General Description - Theoretical Fundamentals and Measuring Principle
11
The temperature of the solid electrolytes and the electrodes is not explicitly
incorporated in the probe signal, although it does determine the internal resistance of
the probe or its limit current and, in turn, the measurable oxygen concentration range.
The probe temperature doesn’t need to be measured or regulated, but it must be
ensured that it does not undershoot a defined critical value, which depends on the
required measurement range. To measure oxygen concentrations of up to 21 vol. %
(atmospheric oxygen) the minimum probe temperature is 800°C (1470°F), for
example. The static probe characteristic I = f (O2) in the diagram shows that the
measurement accuracy is generally stable, regardless of the probe temperature and
oxygen concentration.
Using a current-proportional probe voltage and by compensating pressure and
temperature effects on the flow-control capillary, a measurement accuracy of better
than ± 0.2 vol.% oxygen in flue gases of all common fuels can be achieved, even
when measured values are not compensated. Probe ageing during long-term
operation does not affect the measurement accuracy; it merely narrows the
measurement range. For adjustment monitoring purposes, however, it should be at
least 21 vol.% oxygen.
Probe (measuring cell) aging is compensated by measuring the int. cell resistance
and, in turn, adjusting (increasing) the temperature of the measuring cell over a broad
range.
Long-term experience of operating the device under difficult conditions (e.g.
incineration of industrial waste, waste incineration, bio/sewage gas, etc.) has shown
that the measuring principle is extremely resistant to "contamination" provided that the
LAMBDA TRANSMITTER P Lambda transmitter is used properly.
Cross-sensitivity with non-combustible gas components (e.g. H2O, N2, CO2, NOX,
SO2, etc.) is not an issue. When oxygen is present, combustible gas components are
incinerated on the platinum-coated surface of the sensor, which is approx. 800°C
(1470°F).
Example: 2CO + O2 = 2CO2
This means that with 1 vol. % CO in the measuring gas, 0.5 vol. % oxygen too little is
displayed. For this reason, this measuring principle is not suitable for measuring
oxygen in combustible gases.
The amperometric measuring principle of the Lambda transmitter yields an almost
linear sensor characteristic. This characteristic passes through zero and its gradient
is determined by an adjustment point. When the measurement is taken in ambient air
with an oxygen concentration of ψ
cal
= 20.96 vol. %, this point is normally derived by
determining the probe current I
cal
[mA].
With respect to the measured probe current I, the ideal oxygen concentration (ψ
O2
) of
any measuring gas is governed by the following formula:
ψ
02,ideal
= ψ
cal
x I / I
cal
Gl. (1)
Depending on physical and design aspects, the LAMBDA TRANSMITTER P probe
current depends not only on the oxygen content of the measuring gas, but also on the
gas temperature (T), the differential pressure (∆p), the absolute pressure upstream of
the flow-control capillary, the average molecular weight (M
m
), and a function of the
mean isentropic exponents (F(κ)) of the measuring gas in relation to the adjustment
conditions (index "cal"):
ψ
O2
=ψ
02,ideal
x p
cal
/ p x (T/Tcal)
1/2
x (M
m
/M
m,cal
)
1/2
x F
(κ)cal
/ F
(κ)
Gl. (2)
To compensate these fault effects, the LAMBDA TRANSMITTER P oxygen analyzer
features the following measured value corrections:
• Temperature compensation
• Pressure compensation
• Flow rate compensation
See also "Flow Rate Compensation".