D-Industrial Technology Q200, User Manual, Page: 51 / 95

Q200 User Manual THE Q200 IN USE
10-2 200120/Revision 10
10.3
INSTRUMENT CALIBRATION
In order to obtain accurate and reproducible performance from the Q200, the manner in
which the instrument is calibrated is of the greatest importance. There are a few
general rules that should be borne in mind when performing instrument calibrations:
• When starting from cold, that is, the instrument has been switched off with the
argon flow turned off for some time, for example if the instrument has been left off
overnight, the instrument should be allowed to warm up thoroughly to allow the
operation of the instrument to stabilise. A period of not less than thirty minutes is a
suitable period, preferably with the pump on.
• Where the instrument is in continuous use, calibrations can be performed as
operational requirements or convenience dictates.
• The area in which the calibration is to be performed must have a low background
level of SF
6
in comparison with the level at which the instrument is to be calibrated.
The background level becomes increasingly important as the calibration level
decreases. There are two simple methods of checking the SF
6
background; the one
that is used will depend on individual circumstances:
The first method requires the instrument to be removed to an area where clean
ambient air can be guaranteed. The easiest way of fulfilling this requirement is to
take the instrument outside.
The second method is appropriate if the instrument is used in an essentially fixed
installation; a supply of clean air can be derived from a compressed air cylinder with
a regulator or from a compressed air supply. If a compressed air supply is used,
special care must be taken to ensure that the air supplied to the instrument is free
of both compressor oil mist and water vapour. The presence of either of these
contaminants in the clean air supply will have a deleterious effect on the
performance of the instrument. The air supply should be arranged to flow at a rate
of a few tens of cc/second into a shroud that is a loose fit over the inlet probe of the
instrument. The empty barrel of a 50cc plastic syringe makes a convenient, cheap
and disposable shroud.
Set the instrument to the sample mode with the inlet probe in the supply of clean
air. Allow the instrument to stabilise and zero the instrument. Return the instrument
to the area where the calibration is to be performed without setting the standby
mode or performing another zero. Any positive indication on the display will give a
measure of the SF
6
background either in leak rate or concentration units,
depending on configuration.
If the background reading is a significant proportion of the calibration level, say
10% or higher, measures should be taken to reduce the background, for example,
temporarily increasing the ventilation rate.
ƒ
When calibrations are performed using standard leaks, turn the standard leak on
and remove the blanking cap from the leak face prior to the commencement of the
calibration procedure in order to allow the flow from the leak to stabilise at the
calibrated value.