BA 4510
=
2
,air
= 20.64
U
T
U
T
φ
φ
φ
●
●
e
e
●
O
)
(
)
(
46.42
zF
R
O
2
,sample gas
O
2
,sample gas
Where:
Φ
oxygen concentration in the sample gas in Vol.%
U
: Potential difference in mV
T
: Measuring temperature in K
20.64 oxygen concentration in air with a relative humidity of 50 % in Vol.%
1.4.2 Measuring Requirements
1.4.2.1 General Information
Sample gas can have free or bonded oxygen. Without sufficient "
free oxygen
" in addition to combustible gas components, a
chemical balance will occur on the hot board electrode. The cell then measures the concentration of the "
equilibrium oxygen
".
(The basics are explained in the appendix
Basics of using potentiometric ZrO2 solid electrolyte sensors in optimal combustion
processes
[> page 23].)
Here the following dependencies apply:
U ~ T
Contains free oxygen
U~ 1
T
Contains bonded oxygen
The conditional equation
(II)
applies to both sample gases with free oxygen as well as for reduced gas mixtures which only con-
tain bonded oxygen, e.g. in H
2
/H
2
O or CO/CO
2
mixtures.
1.4.2.2 Sample Gas Flow Rate
The sample gas flow rate must be between 5 and 10 L/h to ensure exact measurement. If the low flow rate is too low, the con-
tamination effects of the gas lines (leaks, permeabilities, desorptions) will cause reading errors. If the flow rate is too high, asym-
metric cooling of the sensor can cause measurement errors.
A differential pressure sensor measures the gas flow. When over- or underrunning the limit, the device will output an error mes-
sage yet continue measuring.
When operating the device with the internal gas pump, the flow rate controls the pump. The gas flow will always be set to an
optimal flow rate of 7 L/h.
1.4.2.3 Measurement Accuracy
The manufacturer only guarantees a measuring error of < 5% (relative error) when measuring oxygen concentrations with an
order of 2*10
5
…10 ppm (this manual uses the unit ppm is used in the sense of Vol.-ppm). When measuring oxygen concentra-
tions of 10…1 ppm, the relative error is under 5 % provided the gas supply has no leaks or permeabilities.
When measuring oxygen concentrations < 10 ppm, user must consider the following aspects when analysing the measured
value:
– composition of the sample gas (e.g. presence of reducing gas components)
– special production process conditions (e.g. material components)
– sample gas temperature
3
Bühler Technologies GmbH
BE550013 ◦ 09/2018
