The Model 421-A Temperature Alarm is a microprocessor-based dual-setpoint temperature alarm suitable for environmental and industrial applications. It can also be configured as a frost alarm. It sounds an alarm and controls the operation of two electrical circuits whenever the temperature reaches either of two independently set limits. The temperature range is -40° to 160° F (-40° to 71° C) and the alarm range is the same. The alarms can be selected to operate either greater than or less than the selected temperature output. A liquid crystal display provides readout of the measured temperatures and is also used to display maximum and minimum temperatures as well as the values of the various alarm parameters using a series of setup menus.

Self-contained relays for each alarm point can be used to actuate external devices such as small pumps or sirens. These internal relays may be used to control external power relays if high current loads are to be switched. An audible alarm, which may be silenced if desired, is also included within the Model 421-A. To eliminate erratic or premature operation of controlled devices, the Model 421-A includes provision for setting a delay or conformation interval prior to the initiation of an alarm and also a delay time prior to negating an alarm.

All setpoints and maximum and minimum temperature values are stored in non-volatile memory so that these values are not lost if there is a power failure. The circuits are designed to have high electrical noise immunity to transient spikes generated by loads being switched with the alarm relays. The unit restarts in the running mode after a power failure.

Temperature sensor: Precision thermistor
Measurement range: -40° to +160°F (-40° to +71°C)
Alarm range: -40° to +160°F (-40° to +71°C)
Resolution: 0.1°
Accuracy: ± 0.5°F (± 0.2°C)
Alarm ON and OFF delay range: 0-99 seconds
Controls: Protected push buttons that select
Run or set variables
Select menu item
Increment alarm setting
Decrement alarm setting
Clear stored max and min values
Indicators: LEDs for alarm point 1 and 2
2 line x 16 character LCD
Display character size 3x8 mm
Operating temperature: Control module only -20° to +50° C
Input power: 12 Vac or dc
Current: 50 mA maximum
Alarm: Contacts rated 3A @ 24 Vdc/115 Vac
Form "C" (SPDT) configuration
Temperature cable length: 40' 2-conductor
Weight/Shipping: 1 lb/2 lbs