**A guide to differential pressure measurement including explanations, applications and choice of products for measuring pressure with an differential pressure.**

Differential pressure is the difference in pressure between two separate points.

A differential pressure can be measured between two points on independent systems or between two different points on the same system.

## Products

Differential pressure sensing devices for measuring the pressure difference between two points across a filter to monitor its condition, or across a restriction in a pipe to measure the flow rate.

- Low Differential Pressure Transmitters - Low differential pressure transmitters for measuring small pressure differences between two measurement points on across ventilation fans, air filters or clean room walls.
- 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.
- Low Range DP Voltage Output Transducers - Low pressure range dp transducers for measuring draft pressures in ventilation systems or air extraction systems.
- High Accuracy DP Sensors - High accuracy differential pressure sensors for measuring with a high degree of accuracy and resolution to obtain precise dp readings.
- Low Range DP Sensors - Low range differential pressure sensors for measuring very small pressure differences.
- Rangeable DP Sensors - Rangeable differential pressure sensors with output signals that can be adjusted by the user to elevate or suppress the zero position and turn down the full range span.
- Intrinsically Safe DP Sensors - Intrinsically safe differential pressure sensors with EEx certification for use in potentially explosive atmospheres with an intrinsically safe zone barrier.
- Air DP Sensors - Air differential pressure sensors for monitoring and controlling air pressures across filters, through ventilation control vanes, in laminar air flow cabinets and between clean room partition walls.
- Digital Interface DP Sensors - Differential pressure sensors with digital interfaces for RS485, RS232 or USB communications with a computer.
- Differential Pressure Voltage Output Transducers - Differential pressure transducers with amplified volts output for measuring the dp of liquids and gases.
- Wet / Wet Differential Pressure Sensors - Wet/Wet differential pressure transducers and transmitters which are compatible with liquids on both positive and negative side process connections.

## Applications

The main use for a differential pressure sensor is to measure the difference in fluid or gas pressure across a restriction in a pipe. The flow can then be determined by converting the differential pressure reading with bernoulli’s equation. Since flow is proportional to the square root of the differential pressure in a closed pipe it is sometimes preferred to use a square root output signal from the differential pressure sensor to simplify the conversion to a flow measurement.

Other applications for measuring differential pressure are as follows:

Hydrostatic level measurement of a tank’s content, where the gas at the top of the tank is not vented. The difference in pressure between the bottom and the top is measured to determine the true hydrostatic pressure.

Leakage monitoring by measuring the difference in pressure between a controlled reference pressure and the component under test pressurised to the same pressure.

Airspeed using a Pitot tube which provides two air channels, one for measuring the total air pressure facing the airflow and the other for the static pressure measured perpendicular to the air-flow. The difference between the two channels provides the air velocity pressure. This method is used on aircraft to measure air speed and in wind tunnels to simulate air velocity for testing the aerodynamics of an object.

## Questions & Answers

### Differential vs Gauge

*When do you need to measure differential pressure instead of gauge pressure?*

You would you use a differential when you need to measure pressure between 2 connection points, and when the reference pressure (negative side) is not the same as atmospheric pressure.

### Which side of DP transmitter to connect to vacuum

*Which side of the impulse line connections for a dp transmitter is the blower vacuum or suction side?*

If the output of the dp transmitter is scaled for a positive pressure range, e.g. 0 to +1 bar = 4 to 20 mA, then you can measure 0 to -1 bar suction but connecting the vacuum to the negative side, and leaving the positive side vented to atmosphere.

## Related Guides

- What does the suffix a, abs, d, dp and diff, g, rel and sg mean after the pressure units in a pressure range?
- Measuring the difference in air pressure between rooms
- Measuring liquid level in a sealed tank with a hydrostatic pressure sensor
- Differential Pressure Sensors
- How do you measure flow rate with a dp cell
- Measuring negative pressure using a positive differential pressure range