Ppm Measurement Suggestions - Ametek Dycor CG1100 Series User Manual

PPM Measurement Suggestions

3-10 | Dycor CG1100 Oxygen Analyzer
This section provides help on how to use the CG1100 in the low PPM
ranges:
Gas Sample Bypass
A good practice in the measurement of PPM gas concentrations (during
calibration or initial start-up) is to provide a bypass on the gas sample
inlet to the analyzer. This will improve the response time of your test
system. A bypass flow rate of 2.0 scfh is recommended.
Leaks
Ensure that all tubing you connect to the CG1100 is leak-tight. Since air
contains approximately 209,000 PPM of oxygen (20.9% O
slightest leak in the sample gas line can cause inaccurate readings. You
can do a vacuum check as shown in the "Leak Check" section of Chapter
6 to check your setup for leaks.
Clean Tubing
Never use rubber, Teflon, or plastic tubing. Use stainless steel tubing
or refrigerator-grade copper tubing. In general, the tubing must be
extremely clean. Even a small amount of contamination in tubing line
can affect PPM oxygen readings.
Quick Response
To get the quickest response from your CG1100 analyzer, use 1/8" stain-
less steel tubing for the sample inlet. Also ensure that the length of the
sample line between the analyzer and the sample gas is as short as pos-
sible.
Effects of Hydrocarbons on Low PPM Oxygen Reading
When hydrocarbons are present in the gas sample, an oxidation process
occurs between the hydrocarbons and excess oxygen. An indication that
hydrocarbons may be present in the gas sample is that the oxygen read-
ing will be lower than expected. For example, if a calibration gas cylinder
has an oxygen value of 20 PPM and 5 PPM of hydrogen (balance nitro-
gen), the oxygen analyzer will read 17.5 PPM of oxygen. The reduction
of oxygen is due to the combustion process where 5 PPM of hydrogen
will combine with 2.5 PPM of oxygen to form water. Thus, the oxygen
analyzer will read 17.5 PPM rather than the actual 20 PPM oxygen in
the sample. The amount of oxygen reduction is dependent on the type
of hydrocarbons present in the sample gas.
If measuring PPM gases, use a packless valve to switch between the
sample gas and calibration gases.
), even the
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