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Section 3
Model 288A Differential Pressure Indicating Switch
3. To test for reverse travel, connect the pressure source to the LP housing
and vent the HP housing. Apply pressures approximately 150% of the DP
range. The pointer should move approximately 5% to 10% below zero.
4. To test for overtravel, connect the pressure source to the HP housing and
vent the LP housing. Apply pressures approximately 150% of DP range.
The pointer should move approximately 5% to 10% above full scale.
5. Apply 0, 50, and 100% of full scale pressure. If indication is within
specifi ed limits, no adjustments are necessary. If indication is not within
specifi ed limits, perform a complete calibration (see Indicator Calibra-
tion, page 17).
6. Make sure the instrument zero indication is correct; otherwise, repeat
step 2.
7. Verify the switch set points (refer to Changing Switch Set Point, page 22).
Pointer Installation and Removal
During adjustment and calibration of the unit, it may be necessary to remove
and reinstall the pointer according to the following procedures.
Pointer Installation
1. Position the pointer on the movement shaft with the pointer set at zero
scale. Obtain 75% to 90% pointer hub engagement on the movement
pinion shaft. The pinion shaft must not extend through the pointer hub.
It may be necessary to enlarge the hub hole, using a tapered broach (in-
cluded in the calibration toolkit, Part No. 9A-0288-1032B).
2. Lightly tap the pointer hub with a hand-set or other fl at-end tool. Use
perpendicular blows to avoid bending the shaft.
3. Check the calibration of the indicating switch over its entire range (refer
to Indicator Calibration, page 17). If the indicating switch is correctly cali-
brated, secure the pointer to the movement shaft by tapping the hub with
a hand-set or other fl at-end tool.
4. Test the pointer for tightness by moving the pointer from the zero to the
50% position manually, and then letting the pointer return freely to zero.
If the pointer indicates a shift, tap the pointer hub to tighten it to the
shaft.
Pointer Removal
The pointer is removed with a pointer puller shown in Figure 3.3, page 17. This
tool is included in the calibration toolkit, Part No. 9A-0288-1032B.
To remove the pointer, perform the following steps:
1. Slide the pointer puller along the pointer until the pin protruding from the
tip of the screw in the pointer puller is directly over the movement shaft
and the arms of the pointer puller are directly under the pointer.
Summary of Contents for 288A
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