Oxygen Monitor SGM7 4 Functional description
HB_SGM7_eng.docx
11
4.3 Gas flow rate
For exact measurements a flow rate between 5 and 10 l/h of the measuring
gases must be ensured (see also chap. 8.1.2)
Contamination effects of the gas tubes (leaks, permeability, desorption) may
cause falsified measuring result if the flow rate is too low.
Asymmetric cooling of the sensor electrodes may cause falsified measuring
results if the flow rate is too high.
The gas flow rate is measured by a pressure difference sensor. The SGM7
displays an error message when the limit values are over- or underrun.
However, the measuring will be continued.
4.4 Accuracy of the measurement
The manufacturer guarantees a measuring error of less than 3% (relative
error) only with measurements of oxygen concentrations within a range of 2
· 10
5
... 10 ppm. For measurements of oxygen concentrations of 10 ... 10
-3
ppm the relative error is less than 5% if the gas inlet tube has no leaks or
permeability.
For measurements of oxygen concentrations less than 10 ppm, the following
aspects must be taken into account during evaluation:
•
composition of the measuring gas (e.g. presence of burning gases)
•
specific characteristics of the production process (e.g. material used)
•
temperature of the measuring gas.
To reduce the measuring error in low oxygen concentrations, the following
conditions must be provided:
•
The measuring gas must be extracted where a formation of layers can
be avoided.
•
The tube from the measuring point to the SGM7 must be as short as
possible in order to avoid a change in the chemical balance in the tube.
•
All gas inlet and outlet tubes must be free of leaks.
•
For measurements of oxygen concentrations of less than 1000 ppm,
the use of steel tubes is necessary.
•
If the measuring gas contains reducing components (e.g. alcohol), the
concentration of free oxygen cannot be determined correctly as
chemical reactions occur at the electrode. In such cases the measuring
gas should be filtered by an active carbon filter before entering the
SGM7. (see chapter 10.4)
NOTE
NOTE
