Teledyne 3060e - Ultra Trace oxygen analyzer - Manual (Page 17)

Teledyne Electronic Technologies
Analytical Instruments
2-7
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Faradaic Calibrator Operation
The calibrator is based on Faraday’s Law of Electrolysis. When applied to the
electrolysis of water, it states that the rate at which oxygen is generated is
directly proportional to the quantity of electric current flowing through the
electrodes. Based on this, the following formula is derived to determine the
current required to generate a mixture with the desired oxygen concentration
(PPM) and flow rate (SCCM).
I (
µµµµµ
A) = 40/150 X O
2
(PPM) X Flow Rate (SCCM)
The desired O
2
concentration is entered through the calibration menu, and the
mass flow controller provides the flow rate of O
2
-free zero gas through the
calibrator. The required current for the electrolysis of water is calculated by the
microprocessor and then supplied to the calibrator electrodes.
A calibrator bypass uses zero gas to flush out any O
2
that has seeped into the
calibration cavity path, eliminating excess O
2
that might otherwise enter the
sampling system.
The accuracy of the calibration gas formed using the calibrator is best in the
0-10 PPM range. Since the sensor has linear output through all ranges, the
analyzer may be calibrated at one or two ranges above the normal range of
operation.
NOTE: For example, if the analyzer is to be used for analysis in the 0–50
PPB range, calibrate the analyzer in the 0–100 PPB or 0–1 PPM
range to minimize calibration error. At this level, errors due to
mass flow controller calibration, measurement of current from a
current source and current leakage are minimized.
To see why, assume the absolute error during calibration on any range is 2 PPB.
If the analyzer is calibrated at 25 PPB, 2 PPB error constitutes an 8% error in
calibration. However, if the analyzer is calibrated at 80 PPB the calibration
error will be only 2.5%.