Setting Switching Points; Direct-Acting Controllers; Reverse-Acting Controllers - Emerson Fisher KC Series Instruction Manual

Instruction Manual
Form 5207
April 2007

Setting Switching Points

Direct-Acting Controllers

The controller output signal will switch from zero
pressure to full supply pressure when increasing
process pressure passes the upper switching point.
The controller output signal will not return to zero
pressure until decreasing process pressure passes
the lower switching point. When making adjustments
as described in the following steps, keep in mind
that:
D Changing the set point adjustment will move
both switching points equally in the direction of
adjustment.
D Changing the proportional band adjustment will
widen or narrow the differential gap between the two
switching points by moving the position of the lower
switching point.
Figure 5-3 shows the relationship between the
percent of sensor range between switching points
and the proportional band setting on the controller.
The following example illustrates how to use figure
5-3.
Example: The sensing element has a range of 30
psi. The lower switching point is to be set at 10 psi
and the upper switching point is to be set at 25 psi.
Proceed as follows:
D Divide the differential gap (the difference
between the upper and lower switching points) by
the sensing element range. Multiply the result by
100 as shown in the following equation.
Differential Gap
Sensing Element Range
D Locate the 50 percent line on figure 5-3. Move
along this line until you intersect the curve. Read the
proportional band setting on the left hand axis. For
this example, the setting is approximately 35
percent.
1. Using the curve in figure 5-3, determine the
correct proportional band setting for the desired gap
(expressed as a percent of the input span) between
the switching points.
2. Set the proportional band knob to the desired
setting determined in step 1.
4195KA, KB, KC, and KS Controllers
15 psi
x 100 = x 100 = 50
30 psi
% SENSOR RANGE BETWEEN SWITCHING POINTS
A2853−1 / IL
Figure 5-3. Curve for Determining Proportional Band Setting
3. Adjust the set point to the desired upper
switching point.
4. Increase the process pressure until the controller
output signal switches from zero pressure to full
supply pressure.
5. Decrease the process pressure to the desired
switching point at which the controller output signal
switches from full supply pressure to zero pressure.
6. Narrow or widen the proportional band slowly
until the output signal switches from full supply
pressure to zero pressure.
7. Repeat steps 4 through 6 until the controller
output switches at the desired points.
8. Observe the process pointer when the output
switches at the upper switching point. The process
pointer indication should be within "2 percent of the
set point indication.

Reverse-Acting Controllers

The controller output signal will switch from zero
pressure to full supply pressure when decreasing
process pressure passes the lower switching point.
The controller output signal will not return to zero
pressure until increasing process pressure passes
the upper switching point. When making
adjustments as described in the following steps,
keep in mind that:
D Changing the set point adjustment will move
both switching points equally in the direction of
adjustment.
D Changing the proportional band adjustment will
widen or narrow the differential gap between the two
switching points by moving the position of the lower
switching point.
SIDE
5-5