MKS 649B - How to Use Trip Points as Error Indicators

How To Use Trip Points as Error Indicators Chapter Four: Operation
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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 [13.33 kPa] unit and your set point is 50 Torr [6.666
kPa]. 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 [0.333 kPa].
This allows the pressure to vary from 47.5 to 52.5 Torr [6.333 to 6.999 kPa].
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 (Tor
r
)
Full Scale Pressure (Torr)
x TP Adjustment Range (V) = Trip Point Voltage (V)
where the full scale pressure is 13.33 kPa and the TP adjustment range is 5 Volts.
TP A:
6.999 kPa
13.33 kPa
5 V 2.625 Volts×=
TP B:
6.333 kPa
13.33 kPa
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 to 52.5 Torr [6.333 to 6.999
kPa]. 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 [6.999 kPa]
and Trip Point B will turn on when the pressure falls below 47.5 Torr [6.333 kPa].