Friday, July 22, 2011

Pressure Gauge Accuracy - Quick Gauge Accuracy Check

Before I start troubleshooting with gauges I check to see if the gauges that are installed  have been calibrated within the last year. I look for a sticker on the back of the gauge with a calibration date on it. If the plant does not calibrate its gauges of the particular gauge is out of calibration I do a quick accuracy check of the gauge. To do this I carry with me two adjustable wrenches, a couple of small (2") but high quality gauges to use as test gauges, and whatever PPE is appropriate for the environment you are working in and fluid you are working with.

Quick Gauge Accuracy Check
  1. Record the pressure shown on the gauge.
  2. Isolate the gauge by closing the valve the gauge is installed on.
  3. Remove the gauge from the valve by putting one wrench on the valve (to keep it from spinning) and one wrench on the gauge.
  4. Slowly remove the gauge, taking care to ensure that the isolation valve is holding and pressurized fluid will not be released.
  5. Once removed, verify the gauge that you removed does not have a zero shift.
  6. Install a test gauge in the same location as the gauge you just removed. Verify that the pressure read is the same as the pressure recorded in step 1.
  7. If the gauge being tested does not have a zero shift and read similar to the gauge reading of the test gauge then I consider the gauge accurate enough for troubleshooting.
If the gauge is found to be accurate I will use it for troubleshooting. If not I replace the gauge if I have a new one handy or continue my troubleshooting using my test gauges and put in  arequest to have the bad gauge replaced. If you must leave the bad gauge in place it is good to mark it as being inaccurate so that others know not to trust its readings.

The most common gauge error found is a zero shift. A gauge has a Zero Shift when it does not return to zero when all pressure is removed from it. This is caused by pressure overload, impact, or frequent pressure surges.

Another common gauge error is not attributed to the gauge, but to the piping the gauge is installed on. Often times the nipple, fittings or valve that that gauge is installed on get plugged up. The pressure shown on the gauge is the pressure on the gauge when the piping became plugged

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.