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 4.2 Pressure Gauges. Pressure gauges can be calibrated by comparing readings
with those of a test gauge of known accuracy. Laboratory test gauges are
Never use test
u s u a l l y accurate to 1/4 of 1 percent of the pressure range.
gauges as service gauges. A convenient pressure source for gauge testing is
an air supply of adequate pressure. I n s t a l l a n a i r f i l t e r a h e a d o f t h e
c o n n e c t i o n to the gauges to obtain clean air during the test. Often, a dead-
w e i g h t tester is used for calibration purposes. The deadweight tester is a
precision built hydraulic unit for obtaining basic pressure standards. It
consists of a manually operated oil pump assembly where a piston supports a
w e i g h t platform. A valved connection is made from the oil pump to the gauge
b e i n g tested. I n operation, calibrated deadweights are placed on the weight
p l a t f o r m and the pump is manually operated until the platform is freely
s u p p o r t e d by the piston. The oil pressure developed, which is shown on the
dial of the gauge being tested, must correspond with the pressure stamped on
t h e calibrated deadweights. To adjust the pointer, remove the bezel ring and
g l a s s plate, or the snap ring on plastic cases, and move the pointer by
t u r n i n g its micrometer adjustment until it is brought to the nearest possible
agreement over the desired range. The pointers of some type of gauges can be
adjusted from the back without removing the cover. R e f e r to paragraph 4.4 for
c a l i b r a t i o n of electronic pressure measurement instruments.
4 . 3 Flowmeters. Flowmeters are generally calibrated by means of a water
column. T h e purpose of the calibration is to make sure that the pressure
d i f f e r e n t i a l caused by the passage of the fluid through the primary element is
i n d i c a t e d correctly by the meter. D u r i n g the test the pressure differential
i s artificially produced by means of a water column, by placing on the high-
- p r e s s u r e side of the meter body a pressure equal to the differential which
should be produced by the primary element under actual flow conditions. The
d e t a i l s of the procedure vary for the different makes of flowmeters and the
m a n u f a c t u r e r ' s instructions should be closely followed. See paragraph 4.4 for
c a l i b r a t i o n of electronic flow measurement instruments.
4 . 4 Electronic Components. O p e r a t o r calibration of most electronic
c o m p o n e n t s consists of zeroing, span adjustments, resistor replacement, or
module replacement. Most electronic components associated with metering are
f a c t o r y calibrated and must be returned for calibration or exchange. Unless
s p e c i f i c calibration instructions are explained in the equipment manual and a
q u a l i f i e d electronics technician is available, contact the manufacturer before
attempting calibration.
6-24
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