Ashcroft a4a Руководство по установке и обслуживанию - Страница 13
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If the gauge is adjusted considerably or if movement parts
are replaced, it will probably be impossible to re-use the
existing dial, and a new dial will have to be plotted as part of
the calibration.
2-Counter-Weighted Pointer
With this method, we begin by producing a "linear" dial. (A
dial upon which the graduations are evenly spaced). Next,
we take a "balanced" pointer and remove the half moon at
the end of the tail. By removing the half moon, the pointer is
no longer balanced because the tail end is now lighter than
the indicating (knife edge) end. This will cause the heavy
pointer tip to "sag" or read "slow" in the first half of the scale
and read "fast" in the second half of the scale, actually over-
compensating for the original non linearity condition.
To achieve the required effect, which is to flatten out the S
curve, a counterweight is added to the tail end of the pointer
to add back some of the weight that was lost when the half
moon was removed.
By adding a counterweight, also called a "poise" onto the
tail-end of the pointer gravity can be used to force the tail of
the pointer "downward", which conversely deflects the tip of
the pointer "upward". In the first half of the scale, the coun-
terweight will cause the gauge to read faster. In the second
half of the scale, the counterweight will cause the gauge to
read slower. Since the gauge was reading slow in the first
half of the scale, the faster readings caused by the counter-
weight will compensate for the non-linearity. Conversely, the
second half of the scale which was reading fast, will be
slowed by the counterweight.
In fact, because of the gravitational effect is decreased as
the pointer approaches a vertical position (due to the
decreased moment of inertia), the counterweight will cause
no effect on the pointer at mid-scale, which is always plotted
in the 12 o'clock position on the dial. This is ideal, as no
compensation is necessary at mid-scale. The counterweight
will cause the greatest effect at ? scale (9:00) and ? scale
(3:00), which is where the most compensation is required.
Properly set, the amount of compensation caused by the
counterweight will exactly offset the amount of inherent non-
linearity as shown by the "S" curve.
Notes:
1-It is important to note that the counterweight can only be
effective when the gauge is used in the vertical or near ver-
tical operating position. Gauges used in the horizontal posi-
tion will always require non-linear dials.
2-Gauges with 6" dials do not have pointers with half moon
tails. Instead, the tail end of the pointer is wider than the
indicating end. The counterbalance offset is accomplished
by trimming the tail end of the pointer until the desired coun-
terbalance effect is achieved.
The following general rules can be used to help determine
whether a gauge has a linear or non-linear dial.
Ashcroft A4A Gauges with Non-Linear Dials
-All vacuum and compound gauges
-All absolute gauges 75 psi or less
-All gauges calibrated in the horizontal position
-Some dual scale gauges
-All gauges originally equipped with "crescent moon"
pointer tail tips
Ashcroft A4A Gauges with Linear Dials
-All that are other than above
-All gauges originally equipped with counter weighted pointers
Note: When performing linearization via pointer trimming it
is important that the trimming be done carefully so as not to
remove too much material which would result in the need to
discard the pointer and start the process over.
7.0
SPECIFICATIONS, RANGES & DIMENSIONS
7.1
STANDARD SPECIFICATIONS
Unless otherwise specified all gauges are calibrated for ver-
tical mounting. If installation is other than vertical, the order
must specify mounting positions to assure proper accuracy.
A certified copy of test results, traceable to the National
Institute of Standards and Technology, is furnished with
each gauge. Standard conditions for calibration conform to
NIST Monograph 8.
Accuracy
±0.1% of span
Sensitivity
±0.01% of span
Repeatability
±0.02% of full scale
Hysteresis
Not greater than ±0.1% of span after
application of maximum scale pressure
Pressure Types
Gauge, compound, absolute* and vacuum
Pressure Ranges
From 0-15 psi through 0-100,000 psi
Bourdon Tube
Material
Bourdon tubes are available in the
following materials:
STANDARD
Beryllium Copper for ranges from 0-15
through 0-40 psi AISI 403 stainless steel
standard for range 0-50 psi through
0-100,000 psi
OPTIONAL
Beryllium Copper for ranges from 0-50
through 0-10,000 psi. AISI 403 stainless
steel for ranges 0-15 psi through 0-40 psi
Standard Process
Connections
1
⁄
NPT female back for ranges to 10,000
4
psi. 9/16-18 UNF-2B female back connec-
tions for ? inch high pressure tubing for
ranges over 10,000 psi
Optional Process
Connections
⁄
NPT female, bottom
1
4
1
⁄
NPT male, back or bottom
8
1
⁄
NPT male, back or bottom
4
1
⁄
NPT female, back or bottom
8
9
⁄
-18 UNF-2B female, bottom or back or
1 6
bottom for ranges below 10,000 psi
MS 33656-4 male, back or bottom
Mil-G-18997D male, back or bottom
MS 33649-4 female, back or bottom
MS 16142 female, back or bottom
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