Teledyne 3350 - Microprocessor based control room monitor for personnel safety - Manual (Page 16)

2-6
2 Operational Theory Model 3350
Teledyne Analytical Instruments
In the presence of oxygen the cell generates a current. The sensor has an
internal thermistor compensation network.
The output of the sensor is converted to millivolts. This output is fed to a
volatge amplifier. The internal thermistor network provides temperature com-
pensation of the sensor output. The resistance of the network changes with
temperature, compensating for the changes of the microfuel cell output to tem-
perature.
The output from the temperature compensation amplifier is sent to an
analog to digital converter (ADC), and the resulting digital concentration signal is
sent to the microcontroller.
The digital concentration signal along with input from the front panel buttons
(KEYBOARD) is processed by the microcontroller, and appropriate output
signals are directed to the display and alarm relays. The same digital information
is also sent to a 12-bit digital to analog converter (DAC) that produces the 0-10
V dc analog concentration signal and the 0-10 V dc analog range ID output. A
voltage to current converter (E–I CONV) produces the 4-20 mA dc analog
concentration signal.
2.4 Alarms
When the alarm setpoints are properly adjusted, they provide an opera-
tional band that covers all acceptable oxygen concentrations. If the oxygen level
at the sensor crosses the adjusted setpoint of one of the alarms, that alarm will
cause the switching of relay contacts. Normally open (N.O.) and normally
closed (N.C.) circuit connections at the interconnection terminal strip will be
reversed. Thus, a circuit that is open (turned off) in a non-alarm condition will be
closed (turned on) when its alarm is activated, and vice-versa.
As per OSHA specifications, the standard factory setting of the two alarms
provides a “CAUTION” (Alarm 1) alarm at 20% oxygen (at the sensor) and a
“DANGER” (Alarm 2) alarm at 19.5% oxygen. To cover special cases, a
limited amount of adjustment is possible.