Ashcroft a4a Panduan Instalasi dan Pemeliharaan - Halaman 12
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Consult Figures 5.1 & 5.2
Move the quadrant slide outward, thereby increasing the
angle, or rotate the pointer and pinion shaft clockwise to
indicate about twice the amount 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 an increase in angle.
Step 4-
Reset the pointer to zero and repeat the linearity check to
evaluate the adjustment. Repeat procedure until linearity is
established within the desired tolerance.
5.3.3
Range Test
Step 1-
Load the gauge to full scale pressure by dead weight and
note the reading on the gauge. If the full scale reading is not
correct within the required tolerance proceed as follows:
Step 2-
If the pointer indicates greater pressure than the dead
weight, turn the Micro-Slide adjusting screw counter-clock-
wise. If the pointer indicates less pressure than the dead
weight, turn the Micro-Slide adjusting screw clockwise.
Should the range adjustment be excessive, loosen the two
Micro-Slide clamping screws and move the slide inward to
increase the range and outward to decrease the range.
Adjustment of the Micro-Slide screw changes the amount of
pointer travel between zero and full scale. It is necessary
after each adjustment to return the pressure to zero, re-
adjust the pointer to zero and repeat to determine the effec-
tiveness of the adjustment.
Adjustment of the Micro-Slide screw requires a movement
of approximately 180 degrees in order to have a pointer
show a change of ? of one percent of full scale reading on
most gauge dials. To correct some gauges an adjustment of
as little as 0.0005" or a turn of approximately 10º of the
Micro-Slide screw is required. The adjusting screw may be
operated with a small Allen wrench.
5.3.4
Final Adjustment
The range and linearity adjustments are interdependent.
With linearity established, fine readjustment may be made
in range and linearity re-checked to bring both adjustments
within the desired limits of accuracy.
5.3.5
Check For Hysteresis
Step 1-
Connect the gauge to the deadweight tester in a vertical
operating position.
Step 2-
If the gauge has been drained, fully or partially, air may be
entrapped in the Bourdon tube and this trapped air must be
removed through the bleeder valve (flushing connector) to
insure positive, accurate response.
If the gauge has been used with and contains mercury, care
should be taken to avoid contamination of brass tubing and
fittings on the tester.
Gauges used in pneumatic systems should, of course, be
cleaned of oil, water or similar entrapped matter before testing.
Step 3-
Set the pointer to zero using the dial adjustment.
Step 4-
Applying pressure slowly, load the gauge to one-half scale
pressure and record the reading.
Step 5-
Load to full scale pressure.
Step 6-
Release to one-half scale pressure and compare the down-
scale reading to the upscale reading.
If the second reading is higher than the first it may be
caused by either hysteresis or friction. With dead weight still
at one-half load, tap the gauge gently. If the pointer returns
to substantially the original one-half load reading, the differ-
ence in readings was caused by friction. If the amount of tap
is not excessive it might be considered satisfactory for use.
It is normal procedure to tap an instrument to observe its
sensitivity. If the tap is excessive it is usually remedied by
cleaning the movement mechanism with solvent such as
ether, carbon tetrachloride, etc.
If after tapping the gauge the pointer still reads higher than
the first reading, the cause is hysteresis. Increased hystere-
sis effect may be brought about by crystallization of the
Bourdon tube due to excessive cycling. A similar condition
may result from exposure to excessively high pressure
causing a partial fracture of the tube. In cases where the
Bourdon tube has been found to have unacceptable levels
of hysteresis a new tube is required. It is recommended that
gauges requiring a new Bourdon tube be returned to the
factory for repair.
Note: If a new Bourdon tube is required for a gauge that has
the optional bimetallic thermal compensation a new thermal
compensation will be required.
6.0
POINTER REPLACEMENT
6.1
GUIDE TO POINTER REPLACEMENT
A "perfect" gauge can be defined as a gauge which yields a
reading which is exactly equal to the actual pressure which
is being applied. Plotted on a graph, these points can be
connected to form a straight line, indicating perfect "lineari-
ty". If a point was plotted that did not fall along this line, it
can be said that this is a "non-linear" reading. Most pressure
instruments exhibit some inherent non-linearity characteris-
tics, due mainly to a disproportionate movement of the
sensing element (Bourdon tube) under different pressures.
Prior to correction, an Ashcroft A4A precision pressure
gauge will generally, in the first half of the scale, indicate a
higher pressure reading than the actual applied pressure
(called "running fast"), and will be the fastest at ? scale. The
gauge will usually read perfectly at midpoint, run "slow" in
the second half of the scale (slowest at ? scale) and read
perfectly at full scale.
In order to maintain high accuracy, some form of compen-
sation must be employed to overcome the additional inaccu-
racy caused by non-linearity. There are two methods of
compensation for non-linearity in Ashcroft A4A gauges:
1-Non-Linear Dials
These gauges are manufactured with balanced pointers
and dials on which the graduations have been plotted to
coincide with the position of the pointer at specific pressure
points. The placement of the graduations is determined by
the use of readings recorded during the preliminary calibra-
tion of each gauge. These readings will then be pro-
grammed into a computer linked to a photo-plotter, which
will produce a negative image of the dial. A positive dial is
then produced from this negative.
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