Teledyne 335 - Analog control room monitor for personnel safety - Manual (Page 11)

2-5 TELEDYNE ANALYTICAL INSTRUMENTS
Oxygen Alarm Monitor Operational Theory 2
Figure 2-3. Characteristic Input/Output Curve for a Micro-Fuel Cell
In addition, since there is zero output in the absence oxygen, the
characteristic curve has an absolute zero. The cell itself does not need to
be zeroed. In practical application, however, zeroing is still used to com-
pensate for a number of other variables in the instrument and in the envi-
ronment of the instrument.
2.3 Circuit Description
Follow this description on the Circuit Schematic in the Drawings
section at the rear of this document.
The measuring cell produces a current signal that is acted upon by the
thermistor, resistor and span potentiometer network to produce a tempera-
ture compensated voltage signal. The voltage signal (about 100mV 20%)
is applied to the CW terminal of the span pot (P1). A portion (about half)
of the voltage, as determined by the position of the span pot slider, is
applied to amplifier A1. Offset zero adjustment is provided at this point by
zero potentiometer P2. Capacitors C1 (RFI filter) and C2 (noise filter)
improve the signal quality to ensure clean amplification by A1. Adjust-
ment of the span pot can now cause the signal voltage, amplified twenty-
fold by A1, to equal 1V full scale at the amplifier’s output.
The amplified 1V signal is fed to the meter, and a full scale meter
response with 1V applied is accomplished by adjustment of potentiometer
P6. Comparator sections A2a and A2b, biased by setpoint potentiometers
P5 and P4, respectively, provide direct activation of the alarm relays and
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