Why is it called "absolute" pressure?
It refers to the fact that the reference is fixed at the lowest possible pressure, a perfect vacuum, so that the pressure reading is always compared to an unconditional and static reference.
How do you convert a gauge pressure to an absolute one?
If you know the local ambient air pressure, the absolute pressure can be determined by adding it to the gauge pressure, i.e. P (abs) = P (atm) + P (gauge).
Does a changes in ambient air pressure affect an absolute pressure reading?
It does not change the reference of an absolute pressure measuring device, but it may affect the pressure reading depending on whether the measured pressure is influenced by changes in ambient air pressure or not.
Can a differential pressure transmitter be converted to measure an absolute pressure?
Yes, it can be modified by evacuating the negative side pressure port and sealing the vacuum inside, although before attempting this the manufacturer should be consulted first to ascertain whether it is ok to apply a high vacuum to the device. Some devices maybe affected by high vacuums, for example those with an oil filled capsule on the negative side may drift due to outgassing of the oil. Also the vacuum maybe spoiled over time due to the internal material surfaces outgass particularly at higher temperatures. The absolute reference should be checked regularly to ensure a stable reference.
Can water depth be measured using an absolute pressure sensor if a vented reference is not possible?
Yes, however the depth of water will need to be compensated by obtaining a reading for local air pressure independently. Also the accuracy of depth measurement will be degraded, more so at low ranges due to the combination of errors from two measurements.
Are absolute pressure ranges used only for measuring vacuum?
No, absolute refers to a perfect vacuum reference and the range can be any value within or beyond the vacuum range.
Is a barometric pressure an absolute reading?
Yes, barometric pressure or atmospheric pressure is measured with respect to a perfect vacuum.
Why is an absolute pressure transducer more expensive than a gauge reference equivalent?
It's not always the case that there is a difference in selling price, but the manufacturing cost for producing an absolute pressure device is often higher because of the following reasons:
- The quantity produced compared to gauge reference devices are much less and therefore the costs of manufacture per device are higher.
- The design of the sensing element is more complex than a gauge reference version.
- Extra production procedures must be carried out to evacuate and seal the vacuum reference.
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