Differential Pressure Transmitters

Differential pressure transmitters for measuring the DP of fluids and gases across particle filters and along a length of pipe to monitor flow.

Find a pressure transmitter for measuring differential pressures with a 4-20mA current loop output signal. Measure air or liquid dp to determine flow in a pipe, negative or positive pressure between rooms or the health of a filter.

Differential pressure transmitters with a 4 to 20mA current loop output signal for measuring pressure drops of liquids or gases.

Contents

Product Help

Liquid & Steam service installation

The transmitter should be installed below the process tapping points to a wall or other rigid mounting using the optional bracket assembly or similar rigid bracket. The transmitter should be piped up in compliance with the pressure transmitter installation guidelines with the process tappings taken to the side of the pipe work as illustrated in the adjacent diagram. The transmitter should be mounted within two degrees of the horizontal, small variations in mounting attitude will affect the transmitter zero point, however, this may be calibrated out during the initial commissioning procedure detailed later. When used on steam service the transmitter and impulse lines must be filled with water before system start up to prevent live steam damaging the transmitter.

Gas service installation

The transmitter should be installed above the process tapping points to a wall or other rigid mounting using the optional bracket assembly or similar rigid bracket. The transmitter should piped up in compliance with the pressure transmitter installation guidelines with the process tappings taken to the top of the pipe work as illustrated in the adjacent diagram. The transmitter is placed above the process measurement tapping to allow condensed liquid to drain back into the process lines. The transmitter should be mounted within two degrees of the horizontal, small variations in mounting attitude will affect the transmitter zero point, however, this may be calibrated out during the initial commissioning procedure detailed later.

Specifying differential pressure transmitter requirements

Some questions to ask when defining the product requirements for a differential pressure transmitter.

Differential pressure range

What is the highest differential pressure you need to measure?

Are the high and low pressure side always the same way round, or could they switch around? If the high and low pressure side is likely to reverse back and forth, then you will need to specify a bidirectional or compound range to cover the changing direction of pressure.

Do you need to change the dp range for a different process or location? Some differential pressure transmitters allow the range to be adjusted by the user. Rangeable dp transmitters are adjusted manually by internal screws or buttons which are accessed by removing a waterproof lid or cap. Intelligent dp transmitters are adjusted via a in-built user interface, digital interface and associated software, or a special user hand terminal.

Static line pressure

What could be the highest pressure measured at either of the two process connections?

This is the general system pressure and is often significantly higher than the differential pressure measurement range. The static line pressure rating of dp transmitters will vary enormously from one type to another, so it is important to pay special attention to this parameter, since it could lead to mechanical failure if an inadequate rating is selected.

Is there a risk that full static line pressure could be applied to one side of the differential pressure transmitter? The line pressure rating does not necessarily mean that this pressure can be applied to one-side only. Most manufacturers will make it clear on their product data sheets whether the maximum static line pressure can only be applied on both sides simultaneously, or is protected from it being applied on one side only.

Overpressure

Is there a chance that the normal operating differential pressure range could be exceeded by a pressure surge or a process problem?

Maximum overpressure is the amount of differential pressure that the transmitter can withstand beyond 100% of the dp range, without pushing the performance outside of specification limits.

The overpressure rating is typically expressed as multiples of the dp range, e.g. 2x, 4x.

Bidirectional or compound differential pressure ranges, which are a combination of a positive and negative pressure range component, should have two overpressure ratings.

If the dp transmitter will only allow full static line pressure to be applied to both sides at the same time, then take care to check the negative overpressure rating is adequate. Depending on the differential sensing technology utilised, the negative overpressure rating maybe significantly lower than the positive overpressure rating.

Where a differential pressure transmitter is protected against the full static line pressure being applied to one side only, then the positive and negative overpressure rating will equal the max static line pressure rating.

Checklist for differential pressure transmitter requirements

Define your differential pressure transmitter requirements using this checklist:

Differential pressure range?
Static line pressure?
Overpressure?
Media type?
Media temperature range?
Environmental conditions?
Signal output? 4-20mA
Power supply?
Measurement accuracy?
Electrical connection?
Process connection?
Certification?