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Methods of Measuring O 2. Methods of Measuring O 2. 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

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methods of measuring o 2
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.
zro2 sensing cell
ZrO2 Sensing Cell
  • Steel Cell Holder
  • Disc-type ZrO2 Cell
theory of operation zro 2 fuel cell
Theory of Operation - ZrO2 Fuel Cell

Cell

Flange

Heater

Diffusion

element

ZrO2 cell

T/C

Cell contact pad

Cell lead wire

1550F

Heater

Flue gases

how does it work theory of operation normal zro 2 o 2 cell
How Does it work?Theory of Operation - Normal ZrO2 O2 Cell

Heater

736 C

(1357 F)

Signal varies with the O2 partial pressure difference between the reference and flue gas sides of the cell

20.95% O2

Cell tube

O2

O2--

-53MV

Thermocouple

Flue Gas

2% O2

+53MV

++

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
Theory of OperationLike 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.

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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.