Transcript Master for Overheads (Latest 4/18/95)

Methods of Measuring O2
Methods of Measuring O2

Orsat - Oxygen absorption into a liquid results in a change
in volume. Chemical, only. Indication, only, No electrical
output. Not suitable for continuous operation. Extractive

Chemical Cell - Used in portable analyzers. Inexpensive
electrolytic cell is consumed by the measurement. Not
suitable for continuous use in Flue Gas conditions.
Extractive.

Paramagnetic - Oxygen is affected by magnetic fields.
Suitable for continuous use in combustion service, but
requires a clean, dry, extracted sample.

Zirconium Oxide - The ZrO2 solid fuel cell has become the
industry standard for measuring combustion O2.
Product Overview - In Situ O2
Probe
Zirconium Oxide Sensing Cell
Permits In Situ Probe Design
ZrO2 Sensing Cell

Steel Cell Holder

Disc-type ZrO2 Cell
Theory of Operation - ZrO2 Fuel Cell
ZrO2 cell
Heater
Cell
Flange
Diffusion
element
T/C
Cell contact pad
Cell lead wire
1550F
Heater
Flue gases
How Does it work?
Theory of Operation - Normal ZrO2 O2 Cell
Heater
20.95% O2
O2 O2 --
736 C
(1357 F)
Signal varies with the O2 partial pressure
difference between the reference and
flue gas sides of the cell
Cell tube
-53MV
Thermocouple
+53MV
Flue Gas
2% O2
++
cell contact pad
and lead wire
(positive charge)
ZrO2 disc
Nernst Equation
EMF = KT log10 (P1/P2) + C
porous platinum electrodes
Theory of Operation
Like a Thermocouple, the Cell Generates It’s own MV Signal
Nernst Equation
EMF = KT log10 (P1/P2) + C
Where:
P2 is the partial pressure of the oxygen in the measured gas on one
side of the cell.
P1 is the partial pressure of the oxygen in the reference gas on the
other side.
T is the absolute temperature.
C is the cell constant.
K is an arithmetic constant.
For best results, use clean, dry, instrument air (20.95%) as a
reference gas.
ZrO2 “Fuel Cell” Technology is Clearly
Favored for Flue Gas Analysis
 Output is inverse and logarithmic.
–
Signal increases at the low O2 levels commonly experienced in
combustion processes.
–
Accuracy is stated as a “percent of reading,” as opposed to the
conventional “percent of full scale.” Accuracy actually improves
at lower O2 levels.
 Sensing cells operate well at elevated temperatures,
permitting the “in situ” design. Response is fast.
 Net/Wet Measurement
 Sensing cells are robust. Cell life can easily exceed 3-5
years. Sulfur is the major cause of shortened cell life.
Output is Inverse and Logarithmic