Pressure Transmitters

Everything you need to select and source a pressure transmitter can be found from this comprehensive product information page.

A complete list of resources to web pages covering pressure transmitter specifications, selection, technical terms, explanations and application notes all in one place.

Pressure transmitters have a 4 to 20 mA current loop output and are used in many industrial and process applications. The robust design of pressure transmitters make them ideal for use in outside installations, often in very harsh environments.

Contents

Products

Find product information for current output pressure transmitters for gauge, absolute & differential pressure sensing where a robust 2 wire 4-20mA current output loop is required which can work over long distances without signal degradation.

If you would like us to select a pressure transmitter for you, then please use this form to specify your requirements.

Description

A pressure transmitter is a pressure sensing device with an output signal that can be sent over long distances without significant signal degradation.

The most common example of a transmitter output is a 4-20mA current loop output, but amplified voltage output signals such as 0-5 or 0-10 volts are also classed as transmitters by some manufacturers, especially where current and voltage options can be specified for the same product type.

The types of current output from a pressure transmitter that you will likely come across are:

4-20mA (2 wire)
4-20mA (3 wire)
0-20mA (3 wire)

A 2 wire configuration means that a total of two electrical wires are used to connect the supply and output connections. The pressure transmitter is connected in series with the components that make up the current loop circuit. There must be a specified minimum voltage supplied between the positive and negative connections to the pressure transmitter to generate a 4-20mA current loop signal. This is the most common configuration used for a 4-20mA output pressure transmitters and is the standard used universally by the process control industry.

3 wire configuration means that a total of three wires are used to connect the supply and output connections. The two output wires are connected in series with other components connected to the current loop output circuit, and the supply positive is connected to the third wire separate to the current loop output circuit. The supply negative is connected to the same wire that the negative side of the current loop is connected.

Installation Tips

Installation tips for improving the measurement accuracy and extending the operational service life of pressure transmitters.

For maximum pressure transmitter life and ease of maintenance the following guidelines are suggested:

If possible the pressure transmitter should be mounted at ground or walk way level to ease access for installation, maintenance and pressure calibration.
When the process conditions allow, pressure transmitters should be connected to the measurement point by suitable isolation valves at the process tapping point and also have isolation valves such as a directly mounted three way valve manifold for DP cells terminating the impulse pipe work to the transmitter.
Avoid mounting pressure transmitters in locations subject to vibration.
Ensure that the thermal operating limits of the pressure transmitter electronics are not exceeded and for the longest life and highest stability run the pressure transmitter well within its rated temperature limits.
Use impulse pipe work that is not lagged to provide better thermal transfer with the ambient temperature. This will help to either cool down or heat up the process media in direct contact with the pressure transmitter so that its operating temperature limits are not exceeded.
Ensure that the wetted parts of the pressure transmitter including ’O’ ring seals and gaskets are chemically and thermally compatible with the process media.
Any impulse piping between the process and the pressure transmitter should accurately transfer the pressure present at the process taps to the transmitter. Possible sources of error in this transfer are leaks, friction losses (especially in purged systems), trapped gas in a liquid line, or trapped liquid in a gas line causing a head error and temperature induced density variations between impulse line legs causing a head error in differential pressure applications.

The following recommendations are made to minimise pressure transmitter errors:

Make the impulse piping as short as possible, taking into account the temperature limitations of the pressure transmitter when used with very hot or very cold processes.
Piping should have a slope of at least 1 in 12 up towards the process connection for liquid and steam service and down towards the process connection for gas service.
Avoid high points in liquid lines and low points in gas lines.
On initial commissioning ensure all gas is vented from liquid impulse legs and all liquid is drained from gas impulse piping legs.
On steam service, fill the impulse lines with water to prevent damage to the pressure transmitter from live steam causing the thermal operating limits of the pressure transmitter to be exceeded.
For differential pressure applications keep both impulse pipe legs at the same temperature.
When a sealing fluid is used, ensure that the system is installed such that both piping legs for differential pressure applications may be filled to the same level and will stay at the same level under operational conditions.
Use impulse piping of sufficient diameter to avoid frictional losses and if purging is used, make the purge connection close to the process taps and for dp measurements purge through equal lengths of the same pipe size. Avoid purging through the pressure transmitter.

Reference documentation for pressure transmitter applications

Useful pages which you can refer to when specifying or using pressure transmitters.

Product Help

Dry air compatible 4 to 20mA pressure transmitter

We are looking for a 4-20mA transmitter for measuring the pressure of dry air, can you recommend a product which would be suitable?

We would suggest the DMP331 which is a 4-20mA pressure transmitter suitable for ranges from 0 to 1000mbar and above or the DMP343 which is a transmitter specifically for low range pressures which will cover ranges form 0 to 10 mbar upto 0 to 1000mbar.

