United Process Controls, Inc.
Oxyfire-LT Installation/Operation Manual
21 |
P a g e
7 APPENDIX
7.1
OXYGEN CALCULATION
A natural phenomenon occurs when zirconia oxide is heated above 550° C. and a reference gas is on one
side and an unknown atmosphere containing the same component as the reference gas is on the other
side. As long as these two atmospheres are kept separate, ions of the reference gas will transfer across
the zirconia boundaries, going from area of greatest concentration to area of least concentration.
Electrodes of various materials are used to measure the ion electrons (+/-) flowing through the zirconia
oxide. This signal (millivolts) is generated as long as a reference gas is continuously supplied. In the
area of combustion control, the reference gas in typically air (20% Oxygen and 80% Nitrogen).
Knowing that there is Oxygen present in the combustion process, allows the in-situ oxygen sensor (it's
substrate being made of zirconia oxide) to measure the oxygen ion transfer across the substrate, and thus
give a millivolt signal that is inversely proportional to the amount of Oxygen present in the combustion
zone. Walther Hermann Nernst, the German physical chemist is credited with developing the "Theory of
Solutions". This theory explains the voltages developed by electrochemical batteries. This universally
accepted equation can be applied to the sensing of Oxygen using a zirconia oxide cell. MMI's oxygen
sensors and their electrical interfaces conform to the Nernst Equation in combustion environments. In
most combustion applications, the level of excess Oxygen range from 0.001ppm to 21% oxygen. The
equation allows the computation and display of the oxygen concentration in the combustion gas stream.
The sensors and electronics are virtually maintenance free, needing no calibration and no maintenance of
sample tubing.
The Nernst Equation is as follows:
E = Sensor Output in Millivolts
T = Sensor Temperature in degrees Kelvin (degrees C + 273)
%O
2_Reference
= Oxygen concentration on the inside of the zirconia substrate (20.9%)
%O
2_Process
= Oxygen concentration on the outside of the zirconia substrate (unknown)
ln() = Natural log
Sample Solution:
T = 1273K
E = 80mV
80 = (0.0215)(T)(ln (20.9/Ln X))
80 = (0.0653)(T) – (0.0215)(T)(ln X)
80 = 83.13 - (0.0215)(1273)(ln X)
(27.37)(ln X) = 83.13 – 80
ln X = 3.13/27.37
ln X = 0.114
X = 1.12% oxygen
E
0.0215
mV
K
T
ln
%O2_reference
%O2_process
