We talked a little last time about oxygen
analysis at the parts-per-million (ppm) level. Electrochemical sensors perform
well in many applications at those levels.
Leaks and diffusion create challenges with ppm O2 applications. In a neutral pressure environment, the gas
analyzer will require a built-in pump. Activation of the pump will create a
slightly lower pressure in the sample tubing ahead of the pump and will promote
inbound leaks at imperfect tubing connections.
Gas diffusion may also occur irrespective
of pressure through polymer-type tubing based on the size of the gas molecule
and the structure of the polymer. Flexible polymer tubing may initially show
decent diffusion resistance. However, other constituents in the sample may
alter the chemistry and structure of the tubing material and make it more prone
to gas diffusion over time.
In many gas analysis applications small
leaks and gas diffusion may not cause significant analytical problems. But with
ppmO2 applications, these problems can have a noticeable effect. Even a small
inbound leak of ambient air will introduce a high level of non-sample O2 and
result in falsely high readings. It may also prevent an accurate Zero
calibration. O2 diffusion may cause the same effects as inbound leaks, but at a
slower rate.
The most effective solutions to these
issues involve removing leaks and points of diffusion. PPM O2 analyzers should
be meticulously built using stainless steel tubing with reliable connections.
We prefer that ppmO2 applications have a positive sample pressure. This removes
low pressure points before the sensor and allows us to use a regulator in place
of a pump. Simple and strong seem to be the best methods for ppmO2.
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| We are currently building a couple instruments with dual-channel PPM / % O2 analysis. The ppm channel can be switched to protect the sensor automatically or manually depending on the customer’s preference. In this photo, the instrument is being purged with N2 in advance of factory calibration. |
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| The ppm side is being purged out with N2 in preparation for factory calibration. Notice that the tubing is narrow diameter flexible stainless steel to minimize leaks or diffusion during this process. |
In most applications, a ppm analysis requirement implies a high proportion of other gases in the sample. Some gases interfere with accurate ppm analysis of other gases. So it is important that we have a good understanding of the sample gas make-up before designing an analyzer.
We have produced portable and a continuous
PPM O2 analyzers which may be suitable for your application.
1-800-295-3771
sales at nova-gas dot com
websales at nova-gas dot com
http://www.nova-gas.com/
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