Ashcroft 1082 Manuale di installazione, funzionamento e manutenzione - Pagina 9
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Calibration chart – After recording all of the
readings it is necessary to calculate the errors
associated with each test point using the fol-
lowing formula: ERROR in percent = 100
times (TRUE VALUE minus READING) ÷
RANGE. Plotting the individual errors
(Figure 1 on page 10) makes it possible to
visualize the total gauge characteristic. The
plot should contain all four curves: upscale –
before rap; upscale – after rap; downscale –
before rap; downscale – after rap. Rap means
lightly tapping the gauge before reading to
remove friction as described in ASME B40.1.
Referring to Figure 1 on page 10, several
classes of error may be seen:
Zero – An error which is approximately equal
over the entire scale. This error can be mani-
fested when either the gauge is dropped or
overpressured and the bourdon tube takes a
permanent set. This error may often be cor-
rected by simply repositioning the pointer.
Except for test gauges, it is recommended
that the pointer be set at midscale pressure to
"split" the errors.
Span – A span error exists when the error at
full scale pressure is different from the error
at zero pressure. This error is often propor-
tional to the applied pressure. Most Ashcroft
gauges are equipped with an internal, adjust-
ing mechanism with which the user can cor-
rect any span errors which have developed in
service.
Linearity – 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 on page 10 is not
linear. The Ashcroft Duragauge
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
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 differ-
ence in readings before and after lightly tap-
ping the gauge case at a check point. Possible
causes of friction are burrs or foreign materi-
al in the movement gearing, "bound" linkages
between the movement and the bourdon tube,
or an improperly tensioned hairspring. If cor-
recting these potential causes of friction does
not eliminate excessive friction error, the
movement should be replaced.
(Continued on page 10)
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