Introduction
Vacuum pressure measurement is common in chemical reactors, drying equipment, and pharmaceutical production systems.
However, pressure readings may become inaccurate or unstable when the instrument is not correctly matched to the vacuum process.
Two important factors should be checked first: the pressure reference and the diaphragm seal system.
1. Check the Pressure Reference
One common mistake is selecting a gauge pressure transmitter when absolute pressure measurement is required.
Gauge pressure is measured relative to atmospheric pressure, while absolute pressure is measured relative to a perfect vacuum.
For example, a vacuum process may be described using:
Absolute pressure
Gauge vacuum pressure
Compound pressure
These values are not interchangeable.
Before selecting the transmitter, confirm the pressure reference used by the process control system.
For applications requiring accurate measurement at low absolute pressure, an appropriate absolute pressure transmitter may be necessary.
2. Check the Diaphragm Seal Filling System
A remote diaphragm seal transfers process pressure through filling fluid inside the seal and capillary system.
Under vacuum conditions, the filling fluid must be carefully selected.
Important factors include:
Filling fluid vapor pressure
Minimum process pressure
Operating temperature
Capillary length
Transmitter mounting position
Ambient temperature changes
If the filling fluid is unsuitable for the vacuum conditions, measurement performance may deteriorate.
3. Installation Position Also Matters
The relative elevation between the process connection and transmitter can create additional hydrostatic effects in a filled capillary system.
Follow the seal manufacturer’s installation instructions and confirm the required mounting position.
Avoid unnecessary capillary bending, mechanical damage, and uneven exposure to temperature.
Conclusion
When reactor vacuum pressure readings are inaccurate, first verify the pressure reference and the diaphragm seal filling system.
Correct instrument selection and installation are essential for reliable vacuum measurement.
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