Sensotec PIM-3, Manual, Page: 78 / 97

12-4 Understanding Limits
Example 1. When these two rules and the above def-
initions are applied, consider a pair of limits set so that
they will indicate if a signal is too high or too low. As an
example, let us say that a signal is ranged from 0 to
10000 pounds, and we wish a limit tripped if the signal
gets outside the range of 2000 to 5000 pounds. A high
limit will trip if the signal exceeds the setpoint, so the
upper limit (5000 pounds) should be a high limit. A low
limit will trip if the signal falls below the setpoint, so a low
limit will work for the lower limit (2000 pounds). Suppose
that it is decided that noise does not exceed 1% of the
full-scale value, or 0.01 x 10000 = 100 pounds. Since rule
2 must apply, the high limit hysteresis is a -100 pounds,
and the low limit hysteresis is +100 pounds. Either limit
may be designated as either value, since the sign of the
hysteresis will tell the PIM-3 which type of limit we want.
Arbitrarily, the high limit will be designated Limit 1, and the
low limit as Limit 2. The setup will then be:
Limit 1 setpoint = 5000
Limit 1 hysteresis = -100
Limit 2 setpoint = 2000
Limit 2 Hysteresis = 100
Figure 12-2 shows these concepts graphically.