Portable Pressure Calibrator

A pressure calibrator is the reference in a comparison, not the instrument under test. It generates or measures a pressure of known value so that another instrument can be checked against it, and the whole exercise depends on the calibrator being more accurate than the device being checked by a margin the quality system accepts. The instrument on this page does that in the field, with an integral hand pump, a 24 VDC loop supply and a battery stated at 15 hours per charge.

Pressure and current are shown together on one display, which is what makes it useful for a two-wire transmitter: the pump applies the pressure while the calibrator supplies the loop and reads the current the transmitter returns, so a verification point and its corresponding output can be read off at the same time rather than in two passes.

How a pressure calibrator is used

The calibrator’s own sensor is the reference, and its accuracy is traced back through successive comparisons to a national standard. The document states that this instrument was verified to JJG875-2005 with a conforming result at 0.05 grade. What the certificate records is the state of the sensor at the time of comparison, which is why the calibrator’s uncertainty is an input into the uncertainty of every check made with it, and why the interval between recalibrations matters as much as the accuracy figure.

Pressure is generated by the hand pump and trimmed to the point being checked. The pump on this instrument covers -90 kPa to 1.6 MPa and reaches a 95% vacuum, and a volumetric trim lets the pressure be brought to a figure without overshooting it. On the five pneumatic ranges the reading resolves to 10 Pa, which is what allows a fine verification point to be set rather than approached.

The electrical side runs in parallel. The rear panel provides 24 VDC to the device under test and terminals for its current, so a transmitter can be powered and read from the same instrument that is applying the pressure. The display shows pressure, current, and the current as a percentage of the 4-20 mA span.

How to select the right calibrator

The pressure range comes first, and it has to cover the device under test without working at the bottom of its own span, where the relative uncertainty is largest. The pneumatic ranges run from a 250 kPa span to 4 MPa, the compound range crosses zero to -95 kPa, and the hydraulic ranges extend to 60 MPa. A device checked across its own range needs a calibrator whose range brackets it, not one that merely reaches its maximum.

Accuracy comes second, and how it is expressed matters as much as its size. This instrument carries a 0.05 grade, with pressure accuracy stated as 0.1%FS and 0.05%FS, so the figure that applies to a specific range has to be established before a ratio against a device under test can be calculated. Where the device under test is specified at 0.5%FS there is room; where it is specified at 0.1%FS the choice of range and the accuracy of that range decide whether the verification means anything.

Then the generation method. Pneumatic generation suits lower pressures and clean dry gas. Hydraulic generation, using pure water in the supplied cup, is used above 4 MPa because a compressed gas at those pressures stores too much energy to release safely, and it costs resolution: 1 kPa rather than 10 Pa.

Working with the hand pump and the hydraulic ranges

Two details separate a calibrator that is straightforward to use from one that is not. The first is the positive and negative switching valve, which is fitted to the small ranges and not to the larger instruments, which have a single output port. Compound work therefore needs the smaller instrument, and the valve has to be in the right position before pressure is applied rather than after. The second is that this is a static system: the pressure falls fairly quickly once it is at full scale, so the documented approach is to pressurise past the point until the over-range alarm holds, wait two to three minutes for the reading to settle, and then trim slowly, adjusting up and down a few times.

Both are also reasons to respect the medium. The sensor diaphragm is thin, so solid particles must not be allowed into the pressure output port, and liquid must not be drawn into the pneumatic system on a vacuum duty, since either can end the sensor rather than the measurement.

Measurement limitations and selection considerations

The overload capability is 1.2 to 1.5 times the range upper limit, with an audible and visual over-range alarm, and the alarm is there to be obeyed rather than tolerated: past the limit the sensor can be permanently damaged. Working pressure also needs the same care in the other direction, since a calibrator used well below its own full scale reports a larger relative uncertainty at the point being checked.

The environment is limited to 80%RH or less, and the operating temperature range is not stated in the material, so a hot or cold site is a question for the enquiry. Where the calibration is performed in a laboratory with an external pressure source, a bench instrument with an interchangeable reference module may cover more ranges than a hand-pumped portable, and where the requirement is a record rather than a check, the documentation has to be established before the instrument is chosen.

The instrument is also not a pressure source for a pressure test. Holding a pipe run at a test pressure for hours and generating a verification point are different duties with different needs, and a test pump is the instrument for the first.

Ordering

A quotation needs the range and whether pneumatic or hydraulic operation is required, the connection on the device under test, the electrical arrangement of that device if its current is to be read, and whether a calibration certificate is needed and to what accuracy. If the instrument is to be verified against a named regulation, state which one, since verification is against a regulation rather than against an accuracy figure in the abstract.