Reference Pressure

Reference Pressure is the pressure present on the reverse or negative side of a pressure sensing element.

For example the pressure reading will be zero when the total pressure measured on the positive side of the diaphragm equals the reference pressure.

Selecting reference pressure is one of the most important parameters to consider when specifying your requirements for a pressure sensing device.

It is often overlooked because the majority of pressure measurement applications have the same reference pressure and this has become the default selection if a pressure reference has not been specified.

The following sections explain what reference pressure is, why it is important and how to switch from one reference pressure type to another.

Contents

Compare Reference Pressure Types

Zero or Atmospheric Pressure?

When pressure is measured you are applying a force per unit area to a surface that is sensitive to changes in pressure. This sensing surface has two sides, one side is exposed to the measurement side, and the other side is exposed to no pressure…or is it?

“No pressure” suggests there is nothing measurable, but really it is an indication that the pressure on each side of the sensing surface is the same which results in a zero reading.

In a vast majority of applications the base pressure is atmospheric pressure, the air pressure created by the pull of earth’s gravity that surrounds everything.

If you have a pressurised vessel and you release the pressure inside to the outside air, the pressure will fall until it equalises with the external pressure. Therefore, although the sensing device would measure a zero reading, there is still a pressure present.

Absolute Reference Pressure

If you wanted to measure this remaining pressure you would need a device that had a sensing surface with true zero pressure on the non-measuring side. True zero pressure is a perfect vacuum, and although this is impossible to achieve, it is possible to get close enough to make adequate measurements of pressure, relative to a near perfect vacuum.

This type of measurement where the pressure on the non-measurement side is close to a perfect vacuum is called absolute pressure measurement.

Devices which measure absolute pressure are described as having an absolute reference.

An absolute reference is normally specified using “abs” or “a” as a suffix to the pressure range and pressure units (e.g. 800 – 1200 mbar a).

Examples of absolute reference pressure measurement are:

Barometric pressure for weather monitoring
Leak testing
Vacuum generation
Altitude/height measurement

Featured absolute reference pressure measurement products

300 mmHg absolute vacuum pressure transducer with 10 volts output - Absolute pressure transducer for precise vacuum measurement. 0-300 mmHg range, 0-10V output, high accuracy.
30 bar intrinsically safe pressure calibration kit - Intrinsically safe pressure calibration kit for accurate testing and verification of gauges and sensors up to 30 bar in hazardous areas. Includes ATEX approved digital test gauge, hand pump, and accessories.

Gauge Reference Pressure

Now if you wished to ignore the remaining pressure left after venting a pressurised vessel, you would use a device where the non-measurement side is open to atmospheric pressure.

This type of measurement where the sensing surface has atmospheric pressure on one side is called gauge pressure measurement.

Devices which measure gauge pressure are described as having a gauge reference.

Gauge (g) pressure is also known as vented gauge (vg), relative (rel) or sealed gauge (sg) pressure, and is normally specified on the end of a pressure range (e.g. 10 bar g).

Examples of gauge reference pressure measurement are:

Tyre pressure monitoring
Hydrostatic level measurement
Water supply pressure

Featured gauge reference pressure measurement products

-0.9 to +2.2 bar g Autoclave pressure transducer - This autoclave pressure sensor provides accurate pressure monitoring in medical, pharmaceutical, and industrial sterilization applications. Its robust design and integrated cooling element ensure reliable performance in demanding environments.
Portable logger for troubleshooting rooftop make-up air unit gas pressure variations - Troubleshoot make-up air unit performance with the DM01 gas pressure logger. Data logging capabilities provide valuable insights into pressure fluctuations.

Differential Pressure Range

So what happens when the pressure on the non-measurement side of the sensing surface is something other than a perfect vacuum or atmospheric pressure?

In this type of measurement, where the pressure is unknown on both sides of the sensing surface, it is called differential pressure measurement.

Differential pressure is often described using the acronyms DP, Diff or ΔP, and they will normally be included after the pressure range and units (e.g. 30 inH2O diff).

Examples of differential pressure measurement are:

Ventilation air flow
Filter monitoring
Air speed
Hydraulic control

Featured differential pressure measurement products

Negative room pressure monitor and logger for multiple zones - Monitoring system to measure negative room pressure inside each of five interior zones to eliminate the spread of dust.
DPS200 HVAC Differential Pressure Transmitter - Differential air pressure sensor with a 10 volt or a 4-20mA output for building ventilation applications. Ranges from 0 to 6 mbar up to 0 to 1000 mbar.

Changing Reference Pressure

When it comes to selecting a pressure range it is important to think about the type of pressure reference required and clearly state it in your list of specification requirements.

Neglecting to specify the pressure reference could lead to the wrong device being supplied, which could lead to significant measurement errors especially at low pressure ranges.

For example if a gauge reference device is used to measure an absolute pressure there will be an offset equivalent to atmospheric pressure with all the readings and vice versa.

Gauge ↔ Absolute Correction

Due to the different way gauge and absolute reference devices are manufactured they are not easily converted to another reference type, and therefore they will have to be returned to the manufacturer for modification or replacement. So neglecting to consider the type of reference pressure required for a pressure range could be costly to rectify.

Differential Adaptability

A differential device has one distinct advantage over gauge and absolute reference pressure devices, as a differential pressure sensing device has two ports for connecting two different sources of pressure.

Differential → Gauge Change

If you want to convert a differential device to gauge reference pressure, you simply leave the low pressure/negative side port open to atmosphere, so the pressure on the high pressure/positive side port can be measured relative to atmospheric pressure.

Differential → Absolute Change

If you want to convert a differential device to absolute reference pressure, it is a little more complicated than the switch to gauge, in that you have to find a way of creating a vacuum on the low pressure/negative side, and ensuring that it does not leak over time.