BT CGA-411 Operation Manual page 17

Coarse Zero
To adjust the zero of a sensor which has drifted too far, remove the instrument from its
carryall, and remove the elongated rubber plug on the right side of the instrument (as you face it)
to access two push-button switches. Place the instrument on a flat surface and set the selector
switch at the % oxygen display. Hold down push-button #1 (see Figure 1) with a pencil, pen, or
other small implement while switching the selector switch to AUTO ZERO. Release button
when zeroing begins. The instrument will proceed to zero all sensors.
Please note that the round rubber plugs to the right of the elongated plug need not be
removed. Please be sure to replace the elongated plug before re-inserting the instrument into the
carryall.
WARNING: The coarse zero procedure overrides the limits for autozeroing.
procedure must be carried out in clean air.
Automatic Sensor Check
If a gas sensor fails (opens up), the display shows "FAIL" (see Troubleshooting Section).
Radio Frequency Interference (RFI)
Gas-Sentry detectors have an interior coating on their cases to suppress RFI.
Fig. 1.
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