Pressure and temperature transducers

Within the broader category of pressure transducers we can recognize, as well as other characteristics, the way of pressure measuring to which the sensitive element is exposed:

  • If measured it as the difference between the atmospheric pressure of the place where the sensor is housed and the other side of the sensitive element, they are called Relative
  • If measured it as an absolute value of pressure to which the sensitive element is exposed, they are called Absolute

Therefore, simplifying, the measurement of relative pressure sensors always refers to the atmospheric pressure present in a given place and at a given time, while absolute pressure sensors, being referred to absolute vacuum (0 bar), are independent of atmospheric pressure conditions.

The relative models (Nastec standard) are cheaper but need to have an air intake (already present at the factory) always open towards a reference environment.

They show zero pressure at normal atmospheric pressure (about 1 Bar absolute, at sea level), but always show a value of 0.0 even when exposed to pressures lower than atmospheric pressure.

The absolute models (Nastec code 2000090500) are more expensive but always give a reading of approximately 1 Bar at atmospheric pressure;

This is not a defect but their way of functioning.

In fact, these sensors are necessary when the measurement point can become depressed compared to the external environment (typically the case of the suction manifolds of pumps in closed circuits or with differential control)


The comparison of the values shown by the various models, in relation to a hypothetical pressure to be measured, can be seen in the following table:

                             Value expressed in Bar
Actual pressure compared to ambient* -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 …..
Relative sensor return value 0.0 0.0 0.0 0.5 1.0 1.5 2.0 ….. Full-scale
Returned value of the Absolute sensor 0.0 0.5 1.0 1.5 2.0 2.5 3.0 ….. Full-scale

* this refers to the pressure inside the pipes, as detectable by a mechanical pressure gauge with a measurement scale capable of reading even negative pressures (depression).

The suggestion is therefore to:

  • use relative sensors for normal pressurization uses with free surface suction (ex-posed to atmospheric pressure)
  • only use absolute sensors if the suction of the pumps can be carried out from a closed place/circuit with the possibility that this environment drops to a pres-sure lower than atmospheric pressure (closed, non-pressurised circulation cir-cuits, or with high pressure drops on suction)
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