reads noise and eventually fails. Mount on a bracket off the pump with a flexible connection.
Frequently asked questions
- What is the difference between gauge and absolute pressure?
- Gauge pressure is measured relative to the atmosphere and reads zero open to air. Absolute is measured relative to vacuum and reads about 14.7 psi open to air at sea level. Most pump and system measurements are gauge; vacuum and barometric work is absolute.
- What does turndown mean on a pressure transmitter?
- The ratio of the transmitter’s maximum range to the span it is calibrated for. A 300 psi unit calibrated for 30 psi has a turndown of ten. High turndown is convenient for stocking, and accuracy can degrade as it increases, depending on how the datasheet states accuracy.
- Why does my pressure transmitter read a few psi with the line drained?
- Usually the liquid column in the connection between the tap and the transmitter, if the transmitter is mounted below the tap, or trapped liquid. Bleed the connection and zero the transmitter with the connection at the condition it will run in.
- Do I need a snubber?
- On a connection that sees pulsation from a reciprocating pump or surges from valves and pump starts, yes. It protects the sensor from spikes and steadies the reading, at the cost of some response speed and a restriction that can plug.
Related topics
- Differential PressureThe differential pressure transmitter as an instrument: what it measures, static and overrange ratings, the three-valve manifold and the order of operations that protects the cell, and the applications from filter head loss to membrane TMP, flow, and level.
- Impulse LinesThe tubing between the process tap and the pressure or DP transmitter, and everything that goes wrong in it: slope and routing for liquid and gas, elevation head and how to correct for it, air and condensate, blocking and manifold valves, freezing, plugging on dirty service, and the diaphragm seal alternative.
- Pressure CalibrationWhat calibrating a pressure transmitter means: as-found and as-left, the five-point test and hysteresis, tolerance and the reference standard, sensor trim versus output trim versus reranging on a smart transmitter, gauge versus absolute, and intervals.
- Distribution Pressure ControlHow a booster or high service pump station holds discharge pressure with variable speed pumps, how pumps are staged, and what stops the system hunting.
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- Pressure Transmitter InstallationWhere and how to install a pressure transmitter so it reads the process and not its own plumbing: tap location, mounting above or below the tap, isolation and bleed valves, line slope, pulsation, elevation, freeze protection, seals, vibration, and sun.
Direct contact
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