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Appendix A.
94 CGA 351 User’s Manual
A.1.3 The CGA 351 Nernst Equation
The Nernst Equation specifically applicable to the CGA 351 analyzer is obtained by substituting the above values into
the general equation, converting the natural logarithm to the common logarithm (base 10) and converting the units
for E
12
to millivolts. This results in the following equation:
The CGA 351 measures the temperature of the oxygen sensor and automatically inserts the correct value into the
Nernst Equation. For example, at the standard oxygen sensor operating temperature of 700°C, Equation A-2 above
becomes:
The voltage drop across the zirconium oxide sensor, as calculated from the Nernst equation, is then sent to the
linearizer circuit. The circuit produces a linear analog output that represents the percentage of oxygen in the sample
gas, and this signal is available for display on the digital panel meter and/or for output to an external device.
For convenience, the standard Nernst equation for the CGA 351 zirconium oxide oxygen analyzer (Equation A-3
above) has been converted into a graphical format. Use this graph (see Figure A-4 on page 95) to quickly correlate
the analog output voltage reading with the corresponding oxygen percentage, at the standard 700°C operating
temperature of the meter. In addition, the data is presented in tabular form in Table 138 on page 95. This table is
useful for looking up the correct mV value to enter during the calibration procedure, if the data will not be entered
directly in either ppm or pcnt oxygen.
E
12
mV ( ) 0.049605 T

0.2093
p
2
----------------
log

=
E
12
mV ( ) 48.273
0.2093
p
2
----------------
log

=