Servomex DF-130E, Operator'S Manual

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Options DF-150E 27
4) Continue to hold the toggle switch lever in the upward position. The
displayed number may be adjusted by using the screwdriver pot located above
the % H
2
(or He) in N
2
label. This number ranges from 0 to -1000,
corresponding to 0% to 100.0 % of the light gas. If the Scale Factor Pot is set
to -000 the instrument will be calibrated for Nitrogen. If the Scale Factor Pot
is set to -1000 the instrument will be calibrated for 100% of the light gas.
Settings between -000 and -1000 will adjust the calibration to compensate for
proportions of the light gas ranging from zero to 100%.
5) Once the toggle switch lever is released allow ten seconds for the analyzer
to return to normal operation.
NOTE: During this ten second timeout period if the switch is thrown upward
again the display will not show the scale factor value.
Disclaimer
The method used to correct the calibration of the Servomex Oxygen Analyzer
for measurement in non-nitrogen background gases is derived from a well
known theoretical mass transfer equation. This equation accounts for the
change in oxygen diffusion rates through different gases.
Although significant empirical work has been done in this field, it is generally
accepted that the equation may be only 85-90% accurate. In addition, there is
further error introduced when correcting for a "multi" component background
gas. This may result in an additional 3-5% error. Correcting the calibration
(for all combinations of background gases) using theoretical means has its
limitations.
An alternate method when using a non-nitrogen or "multi" component
background gas is to obtain a certified oxygen calibration standard which has
been prepared in a background gas which models the average process sample.
In this case any possible error introduced in using the theoretically derived
correction factor is eliminated. Caution must still be used, however, as
certified standards may also have inaccuracies associated with them.
Questions regarding the calculation of a background gas correction factor for a
specific application should be directed to your local Servomex Business
Center.
NOTE: In light gas (H
2
or He) backgrounds, the diffusion rate of oxygen will
be greater than that in nitrogen, resulting in a higher absolute current generated
by the sensor. If the sample contains an oxygen concentration near the high-
end of the instrument (e.g. 80 ppm on a 0-100 ppm unit), and consists of a
light gas background, the current generated by the sensor may be too much for
the electronics to source and will effectively put the instrument out of range. In
such a case, it would be appropriate to use an analyzer of the next highest
range (e.g. 0-500 ppm). Consult Servomex for application specific details.