Measuring gas concentration

Joosje van Bennekom writes

>For a project I need to know
>if there exist do-it-yourself tests to measure the concentration of
>some specific gases (H2S, SO2, formic acid, etc.) in, for example,
>show-cases. If these tests exist, does anybody know where I can get
>them?

I'm assuming that you're looking for something inexpensive. (We use
a set of Monitor Labs gas analyzers, such as their UV Fluorescence
SO2 Analyzer, but they run close to US$10,000 each.) Both Drager and
Sensidyne put out detector tubes that sound more like what you want.
Their most common type of tubes consist of a sealed glass tube with
a chemically treated open-cell foam inside and a calibration scale
written on the tube wall. There is also a fixed volume hand pump
(electric is available) with a counter. The two ends of the tube are
broken and the indicated end is inserted into the pump. Each tube is
marked with the number of "pumps" required. The foam changes color
in the presence of the specified gas (each tube discusses
cross-sensitivity) and the interface line between "changed" and
"unchanged" foam is read on the scale. The notes with the tubes also
discuss useable temperature range, humidity and correction for
atmospheric pressure.

For your use, you may be more interested in the diffusion tubes. The
two ends of the diffusion tube are broken and left for a known
number of hours. Readings are made as above. The value read off the
tube is then divided by the number of hours to get the TWA value
(Time-Weighted Average) per hour. Wall-mount or "lapel-clip" holders
are available for these tubes. In theory, if the gas concentration
remains constant, both types of tubes should produce the same
readings. However, the short-term tubes are often used to measure
gas levels when you need a fast reading (high levels.) The diffusion
tubes have a much lower scale. This makes sense since, if you're at
the upper short-term limit for a gas, you don't have 8 hours (or
even an hour) to figure it out. Drager says that their diffusion
tubes should be used for a maximum of 8 hours while Sensidyne allows
up to 24 hours. SInce their technology should be the same, I don't
see why the Drager tubes could not also be used up to 24 hours. Just
remember that long-term readings (measuring very low levels) will be
less accurate and more prone to error (one possible reason why
Drager has set an 8 hour limit.)

I suspect that Drager is closer to you in Europe then Sensidyne. The
literature enclosed with the tubes don't have a real address though.
All it says is Dragerwerk Aktiengesellschaft - Germany.

Good luck,

Douglas Nishimura
Image Permanence Institute

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                  Conservation DistList Instance 10:45
                Distributed: Thursday, November 7, 1996
                       Message Id: cdl-10-45-004
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