Measuring Suction and Vacuum Pressure

Explanations for technical queries relating to measuring negative gauge and absolute pressures in the vacuum range.

Do you have a question about suction or vacuum pressure?

How is suction pressure measured?

Use a gauge reference pressure measuring device with a range scaled in the negative direction.   The zero to full scale range will depend on the maximum suction pressure expected.

How is vacuum measured?

A vacuum is any pressure that is below atmospheric pressure.  A vacuum can be measured using an absolute pressure sensing device.  Zero absolute pressure is a perfect vacuum, the higher the absolute pressure the lower the vacuum.

Can a differential range be used for measuring suction pressure?

Yes, there are two ways of using differential pressure sensing instruments (DP) for measuring suction pressure:

  1. Leave the negative side port open to atmosphere and connect the positive side to the suction pressure. The DP should be scaled and calibrated to read in the negative direction.
  2. Leave the positive side open to atmosphere and connect the negative side to the suction pressure so that the DP will measure the difference as if it were a positive pressure.

Is zero bar absolute the same as minus 1 bar gauge?

No, only when atmospheric pressure happens to correspond exactly to 1 bar absolute which rarely happens.

Is a minus 1 bar range adequate for measuring over the vacuum range?

For most applications yes, especially if measuring low suction pressures.  If measuring a very high vacuum to some degree of accuracy, it is best to measure with a positive absolute range, since a negative gauge device will give different readings due to the changes in atmospheric pressure.

What type of sensing technology is used for measuring suction pressure?

For suction pressures in the range of 0-10 mbar (~0.15 psi) up to
0-1000 mbar (~15psi) it is possible to use strain gauge diaphragm technology.  For ranges 0-0.1 mbar up to 0-10 mbar the sensitivity is too small and errors are too great to use a strain gauge diaphragm, so for very low pressures a more sensitive technology such as variable capacitance, inductive or LVDT is used.

What type of sensing technology is used for measuring in the vacuum range?

For vacuum ranges 0-100 mbar up to 0-1000 mbar absolute it is usual to use a lower cost strain gauge diaphragm type.  For ranges from 0-1 mbar up to 0-1000 mbar absolute, capacitive or inductive sensing techniques are the typical methods for measuring higher vacuums more precisely and with less drift.   For ultra high vacuum ranges below 0-1 mbar it is not possible to measure this low a pressure with electromechanical devices and it is necessary to use less direct methods such as thermal conductivity and ionisation techniques.

How do you calibrate the zero reading for an absolute pressure measuring device?

In most situations it is not possible to generate a high enough vacuum due to the limitations of the available vacuum pump, so instead the zero reading must be extrapolated.  To avoid introducing span errors the vacuum should be less than 10% of the vacuum range being calibrated.

What pressure units are used for measuring vacuum pressures?

What happens inside a pressure transducer when negative pressure is applied?

Before negative pressure is applied, there is an equal pressure on the front and rear side of the sensor diaphragm.  When the negative pressure is applied, the rear side pressure is now higher than the front side pressure.  This causes the sensing diaphragm to flex in the reverse direction resulting in a negative output signal.  The negative output signal is often converted to a standardised positive signal such as 4-20mA, 0-10V or 0-5V.

Will the vacuum level be the same at the opposite end to the suction end of a chamber where a vacuum pump is connected?

If there is flow of gas, then you should expect there to be some difference in pressure, but it will depend on flow rate, size of volume and any restrictions between pump and end of chamber. If the suction pressure is static with no flow, then the vacuum pressure will equalise it all points.

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