Customs commodity coding

What is the customs tariff code for a pressure transmitter?

In the UK a pressure transmitter falls within the UK trade tariff commodity coding for section XVIII chapter 90 which includes:

Optical, photographic, cinematographic, measuring, checking, precision, medical or surgical instruments and apparatus; parts and accessories thereof.

The applicable heading for pressure transmitters is 9026 which includes:

Instruments and apparatus for measuring or checking the flow, level, pressure or other variables of liquids or gases (for example, flow meters, level gauges, manometers, heat meters), excluding instruments and apparatus of heading 9014, 9015, 9028 or 9032.

There is also an additional sub-heading code of 20 which includes:

For measuring or checking pressure.

This is followed by a further sub-heading of 20 which includes:

Electronic.

And finally 90 which includes:

Other

The final customs tariff code to use is 9026202090 for importing into the UK, and 90262020 if exporting from the UK.

Measure suction pressure

How do you use a dp transmitter to measure suction pressure?

Since differential pressure transmitters measure pressure between two points of high and low pressure, the suction pressure is simply the pressure difference measured on the low port compared to the high port. A standard differential pressure transmitter setup to measure a positive delta pressure and output 4-20mA can be easily converted to measure suction over the equivalent range in the reverse direction by swapping over the process connections to the high and low ports.

The DMD341 low range pressure transmitter has a selection of ranges from 6 to 1000 millibar which can be used to measure suction pressure.

Pressure switch vs transmitter

What is the difference between a pressure transmitter and a pressure switch?

Pressure transmitters provide a continuous amplified analogue output signal of typically 4-20mA which varies proportionally with changes in pressure.

Pressure switches have open/close voltage free switch contacts or an on/off fixed level signal which is triggered at a pre-configured pressure setpoint.

Operating Intrinsically safe pressure transmitter without an IS barrier

If you are installing an intrinsically safe pressure transmitter, can you wire it up without the intrinsically safe barrier and still get a correct 4-20mA value out of the transmitter?

Yes, if an IS pressure transmitter is installed in a safe area without an IS barrier, it will function in the same way as a non-IS approved pressure transmitter.

Span limit

What is the span limit of a pressure transmitter?

The span limit typically refers to the highest and lowest possible span configuration for a rangeable pressure transmitter.

A rangeable pressure transmitters will have a nominal pressure range and an adjustable output signal. The adjustment is normally limited to a ratio of the nominal pressure range.

For example a rangeable pressure transmitter with a range of 20 bar and span turn-down ratio of 10:1, would have an upper span limit of 0 to 20 bar and a lower span limit of 0 to 2 bar.

Pneumatic vs Electronic pressure transmitters

What is the difference between electronic pressure transmitter and pneumatic pressure transmitter?

Electronic pressure transmitters are designed to measure pressure and typically output a 4 to 20 milliamp current loop signal.

Pneumatic pressure transmitters are designed to measure pressure and typically output a 3 to 15 psi pressure signal which is generated by regulating an incoming air supply pressure of at least 15 psi. Pneumatic pressure transmitters are also called pneumatic, pressure-to-pressure, P-to-P or P2P transmitters.

Measuring suction with gauge pressure transmitter

How are gauge referenced pressure transmitters configured to measure suction pressure?

All types of suction pressure transmitters with a gauge reference will measure the amount of suction below atmospheric pressure. Typically the 4mA output is set to zero suction pressure (ambient air pressure) and 20mA output is set to max suction pressure. It is also possible to reverse the output on some devices to set 4mA to the maximum required suction pressure, if preferred.

What is a compound range pressure transmitter

We have a system specification which includes a pressure transmitter that has a compound pressure range, please could you explain what is meant by compound?

A compound pressure ranges is one that has an output signal which combines a negative with a positive pressure range.

e.g. If you wanted to measure an air suction pressure of 0 to -200 mbar g and an positive air pressure of 0 to 400 mbar g with a 4 to 20 mA, the appropriate compound pressure range would provide a measurement signal as follows: -200mbarg = 4mA, 0mbarg = 8mA and +600mbarg = 20mA.

Using ATEX pressure transmitter with an IS barrier

Does an ATEX approved intrinsically safe pressure transmitter need to be used with an IS barrier?

If you are going to use the sensor in a hazardous area you will also need to install a suitable IS barrier which is compatible with the required entity parameters for current, power consumption etc…, we would suggest the MTL 5500 Isolator or the MTL7700 Zener IS Barrier series for compatibility with most ATEX approved pressure transmitter types.

An ATEX approved intrinsic safety pressure transmitter does not need to be used with a IS barrier if installed in a safe (non-hazardous) area and should operate as normal without the barrier.