PureAire Monitoring Systems, Inc.
10/25/07
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7.2 Alarm Relays
The Alarm option provides 5-amp resistive form C relays for Alarm 1, Alarm 2,
and Alarm 3 conditions.
IMPORTANT: The relay contacts are rated for 5 amp resistive loads.
Appropriate surge suppressors should be installed across loads to prevent arcing
on the contacts. Arcing generates high levels of RFI that may interfere with
measurement signals.
7.2.1 Alarm 3 (Sensor Fault Alarm)
Alarm 3 is typically a fail-safe Fault relay and is activated by a sensor fault, such
as low electrolyte or a missing sensor. Since it is fail-safe, it also indicates loss of
power conditions. If a Fault alarm is not needed, this relay can be set to trip with
the Alarm 2 relay (see 7.2.3 below).
7.2.2 Alarm 1 and Alarm 2 (Concentration Alarms)
Alarms 1 and 2 are intended as low and high concentration alarms. They are
factory set at one and two times LEL respectively. Alarm 1 is non-latching;
Alarm 2 is latching.
Changing the Alarm Setpoint
The alarm setpoints for Alarms 1 and 2 are controlled by 16 position rotary DIP
switches. Both alarm setpoints incorporate approximately 1.5% hysteresis;
therefore, the signal must drop about 1.5% of the trip level to reset the alarm.
This prevents “chatter” when the input signal equals the trip level.
The trip points are various rotary switch positions are as follows:
Switch Position
Trip Level
(% full scale)
0
Alarm Inactive
1
5%
2
10%
3
15%
4
20%
5
25%
6
30%
7
35%
8
40%
9
45%
A
50%
B
55%
C
60%
D
65%
E
70%
F
75%
