Ashcroft a4a, Installation And Maintenance Manual

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imized or eliminated through the use of commercially avail-
able pressure snubbers.
When the application for which the gauge is to be used
involves the sudden release of pressure, the “Slotted Link”
option should be specified at the time of order to protect the
movement assembly.
Temperature Error
Ashcroft A4A gauges are typically used where tempera-
tures are maintained within close limits. In cases where
extreme temperature variations will be encountered, correc-
tions may be calculated and applied to the gauge readings.
When temperatures rise the Bourdon tube reads faster and
when they drop it reads slower at a rate of one percent per
50ºF. In these cases, the automatic bimetallic thermal com-
pensation option is recommended for greater accuracy and
convenience. When equipped with the bimetallic compen-
sator, the gauge accuracy will be maintained through tem-
perature from -25ºF to +125ºF.
Loose Pointer
Gauge error may be caused by a loose pointer. If pointer
shift is suspected, simply release pressure and check that
the pointer returns to the zero position. If an adjustment is
necessary first position the zero adjust mechanism in the
middle of its full range of adjustability. Once this is accom-
plished, move the pointer to the proper position and, using
your fingers, press it firmly onto the shaft. The pointer
should never be hammered or tapped in place.
5.0 CALIBRATION
5.1 CALIBRATION STANDARDS
In order to properly check and adjust the calibration of a
precision gauge, a reliable standard of reference is
required. Care should be taken in the selection of a dead-
weight tester since the resulting operation of the precision
dial gauge can be no more accurate than the standard on
which it is based.
A primary pressure standard with accuracy of ±0.025 per-
cent of reading or better is recommended to properly check
or calibrate the Ashcroft A4A. Corrections for local gravity
effects, air buoyancy and temperature should be applied to
the pressure standard to compensate for environmental
conditions.
Note: If a secondary transfer standard is used, it should be
of the same range as the instrument being tested and have
accuracy of ±0.025 percent of span or better.
If you would prefer, NIST traceable recalibration of your
Ashcroft A4A Precision Pressure Gauge can be performed
at the Ashcroft Inc., Stratford, CT facility. Priority Recertifica-
tion can be arranged to assure the gauge is calibrated and
shipped for return to your facility within 5 days of receipt at
the Stratford Operation. Contact Ashcroft Inc. Customer and
Support Service at (203) 378-8281 for further details on
available calibration services.
5.2 CALIBRATION CHECK
Apply full pressure on the gauge for several minutes and
then release the pressure. Re-zero the gauge, if necessary.
Using an appropriate pressure standard generate pressures
and take readings at zero to full scale in 10% increments,
both upscale and downscale. If the error exceeds the
amount permissible for the intended service, follow the
recalibration procedure (Section 5.3).
5.3 RECALIBRATION PROCEDURE
Prior to beginning calibration on a fluid media the Bourdon
tube must be completely free of entrapped gas or, if calibra-
tion is to be done on a gas system, the Bourdon tube must
be free of any liquid.
5.3.1 Linearity Test
Step 1-
Load the gauge to full scale pressure and return to zero.
Step 2-
Adjust the pointer to zero with the dial centered. If neces-
sary, remove pointer then center the zero adjust mechanism
and replace pointer.
Step 3-
Load the gauge to one-half scale pressure and record the
reading.
Step 4-
Load the gauge to full scale pressure and record the read-
ing. If linearity is normal, the reading at full scale pressure
should be double the reading at one-half scale pressure so
as to produce a straight line relationship.
If the readings are not linear, recalibration procedure as fol-
lows is indicated:
5.3.2 Linearity Adjustment
Step 1-
First the gauge must be “straight lined”. That is to make the
readings linear or proportional to the load. At this point the
actual value of the readings at any given load is secondary.
Once linearity has been established, range adjustment to
bring the full scale reading into agreement with the full scale
load will complete the calibration.
Unscrew the zero adjust locking knob and remove the two
knobs and then remove the bezel and window by lifting the
bottom up and away from the gauge face.
Step 2-
Remove the back blow out cover by unscrewing the knurled
knob on the center back of the gauge and lifting away the
stainless steel blow out disc.
CAUTION:
Prior to proceeding it is recommended that the tooth rela-
tionship between the sector and its mating pinion be
marked (see figure 5.1). If the sector and pinion are acci-
dentally disengaged these marks will assist in proper re-
engagement. For optimum performance the original tooth
engagement should be maintained.
Step 3-
To compensate for a full scale reading greater than twice
the one-half scale reading:
Consult Figures 5.1 & 5.2
Loosen the two clamping screws on the quadrant slide and
slide it upward, thereby decreasing the angle. Or, if the error
is slight, rotate the pointer and pinion shaft counter clock-
wise to indicate about twice the extent of the error. Holding
the pointer in this position, release the quadrant clamp
screw permitting the Bourdon tube to adjust its position,
then re-clamp. This also results in a decrease in angle.
To compensate for a full scale reading less than twice the
one-half scale reading use the same procedure outlined in
step 3 except as follows:
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