Conveniently Calibrate Plant Vacuum Gauges
Why do vacuum gauges need to be calibrated?
The largest uncertainty when using vacuum gauges on a
process plant 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 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.
Chell’s CalCube is a convenient solution to vacuum gauge calibration
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, so diagnosis is down
to a skilled operator recognising when a batch of product does
not conform.
This 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.
With the inherent lack of ruggedness of many vacuum gauges, 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.
The difficulties of calibrating vacuum in-situ
Unfortunately, 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. Not the least of concerns is the possible
cross-contamination of the standard by a contaminated sensor
under test. Another is the difficulty of access to many on-plant
gauges. So it has 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.
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.
The development of a technique
Having its own ISO IEC 17025:2005 accredited UKAS laboratory
with the lowest uncertainties for Vacuum in the UK, Chell are
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.
CalCube comparing two manufacturer’s Capacitance Manometers
CalCube 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. 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.
Fitting easily into almost any estate car, CalCube comes with
lifting handles to aid safe removal from a vehicle or lifting up
steps, etc. A removable side panel is foam lined with cut-outs
to hold all the common mating vacuum connector systems - of
course, custom connectors may be purchased and housed for
specific applications.
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 Chell’s
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 l evel. At the
atmospheric pressure end of the scale, the vacuum straight line
generated by the capacitance manometers is calibrated against
our 32” Schwien Merco-Master manometer giving us the lowest
uncertainties in that pressure regime.
Chell’s customers thus enjoy the accumulated vacuum calibration
skills of over 40 years and, should they have specific different
requirements, Chell has the skills to customise CalCube to suit.
For more information contact:-
CalCube showing its useful vacuum coupling storage
With the development of Chell’s on-site UKAS calibration service,
they have developed CalCube both as a portable vacuum
calibration system with which to offer an on-site service but,
for customers who would prefer their own facility, a convenient,
tried and tested system.
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Chell Instruments Ltd
Folgate House
Folgate Road
North Walsham
Norfolk
NR28 0AJ
Tel: 01692 500555
Fax: 01692 500088
e-mail: [email protected]
Website: www.chell.co.uk