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What does negative and positive gauge pressure mean

You may not be aware but nearly every pressure measurement device you are using has a positive or negative gauge reference. It is so ubiquitous, that a gauge reference is often assumed and considered the default reference, neither mentioned in specification requirements or on manufacturers data sheets.

Although fundamentally pressure is defined and measured relative to a perfect vacuum in most scientific textbooks, it is impractical in the majority of industrial applications to measure pressure relative to an environment that does not exist naturally anywhere on terra firma. Instead most industrial equipment is located in an environment surrounded by air. To create movement of components on a machine, or to transfer a gas or fluid from one part of a process to another, a pressure has to be applied which is either greater or less than the surrounding ambient air pressure.

Local air pressure is the natural base pressure for all pressurised systems, before they are sealed and isolated from other components. This base pressure has no defined pressure value, and since local air pressure varies constantly, it is necessary to provide a way to track the gauge reference changes, to ensure a pressure reading is measured relative to the base pressure at all times.

The easiest and most practical way to continuously track a gauge reference is to provide a way to vent the reverse side of a pressure sensing device to the surrounding ambient air pressure. This will ensure that any change in the atmospheric pressure during measurement is compensated instantaneously.

Positive gauge pressure is any pressure that is measured above the current atmospheric pressure.

Negative gauge pressure is any pressure that is measured which is below the current atmospheric pressure.

Positive gauge pressure measurement products
  • 5000 psi data logging gauge for measuring hydraulic pressures
  • Measuring 125 mm of water level range to 1mm accuracy
  • 60 barg range RS485 Modbus RTU seawater pressure sensor for flow control use
  • Saltwater spray resistant 100 barg range 1-10V output ceramic pressure sensor
What does negative and positive gauge pressure mean
Negative gauge pressure measurement products
  • -20 to +50 psig 0-10V out compound range liquid dispensing pressure sensor
  • Vacuum Logger for sewer system with flush diaphragm & intrinsic safety
  • 10 psi vacuum air and 50 psi water pressure gauge and data logger
  • Vacuum suction sensor & wall mount digital readout with switched contacts

If positive or negative is not mentioned when specifying a gauge reference pressure instrument, it is safe to assume that it is a positive gauge range, since the majority of applications require a positive pressure range. A negative gauge pressure range is normally identified in requirements by the way the pressure range is described, such as minus 2 psi or negative 1 bar gauge range.

You will also see something like 10 psi vacuum ranges or vac for short, but this can lead to confusion, since a vacuum range could be defined as an absolute or a gauge reference range, both requiring different types of measurement instrument. If a g, gauge, negative or minus is mentioned as part of the vacuum range description then it is reasonable to assume it is a negative gauge range and not an absolute one.

Gauge pressure measurement products
  • Depth measurement of surface water flowing to sea
  • 30 mbar pressure logger for monitoring natural gas pressures
  • 16 bar air compressor pressure transducer with 4 to 20 milliamp output
  • Borehole water level transducer to determine recharge rates

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