In TTL mode (transistor-to-transistor logic), the instrument sends a +5 V signal
when the alarm is off and a 0 V signal when it is on. The alarm can be used to
activate a relay switch or send a digital signal, for example. In Open collector mode,
the alarm works like a switch. There is infinite resistance when the alarm is off and
low resistance when the alarm is on, allowing current to flow between the pins.
Alarms are configured under Settings > Alarm Thresholds. To configure the alarms:
1
Check Enable Alarms.
Select whether the alarm should be for CO
2
or H
2
O (with the LI-850). Alarms can
be set for one or the other, not both.
2
Configure the High Alarms.
l The ON settings is the high level at which the alarm is activated.
l The OFFsetting is the level at which the alarm is deactivated. The deadband is
the range between the ON and OFF settings.
3
Configure the Low Alarms.
l Again, the ON setting is the low level at which the alarm is activated.
l The OFF setting is level at which alarm is deactivated. The deadband is the range
between the ON and OFFsettings.
4
Click Apply when you are done.
Figure 2-4. In this example, the high alarm is used to activate a relay, which turns on a
fan.
Section 2. Configuring the gas analyzer
2-9Using alarms
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