Thursday, July 21, 2011

Zero Shift - Pressure Gauges

For pressure gauges, the most common indicating error that develops over time is a zero shift.  Zero shift can be due to a variety of causes.  Some merely cause a constant deviation which can be eliminated by resetting the pointer to  zero and then comparing the reading  with that of the reference gauge at full scale.  If the difference in readings is, in the user's judgment, within the required tolerance, re-calibration can be  completed by adjusting the pointer of the gauge under test to full scale and double checking that the pointer comes to rest within the zero band when the gauge is depressurized.

If there is a significant difference in readings, the span of the gauge under test has to be adjusted.  This is done by changing the leverage of the sector tail.  If the reading of the gauge under test is too high, the link nut assembly on the sector tail has to be moved away from the segment arbor.  If the reading is too low, it must be moved towards the segment arbor.  Note that this should be done in very small steps and repeated until the reading is satisfactory.  Make sure the pointer returns to zero.  Otherwise, re-set the zero and repeat the procedure.  In most cases, the link nut assembly can be moved (levered) without loosening the link screw.  Only rarely does the tension of the link screw have to eased very slightly.  When adjustment at full scale is completed, check the reading at mid-scale.  If it does not coincide with that of the reference gauge, loosen the dial screws slightly and tilt the dial until the pointer is at mid-scale.

No comments:

Post a Comment