Calibrating vacuum gauges

Vacuum gauges used on process plants will need to be calibrated regularly, but carrying out this process in-situ is often very difficult. To help, Chell Instruments has developed the portable CalCube vacuum system which enables convenient on-site calibration.

When using vacuum gauges on a process plant, the largest uncertainty is knowing when the process has caused drift or shifts of calibration. Many processes handle vapours, and if any condensation or deposition happens the chances are it will also happen in the vacuum gauge. Many such gauges – Pirani, cold cathose and ion gauges – use thermal or ionisation cross-section measurements to infer the level of vacuum being experienced, and they have filaments, cathodes and other hardware, all of which may be coated with by-products from the process.

Even more rugged gauges – such as capacitance manometers – will be affected by depositions or particulates. This is because any deposition will have mass and the gauge will weigh it, especially if it has a low range and therefore a very thin diaphragm.

As processes move away from their design parameter due to gauge ‘drift’, little about the quality of the process can be inferred from the instrumentation readings. This means diagnosis is down to a skilled operator recognising when a batch of product does not conform – which is too late!

If the process is sensitive to the level of vacuum being achieved, then the cost of losing a batch could be astronomical – many times the cost of maintaining the plant properly in many cases.

Many vacuum gauges lack ruggedness, so it is imperative that plant gauges are regularly calibrated and that the operator has spare, calibrated gauges, ready to substitute as soon as he fears the process may have drifted away from specification.

A portable system

It is very difficult to carry out in-situ vacuum calibrations due to the amount of hardware needed to establish a suitable level of vacuum in a situation where a calibration standard gauge may be compared. Concerns include the possible cross-contamination of the standard by a contaminated sensor under test; and the difficulty accessing many on-plant gauges. It has therefore become tradition for gauges to be removed regularly from the plant and shipped to an external calibration laboratory for re-calibration and, in many cases, adjustment to compensate for the effects of contamination.

Chell has an on-site UKAS calibration service and has developed CalCube, which is both a portable vacuum calibration system with which to offer an on-site service and, for customers who would prefer their own facility, a convenient, tried and tested system.

CalCube complies with BS ISO3567:2011 which specifically covers the design and use of vacuum calibration systems and incorporates an ISO/Dis stainless steel calibration chamber with geometry optimised to ensure that all gauges connected see the same level of vacuum so that comparisons may be made.

On-site calibration

Having its own ISO IEC 17025:2005 accredited UKAS laboratory, Chell is well placed to prepare CalCube for on-site gauge calibrations. The Transfer Standards fitted to CalCube are calibrated in the laboratory both before and after an on-site calibration job so that the confidence level of the calibration is the highest possible.

Over 40 year’s experience of calibrating vacuum gauges have gone into CalCube’s design, which includes such capabilities as calibrating both rising and falling pressures very accurately using a pair of ultra-fine Chell CMF valves. It is constructed within a rugged, wheeled flight-case type enclosure having just a 13A mains plug yet containing dry diaphragm backing and turbomolecular pumps with high vacuum isolation valves.

Fitting easily into almost any estate car, CalCube comes with lifting handles and a removable side panel which is foam lined with cut-outs to hold all the common mating vacuum connector systems. Custom connectors can, of course, be purchased and housed for specific applications.

The choice of vacuum Transfer Standards will be an important decision, which is aided by Chell’s years of experience. These standards, when not fitted to CalCube, are housed in another, matching flight-case, ready for convenient return to its calibration lab for scheduled UKAS calibration.

Chell’s in-house vacuum standards consist of three high-accuracy capacitance manometers having 20 years of continuous calibration history at national Standard level, and these are monitored on a daily basis by a Spinning Rotor Gauge whose ball-flange assembly is also calibrated annually at National Standard level. At the atmospheric pressure end of the scale, the vacuum straight line generated by the capacitance manometers is calibrated against a 32” Schwien Merco-Master manometer, giving the lowest uncertainties in that pressure regime.

Should customers have specific different requirements, however, Chell has the ability to customise CalCube to suit.

Chell Instruments

 

 

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