BT CGA-411 Operation Manual page 38

E. Adjustment of the Block Limit of the Pump
If a "bloc" signal is not displayed when the air intake to the instrument is blocked, the
problem may be corrected by adjusting the block limit used to detect this condition.
Before any adjustments are attempted, determine that the absence of a block signal is not
due to a leak in either the probe, the hose, or some connection along the way. Remove the hose
and block the air intake directly to make certain the air intake is actually blocked.
1. Observe Pump Current in Normal Operation and When Blocked
If no mechanical reason for the absence of a "bloc" signal is found, determine the
behavior of the pump by entering the set up mode described at the beginning of this Section 12
(press switches #1 and #2 simultaneously while switching the front panel switch from % Oxygen
to AUTO ZERO) and note the reading that appears on the display following "Pu" (see Table 5).
The reading represents in arbitrary units the current supplied to the pump; it should be between
900 and 1200 with the hose and filter attached. If the end of the probe is blocked, a reading of
about 320.0 to 350.0 is normally displayed. The decimal point appears if the pump current
exceeds the block limit (2500).
If a number greater than 250.0 is not displayed when the air intake is blocked and if the
number is above 2000, adjust the block limit, as described below, to some lower value, for
example, 2000. The block limit should not be set to a value less than one and a half times (1.5x)
the value observed when the probe is not blocked. Otherwise, a "bloc" signal may appear in
normal operation, even when the probe is not blocked. If this margin between normal operation
and a "bloc" condition cannot be maintained, the pump must be mechanically re-adjusted.
2. Adjust Block Limit
To adjust the block limit, place the front panel switch to % Oxygen and then back to
AUTO ZERO and press switch #2. The instrument will be in the alarm setting and option
selection mode (Section 11). Press switch #2 until "Pu" appears on the display followed by
2500. Press switch #1 to show "On" on the display, then press switch #2 to show 2500 with the
underlined digit flashing. Press switch #2 again to advance the flashing digit to the second
position. Now press switch #1 to advance this digit to 0 and then press switch #2 three more
times until Pu appears followed by 2000. Exit by either pressing switch #2 again or by placing
the front panel switch at some position other than AUTO ZERO.
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