User Manual

dropped or overpressured and the Bourdon tube takes a
permanent set. This error may often be corrected by simply
repositioning the pointer. Except for test gauges, it is recom-
mended that the pointer be set at midscale pressure to “split”
the errors.
Span – A span error exists when the error at full scale pres-
sure is different from the error at zero pressure. This error is
often proportional to the applied pressure. Most Ashcroft
gauges are equipped with an internal, adjusting mechanism
with which the user can correct any span errors which have
developed in service.
L
inearity – A gauge that has been properly spanned can
still be out of specification at intermediate points if the
response of the gauge as seen in Figure 1 (Typical
C
alibration Chart) is not linear. The Ashcroft Duragauge
®
pressure gauge is equipped with a rotary movement feature
which permits the user to minimize this class of error. Other
Ashcroft gauge designs (e.g., 1009 Duralife
®
) require that the
dial be moved left or right prior to tightening the dial screws.
Hysteresis – Some Bourdon tubes have a material property
known as hysteresis. This material characteristic results in
differences between the upscale and downscale curves. This
class of error can not be eliminated by adjusting the gauge
movement or dial position.
Friction – This error is defined as the difference in readings
before and after lightly tapping the gauge case at a check
point. Possible causes of friction are burrs or foreign material
in the movement gearing, “bound” linkages between the
movement and the bourdon tube, or an improperly tensioned
hairspring. If correcting these potential causes of friction
does not eliminate excessive friction error, the movement
should be replaced.
6.1 Calibration – Rotary Movement Gauges and Type
1259 Gauges – Inspect gauge for accuracy. Many times
gauges are simply “off zero” and a simple pointer adjustment
using the micrometer pointer is adequate. If inspection shows
the gauge warrants recalibration to correct span and/or linearity
errors, proceed as follows:
a. Remove ring, window and, if solid front case, the rear
closure assembly.
b. Pressurize the gauge once to full scale and back to zero.
c. Refer to Figure 2 (Ashcroft System Assembly w/Rotary
Gear Movement) for a view of a typical Ashcroft rotary
system assembly with component parts identified. Refer to
Figure 2A for link configuration of Type 1259 gauge.
d. Adjust the micrometer pointer so that it rests at the true
zero position. For open front gauges the pointer and dial
must also be disassembled and the pointer should then
be lightly pressed onto the pinion at the 9:00 o’clock
position.
e. Apply full scale pressure and note the magnitude of the
span error. With open front gauges, ideal span (270
degrees) will exist when at full scale pressure the pointer
rests exactly at the 6:00 o’clock position.
f. If the span has shifted significantly (span error greater
than 10%), the gauge should be replaced because there
may be some partial corrosion inside the Bourdon tube
which could lead to ultimate failure. If the span error
exceeds 0.25%, loosen the lower link screw and move
the lower end of the link toward the movement to
increase span or away to decrease span. An adjustment
of 0.004 inch will change the span by approximately 1%.
This is a repetitive procedure which often requires more
than one adjustment of the link position and the subsequent
rechecking of the errors at zero and full scale pressure.
g. Apply midscale pressure and note error in reading. Even
though the gauge is accurate at zero and full scale, it
may be inaccurate at the midpoint. This is called linearity
error. For corrections to linearity with the Type 1259
gauges refer to Figure 2B. For rotary movement gauges,
note the following: if the error is positive, the movement
should be rotated counter clockwise. Rotating the move-
ment one degree will change this error by approximately
0.25%. Rotating the movement often affects span and it
should be subsequently rechecked and readjusted if nec-
essary according to step 6.1e and 6.1f.
h. While recalibrating the gauge, the friction error – differ-
ence in readings taken with and without rap – should be
noted. This error should not exceed the basic accuracy
7
BOURDON TUBE
SEGMENT
TIP
LINK
BACKPLATE
SOCKET
ROTARY MOVEMENT
PINION
HAIRSPRING
TIP
LINK
BOURDEN TUBE
MOVEMENT
HAIRSPRING
BACKPLATE
SOCKET
FIG. 2 ASHCROFT SYSTEM ASSEMBLY W/ROTARY GEAR MOVEMENT
FIG. 2B LINEARITY ADJUSTMENT & LINK CONFIGURATION FOR
ASHCROFT TYPE 1259
FIG. 2A ASHCROFT TYPE 1259 SYSTEM ASSEMBLY
To increase pointer indication
at mid-scale, use a pliers to
open the link.
To decrease pointer indication
at mid-scale, use a pliers to
close the link.