Chapter Four: Operation
Type 623H End Cap
37
Type 623H End Cap
Figure 8 shows the endcap of a Type 623H instrument.
Figure 8: Type 623H Endcap
The endcap provides easy access for zero adjustment, electrical connections to terminal blocks
TB1 and TB2, and trip point adjustment screws. The LEDs illuminate to indicate when the
transducer is powered, and to indicate the status of the pressure relative to each of the trip points.
A green LED indicates the pressure is
below
the alarm trip point, and a red LED indicates the
pressure is
above
the alarm trip point.
Hysteresis of Type 623H Trip Relays
Hysteresis is built into the operation of the two trip points to help compensate for the noise
inherent in all systems. Without hysteresis, the noise may cause the relays to repeatedly switch
states, a condition known as “relay chatter.” Each trip point has a separate hysteresis setting.
Setting the hysteresis too high creates a
deadband
around the trip point. The deadband prevents
the trip point relay from responding to changes in the pressure signal around the trip point.
Ideally, the hysteresis should be close to, but not less than, the peak-to-peak noise to provide
maximum immunity from relay chatter while providing the best possible accuracy. It may take
some trial and error efforts to determine the best hysteresis setting for your system.
Figure 9, page 38, shows a system in which the relay is energized when the pressure falls
below
the trip point setting.
Trip Point B
Indicator LED
Trip Point A
Adjustment Screw
Trip Point B
Adjustment Screw
Zero
Potentiometer
Adj
Zero
TYPE 624
8
Pressure Transducer
5-Pin Terminal
Block (TB1)
8-Pin Terminal
Block (TB2)
Trip Point A
Indicator LED
TB1
TB2
1
5
1
C
h
a
s
s
is
G<<R
G<<R
Adj
Trip pt. B
Trip pt. A
