Oxygen Analyser Zirconia Cell

1. LALIT SINGH
SR. ENGINEER
OXYGEN ANALYZER

2. Zirconia oxygen analyzers use a gas whose oxygen concentration is known and
always consistent, as the reference gas. In general, air is used as the reference
gas. The oxygen concentration of dry air is constant at 20.95%; however, air
generally contains water vapor, in which case the oxygen concentration varies
with temperature and humidity. In zirconia oxygen analyzers, a measurement
error is caused if the temperature or humidity of the reference gas (air) varies
significantly between calibration and measurement. When instrument air is
used as the reference gas, this error can be ignored, but if it cannot be used,
care is required.
ZIRCONIA OXYGEN ANALYSIS

3. Concentration cell system:-A solid electrolyte like zirconia exhibits conductivity of oxygen ions
at high temperature. As shown in the figure below, when porous platinum electrodes are attached
to both sides of the zirconia element to be heated up and gases of different partial oxygen
concentrations are brought into contact with the respective surfaces of the zirconia, the device
acts as an oxygen concentration cell. This phenomenon causes an electromotive force to be
generated between both electrodes according to Nernst's equation.
Limiting Current type:- As shown in the figure below, if the flow of oxygen into the cathode of
a zirconia element heated to high temperature is limited, there appears a region where the current
becomes constant even when the applied voltage is increased. This limited current is proportional
to the oxygen concentration
ZIRCONIA TYPE MEASUREMENT
: CONCENTRATION CELL SYSTEM

4. The zirconia cell is a high temperature ceramic sensor. It comprises two
electronically conducting, chemically inert, electrodes attached to either side of a
solid electrolyte tube. This is shown schematically in Figure 1., overleaf.
The tube is completely gas tight and made of a ceramic (stabilized zirconium
oxide) which, at the temperature of operation, conducts electricity by means of
oxygen ions. (Note: In sensors of this type, the temperature has to be above
450oC before they become active as an electrolyte conductor). The potential
difference across the cell is given by the Nernst equation.
where:
E- is the potential difference (volts)
R- is the gas constant (8.314 J mol-1 K-1)
T- is the absolute temperature (K)
F- is the Faraday constant (96484 coulomb mol-1)
P1 & P2 are the partial pressures of the oxygen on either side of the zirconia tube
THEORY

5. Separate type zirconia oxygen analyzers need not use a sampling device. They
can also be used in a variety of manufacturing applications.
They allow direct installation of the probe in the wall of a flue or furnace to
measure the concentration of oxygen in the stack gas to 700℃. In addition,
with the high temperature detector, they can allow to 1400℃.
The converter uses a digital display and incorporates a LCD display with
touchscreen for ease of operation.
ZR22G/ZR402G DIRECT IN-SITU ZIRCONIA
OXYGEN ANALYZER(SEPARATE TYPE)

7. The cell (sensor) voltage will be an index to determine the amount of
degradation of the sensor. The cell voltage corresponds to the oxygen
concentration currently being measured. If the indicated voltage approximates
the ideal value (corresponding to the measured oxygen concentration), the
sensor will be assumed to be normal. The ideal value of the cell voltage (E),
when the oxygen concentration measurement temperature is controlled at
750℃., may be expressed mathematically by:
E = -50.74 log (Px/PA) [mV]
where, Px: Oxygen concentration in the measured gas PA: Oxygen
concentration in the reference gas, (21% O2)
CELL VOLTAGE

8. OXYGEN CONCENTRATION VS. CELL
VOLTAGE (CELL TEMPERATURE: 750℃ )

9. Another factor of importance is the degradation of the sensor due to ageing
of the system, where the main change is an increase in the response time and
a decrease in the EMF output. Poisoning of the catalytic activity of the Pt
electrode can occur by the deposition of lead oxides or the formation of oil
rich deposits on the sensor.
Cell's Internal Resistance: -A new cell (sensor) indicates its internal resistance
of 200 Ω maximum. As the cell degrades, so will the cell's internal resistance
increase. The degradation of the cell cannot be found only by changes in cell's
internal resistance, however. Those changes in the cell's internal resistance will
be a hint to knowing the sensor is degrading. The updated values obtained
during the calibration are displayed.
DEGRADATION OF THE OXYGEN
SENSOR

10. There are two calibrations for zirconia oxygen analyzers
(1) Two-point Calibration:- The following figure shows a two-point calibration using two
gases: zero and span. Cell electromotive forces for a span gas with an oxygen
concentration p1 and a zero gas with an oxygen concentration p2 are measured while
determining the calibration curve passing between these two points. The oxygen
concentration of the measurement gas is determined from this calibration curve. In
addition, the calibration curve corrected by calibration is compared with the theoretical
calibration curve for determining the zero-point correction ratio represented by B/A
100 (%) on the basis of A, B and C shown in the bellow figure and a span correction
ratio of C/A 100 (%). If the zero-point correction ratio exceeds the range of
100 30 % or the span correction ratio becomes larger than 0 18 %, calibration of the
sensor becomes impossible.
(2) One-point Calibration The next figure shows a one-point calibration using only a span
gas. In this case, only the cell electromotive force for a span gas with oxygen
concentration p1 is measured. The cell electromotive force for the zero gas is carried
over from a previous measurement to obtain the calibration
curve.
The principle of calibration using only a span gas also applies to the one-point
calibration method using a zero gas only.
CALIBRATION

11. THANK YOU