312 AO2000 CONTINUOUS GAS ANALYZERS | OI/AO2000-EN REV. B
ZO23: Function test
Description
The function test is used to carry out a rapid and regular check of the re-
sponse time of the sample cell. The function test can be carried out without
any test gases by feeding the sample gas with constant concentration. It
has a very high correlation with the check using gas. In case of doubt, the
latter is decisive, however. The function test facilitates the preventive
maintenance of the gas analyzer, since a requisite exchange of the sample
cell becomes plannable through the change in the response time. The values
determined in the function test are stored in the log. As a result, the pro-
gression of the response time of the measuring cell can always be traced
back.
Procedure
In the function test, the build-up of an oxygen potential in the sample cell is
simulated through the connection of an electrical test current. The change in
the oxygen potential correlates with the response time of the sample cell. A
small change in the oxygen potential points to a relatively fast response
time of the measuring cell. In this case, the result of the function test is
"Test Passed". If the value deviates from the value before the completion of
the test by more than 10 % of the value before the beginning of the test, the
result is rejected, since it is assumed that the change in the sample gas was
too high during the test.
Test factor
The test result can be tailored to the required response time. In this connec-
tion, the test factor can be set by the user within the range 1 to 200 %. A test
factor of 100 % is set ex works.
Test factor 1 to 99 % Test requirements lower T90 > 60 s
Test factor 100 to 200 % Test requirements higher T90 ≤ 60 to 20 s
If the test criteria are not met, the test factor must either be changed or the
sample cell checked by means of test gases.
Check of the measuring cell using test gases
Two test gases (see page 67) with different concentrations within the
measuring range are required to check the T95 time, e.g. a test gas with 2
ppm O
2
and one with 8 ppm O
2
. The T95 time is determined through alter-
nate application of the test gases. Beforehand, purge the test gas valves and
the gas supply line with oxygen-free gas (e.g. with nitrogen from a loop
feeder) or with sample gas (flow rate 5 to 10 l/h, duration approx. 2 h).
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