Chemglass AF-0304 - Manual (Page 17)

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4.5.1 Offset & Span Adjustment Using Reference
Calibrator
Purchase a reference gauge tube calibrator from the
manufacturer or your supplier. The calibrator is marked
with the three proper settings for adjustment of zero,
span, and offset. A simple toggle switch selects the
“zero” and “span” and “offset” circuits so that you can
make the proper adjustments using the associated zero,
span and offset pot. All three adjustment pots are located
on the rear of both the indicator and controller instru-
ments. The pots are clearly marked.
Make the adjustments in the following sequence:
1) Adjust offset (A calibrator must be connected to the
gauge to make this adjustment.
2) Adjust zero
3) Adjust span
Please note that the “zero” adjustment is not truly a zero
setting, but a power adjustment at which a gauge tube
will read zero when pumped to a pressure below its
sensitive range. This is usually referred to as “hard
vacuum.” For the gauge tubes supplied with these
instruments, hard vacuum is any pressure one decade
below the lowest reading for that range, or lower.