How Pressure is measured

Pressure sensors work on a differential basis by measuring the difference between the applied pressure and a reference pressure. The reference can either be absolute vacuum pressure or atmospheric (ambient air) pressure. The sensor is therefore said to measure either 'absolute' or 'gauge' pressure.

Atmospheric pressure varies constantly from place to place, moment to moment depending on ambient weather conditions. The standard atmospheric pressure is approximately equal to typical air pressure at sea level and standardised to 101.325 kPa [or 0 psi(sg), 14.7 psi(a), 1.01 bar].

Absolute Pressure

An absolute pressure sensor measures pressure relative to absolute vacuum.
A sensor held out in normal air at sea level will show around 101 kPa [14.7 psi(a), 1.01 bar].

For example, manifold pressure (MAP) sensors are absolute pressure sensors, as these sensors are required to measure pressures both below (vacuum) and above atmospheric pressure.

Gauge Pressure

Pressure can be measured relative to atmospheric pressure. This is known as gauge pressure and in technical work, should be written as psi(g).

For example the air pressure in a tire of a car, which might be said to be '30 psi', is actually 30 psi above atmospheric pressure and should be more accurately referred to as 30 psi(g).

Sealed Gauge

Sometimes the rear of a sensor is sealed to prevent contamination and improve the repeatability of readings.

The zero point of the sensor is set to standard atmospheric pressure. This may not be exactly the same pressure as where you are on any given day, resulting in offset readings. Changes in temperature and atmospheric pressure will not effect the reading from sealed gauge sensors, it is an accurate measurement of actual gauge pressure above standard atmospheric. 

Sealed gauge measurements are labelled as psi(s) or psi(sg).

Pressure Sensors

MoTeC supplies a wide variety of quality pressure sensors, all of which have been tested and calibrated to suit MoTeC systems. These sensors come in a variety of pressure ranges, and can be set up within the Dash Logger or ECU to show many units, including psi(g), psi(a), kPa(g), kPa(a) and bar. Some sensors are available to suit specific applications such as Nitro methane compatible sensors.

MoTeC supplies the following types: