Ashcroft 4480 - Pneumatic Transmitter - Manual (Page 12)

12
General
Calibration of the transmitter Is very similar to calibration of a
gauge. A coarse zero adjustment corresponds to the setting of
the pointer on the shaft of a gauge. A fine zero-adjustment cor-
responds to the micrometer adjustment on the pointer of a
gauge. A span adjustment corresponds to the slide on a gauge
movement. No linearity adjustment is required, since the trans-
mitter is inherently linear In Its action.
Procedure
To calibrate span indicating pressure transmitters, perform the
procedures outlined below. To calibrate non-indicating pressure
transmitters, perform the procedures outlined below but omit-
ting steps g through i.
a. With pressure transmitter mounted in final position, con-
nect a variable source of process pressure to input sys-
tem assembly. An accurate test gauge, deadweight
tester, or manometer must be provided in this line to
establish the required known input pressures.
b. Connect a source of supply air to left-hand connection
marked “supply.” Use a 20 psig air supply if output pres-
sure range is to be 3 to 15 psig. Use a 30-psig air sup-
ply if output pressure range is to be 3 to 27 psig.
c. Connect an accurate receiver gauge to air connection
marked “output.”
d. Loosen two captive screws (31), Fig. 8) and remove
cover (36), removing ring/case gasket (52) from ring (50)
(on indicating pressure transmitters).
e. With supply air turned off, move span adjusting disc (10)
on nut assembly (9) to approximate center of adjustment
range.
f. Turn adjusting screw on zero adjusting assembly (3)
until bottom edge of vane is approximately flush with top
surface of nozzle plate of flexure assembly (13). Check
to be certain that vane is centered between nozzles. If
not, bend vane slightly to center it.
g. Remove ring (50), pointer (47), and dial (44).
h. Calibrate indicating portion of transmitter to required
accuracy (normally ASA Grade AA accuracy). This cali-
bration is identical to Ashcroft Duragauge calibration.
i. Install dial (44), position pointer (47), and install ring
(50).
j. Turn on supply pressure and turn adjusting screw on
zero adjusting assembly (3) until receiver gauge indi-
cates 3 psig.
k. Apply process pressure and adjust pressure for full
scale indication.
l. Check to be certain that tubing (22c) does not touch any
stationary part within 1
1 ⁄
2
inches of nozzle.
m. Check that vane does not touch either nozzle at any
time.
n. Move span adjusting disc (10) on nut assembly (9)
toward bellows if receiver gauge indicates less than full
scale indication desired (15 or 27 psig). Move range
adjusting disc away from bellows if receiver gauge indi-
cates above full scale indication desired.
o. Remove process pressure and turn adjusting screw on
zero adjusting assembly (3), if necessary, to obtain a 3-
psig indication on receiver gauge.
p. Apply full scale process pressure and check receiver
gauge indication. If necessary, repeat step n.
q. Repeat steps 0 and p until zero and full scale indications
are correct.
r. Decrease process pressure to 50 percent of full scale
and check that receiver gauge indication is correct within
plus or minus
1 ⁄
2
percent of full scale.
s. Reduce process pressure to zero. Then increase
process pressure to 50 percent of full scale and check
that receiver gauge indication is correct within
1 ⁄
2
percent
of full scale.
CALIBRATION
I&M008-10061-1-01 250-1969-C_I&M008-10061-1/01 250-1969-C 12/20/13 9:09 AM Page 12