Type 4000
8
ZERO
ADJUSTMENT
LOWER ARM
Figure 10. Positioner Span and Zero Adjustment Locations
SPAN
ADJUSTMENT
SPRING GUIDE
LOCKING SCREW
A7198 / IL
ZERO ADJUSTMENT
COVER
Zero and Span Adjustment
Refer to figure 10.
1. Remove the positioner front cover.
2. Apply supply pressure to the positioner.
3. Set the input signal to the 0% value [0%=3 psig
(0.2 bar)] and wait for the unit to reach steady state.
4. Rotate the zero adjustment until the actuator stem
is at its starting position. Zero is adjusted by turning
the silver thumb wheel located on the lower arm or
externally with a screwdriver through the zero adjust-
ment opening.
5. Slowly increase the input signal until the input
reaches the 100% value [100%=15 psig (1.0 bar)].
Observe the actuator travel and valve position as the
input increases. Note the actuator travel at 100% input
signal.
6. Set the input signal to the 0% value.
7. If necessary, loosen the locking screw and adjust
the span as follows (span is adjusted with the brass
colored thumb wheel located on the feedback spring):
Figure 11. Principle of Operation
1
2
3
4
5
6
7
8
A7199 / IL
If the actuator travel recorded in step 5 is short of
the desired travel at 100% input signal, rotate the span
adjustment down to increase span. Tighten the locking
screw.
If the travel recorded in step 5 reaches the de-
sired travel before the input signal reaches the 100%
value, rotate the span adjustment up to decrease
span. Tighten the locking screw.
After the span adjustment, recheck to be sure the end
of the feedback spring is not touching the spring guide.
8. After a span adjustment, recheck the zero and ad-
just if necessary.
9. Repeat steps 4 through 8 as necessary to achieve
correct actuator travel.
10. Replace the positioner front cover.
11. If you used the zero adjustment opening, remem-
ber to install the zero adjustment cover to ensure the
unit is sealed.
Principle of Operation
Refer to figure 11.
The pneumatic positioner operates on a force balance
principal. Force is originated by the signal pressure
transmitted through a diaphragm on to the balance
arm. The opposing force is achieved through the feed-
back spring and is proportional to the position of the
lower arm. The lower arm position is determined by
the position of the cam which is secured to the spindle
