Chemglass AF-0304 - Manual (Page 18)

4.5.2. Setting Zero And Span Using Manometer
16
This method requires a high vacuum system and a
precision calibration “standard” such as a capacitance
manometer. If these are not available, use the reference
tube technique described previously. The procedure
below lists ranges and settings for a 2000 mTorr instru-
ment as an example.
To set zero and span using a capacitance manometer
for the 2000 millitorr instrument requires a high vacuum
system capable of pressures less than 1 x 10
-4
Torr,
and a precision calibration “standard” such as a
capacitance manometer with a 1 Torr or 10 Torr head.
First, pump the tube to hard vacuum and hold it there for
about 20-30 minutes to thoroughly outgass the tube.
Adjust the zero adjustment until it reads 000 on the meter.
Second, raise the pressure to 900 mTorr (for instruments
of other ranges choose a pressure near mid-scale), and
hold that pressure constant. Adjust the span adjustment
pot to read the same pressure as the calibration stand-
ard. The instrument is now calibrated. If the span adjust-
ment was far off, repeat the procedure.
The span adjustment permits you to trim the instrument
for precise indication at a critical pressure, increasing
the absolute accuracy at that point, although possibly
sacrificing tracking at other points on the scale. The 900
mTorr point is best on a 2000 mT instrument for all-
around average tracking throughout the 1 to 2000 mTorr
range.
NOTE: If a reference tube or precision calibration
standard are not available, but you feel there is a
need to calibrate the instrument, then pump to hard
vacuum and set the zero only. Do not attempt to
make the reading agree with other typical analog
thermocouple or Pirani gauges upscale as this will
result only in degraded performance of this gauge.
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