Servomex NanoTrace DF-760E, Instruction Manual

Page: 125 / 182

Sample Gas Preparation and Delivery DF-760E 121
WARNING
Always maintain a minimum of .25 lpm flow through the bypass
when doing a flow sensitivity test.
8.4
Sample Gas Scale Factor
The Gas Scale Factor (GSF) is used to correct for effects when background gases other
than nitrogen are present in the sample stream. The Sample GSF can be adjusted by
entering the composition of the background gas in percent for either oxygen or moisture in
their respective menus. In the case of the oxygen sensor, the GSF corrects for the
difference in the diffusivity of oxygen with respect to the particular background gas. For
information on setting the oxygen GSF see page 103. In the case of the moisture cell, the
effects of molecular interactions are accounted for.
For information on setting the moisture
GSF see page 103.
8.4.1.1 Disclaimer
The method used to correct the oxygen calibration of the DF-760 Moisture/Oxygen
Analyzer for measurement in non-nitrogen background gases is derived from well-known
theoretical equations.
Although significant empirical work has been done in this field, it is generally accepted
that the equation may be only 85-90 percent accurate. In addition, there is further error
introduced when correcting for a "multi" component background gas. This may result in
up to an additional 3-5 percent error.
An alternate method when using a non-nitrogen or "multi" component background gas for
spanning is to obtain a certified Calibration standard that has been prepared in a
background gas that models the average process sample. Care must still be used, however,
as certified standards may also have an inaccuracy associated with them.
Questions regarding the calculation of a background gas correction factor for a specific
application should be directed to Servomex.
8.4.2 Background Gas Effects on Indicated Flow Rate
If the molecular weight of the background gas is significantly different from N
2
, the
indicated flowrate as displayed on both the bypass flowmeter and the front panel display
will not be accurate. The flowmeters used in the DF-700 series analyzers are calibrated for
use in air (or N
2
). Most other gases have molecular weights within ± 25 percent of air.
Since the required flow rate is not extremely critical most gases produces reasonably
correct readings. The exceptions are light gases such as Helium and Hydrogen whose flow
rates should be set to approximately one-third that of Nitrogen.