How To Use Trip Points as Error Indicators Cha
ter Four: O
eration
40
How To Use Trip Points as Error Indicators
You can use the trip points to indicate when the error signal deviates from a given range. The
error is defined as the difference between the actual pressure reading and the set point.
For example, assume you have a 100 Torr unit and your set point is 50 Torr. You want the trip
points to illuminate when the error is more than ±5% of the set point value, which indicates that
the pressure reading has deviated by more than ±2.5 Torr. This allows the pressure to vary from
47.5 to 52.5 Torr.
The 649 controller is initially configured with TP A on
above
the trip point and TP B on
below
the trip point. If you have not changed the action of either trip point, you may follow the steps
below. If you have changed the action of the trip points, you need to reset them back to the
initial configuration for this example. Refer to
How To Adjust the Trip Point Values,
page 37,
for instructions.
1. Calculate the trip point voltage that corresponds to each trip point value:
Trip Point Pressure (Torr)
Full Scale Pressure (Torr)
x TP Adjustment Range (V) = Trip Point Voltage (V)
where the full scale pressure is 100 Torr and the TP adjustment range is 5 Volts.
TP A:
52.5 Torr
100 Torr
5 V 2.625 Volts
×=
TP B:
47.5 Torr
100 Torr
5 V Volts
×=
2 375.
2. Measure the value of TP A by inserting a positive test probe into the test jack labeled
“TP A” and the ground probe into the test jack labeled “Gnd.”
The ground connection and the test jacks are located inside the unit on the Transducer
board, as shown Figure 14, page 39. A 0 to 5 V signal corresponds to a 0 to 100% full
scale pressure.
3. Use a small screwdriver to adjust the pot for TP A, located on the top of the unit, to set
TP A to 2.625 Volts.
Refer to Figure 6, page 21, for the location of the trip point adjustments.
4. Measure the value of TP B by inserting a positive test probe into the test jack labeled
“TP B” and the ground probe into the test jack labeled “Gnd.”
5. Use a small screwdriver to adjust the pot for TP B, to set TP B to 2.375 Volts.
The trip points will be off when the pressure reading is between 47.5 and 52.5 Torr. Should the
pressure deviate from this range the appropriate trip point will turn on and its LED will
illuminate. Trip Point A will turn on when the pressure exceeds 52.5 Torr and Trip Point B will
turn on when the pressure falls below 47.5 Torr.
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