Teledyne 3060e - Ultra Trace oxygen analyzer - Manual (Page 15)

Teledyne Electronic Technologies
Analytical Instruments
2-5
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When the O
2
is reduced at the cathode, cadmium is simultaneously oxidized by
the following mechanism:
Cd
→
Cd +2 + 2e- (anode)
The electrons released at the anode surface flow to the cathode surface via an
external circuit. This current is measured and used to determine the O
2
concentration in the gas mixture. The resulting current is directly proportional
to the O
2
level in the gas mixture.
The current output of the sensor changes with temperature therefore, in order
to minimize the effects of ambient temperature variation on the current output,
the sensor is housed in a temperature-controlled oven maintained at 28°C
±2°C. Variations in temperature affecting the sensor are detected by a resistive
thermal device (RTD) attached to the sensor, which signals the temperature
controller to adjust the oven temperature accordingly.
A thermistor installed in the sensor monitors electrolyte temperature. Changes
in electrolyte temperature above 30°C are relayed to the microprocessor, which
adjusts the current output through a compensation algorithm.
NOTE: Since the gas to be analyzed does not flow through the bulk
electrolyte of the sensor, the probability of contaminating the
electrolyte by the particulates (if any) in the sample gas is very low.
This eliminates the requirements for frequent replacement of
sensor electrolyte. This is in contrast to Hercsh type galvanic cells
where gas is bubbled through the electrolyte and frequent replace-
ment of electrolyte (3-6 month intervals) is essential to maintain
sensor performance.
Faradaic Calibrator
The analyzer may be calibrated by using either the internal Faradaic calibrator
or an external span gas. The calibration using an external span gas is simple and
self-explanatory. The internal calibrator generates O
2
at a precisely controlled
rate. The unique design of the calibrator allows repeated and reliable calibration
of the analyzer.
Faradaic Calibrator Components
Figure 2-3 illustrates major calibrator components. The calibrator is comprised
of a top electrode assembly and a bottom mounting block containing a cup for
electrolyte. An O-ring is used to make a leak-free seal between these two
components.