Alarm Action; Latching Or Non-Latching Hi And Lo Alarms - Lakeshore 818 User Manual

3.5.1 Checking the Alarm Setpoints
To check the HI alarm setpoint, press the front panel HI ALARM key. The HI alarm setpoint displays in the
chosen temperature units. If the units are V (volts), the units indicator changes to K (kelvin) and the setpoint
displays in kelvin. Similarly, to check the LO alarm, press the LO ALARM key.
3.5.2

Alarm Action

The Model 818 and 819 have two different alarms that can be selected when the sensor curve is selected.
Alarm action 0 is the Model 818 operation and alarm action 1 is the DIGI-K operation. See Table 3-2A and
3-2B for a truth tables of the respective alarm actions.
Display Reading Compared to:
LO Alarm
Higher
Higher
Lower
Lower
Power OFF
Display Reading Compared to:
LO Alarm
Higher
Higher
Lower
Lower
Power OFF
3.5.3

Latching or Non-Latching HI and LO Alarms

When the rear panel ALARM LATCH DIP switch is OFF (0), relays close or open as specified in Table 3-2.
If the sample sensor temperature oscillates around the HI or LO alarm setpoint temperature, then the relay
opens and closes in an oscillatory fashion following the alarm action. There is a deadband around the setpoint
as specified in Paragraph 3.5 to eliminate chattering of relay contacts.
When the ALARM LATCH switch is ON (1), the alarm relays take on a latching configuration. When the
sample sensor temperature forces the alarm to go from NO to NC, the relays remain closed. There are three
ways to reopen the relay.
1. From the Front Panel: hold the HI ALARM and LO ALARM keys down for more than 2 seconds. The
ALARM STATUS light(s) go out indicating alarms are reopened.
2. From the Rear Panel: disable the MARK LATCH switch (Configuration DIP Switch 2) momentarily.
3. Send the R (RESET) command over the RS232C interface.
Operation
Lake Shore Model 818 / 819 Cryopump Monitor User's Manual
Table 3-2A. Alarm Action 0 (Model 818)
HI Alarm NO - COM
Higher Open
Lower Open
Higher Closed
Lower Closed
Open
Table 3-2B. Alarm Action 1 (DIGI-K)
HI Alarm NO - COM
Higher Closed
Lower Closed
Higher Open
Lower Open
Open
Contact Positions
LO Alarm
COM - NC NO - COM
Closed
Closed
Open
Open
Closed
Contact Positions
LO Alarm
COM - NC NO - COM
Open
Open
Closed
Closed
Closed
HI Alarm
COM - NC
Closed Open
Open Closed
Closed Open
Open Closed
Open Closed
HI Alarm
COM - NC
Closed Open
Open Closed
Closed Open
Open Closed
Open Closed
3-3