Installation
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3-7
PPM Measurement Suggestions
This section provides help on how to use the CG1000 in the low ppm
ranges:
Gas Sample Bypass
- A good practice in the measurement of ppm gas con-
centrations (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 3.0 scfh is recommended.
Leaks -
Ensure that all tubing you connect to the CG1000 is leak-tight.
Since air contains approximately 209,000 ppm of oxygen (20.9% O
2
), even
the slightest leak in the sample gas line can cause inaccurate readings. You
can do a vacuum check as described in the Troubleshooting chapter to
check your setup for leaks.
Clean Tubing
- Never use Teflon, rubber 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 CG1000 analyzer,
use as small a sample tube diameter as possible. The smaller the inside
diameter of the sample tubing, the better the response time will be. Also
ensure that the length of the sample line between the analyzer and the
sample gas is as short as possible.
Effects of Hydrocarbons on Low PPM Oxygen Reading -
When hydrocarbons
are present in the gas sample, a mini-combustion process occurs between
the hydrocarbons and excess oxygen. An indication that hydrocarbons
may be present in the gas sample is that the oxygen reading 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 nitrogen), 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.
