[aprssig] Chemical sensors

[Ray McKnight]

Russ, there are several fundamental issues you have hit on that must be
addressed.  First, the detection of a chemical "family" of gases is
certainly possible.  However, if we're talking about a potential release by
deliberate attack, it might be very difficult to choose and position a
sensor with any high probability of accuracy.  For instance, the detection
of aromatic hydrocarbons is fairly easy (substances such as benzene,
toluene, styrene, refined petroleum distillates etc).  They make general
purpose sensors that do that sort of thing.  The instruments are not
inexpensive though, and until you get to the level of laboratory reference
such as a gas chromatograph, rarely if ever have serial/data output to
permit interfacing such as would be required for APRS.

The most common detector system is the "colorimetric" tube, such as made by
International Draeger.  They have over 200 tubes which can be either a
general purpose tube or one for a specific substance and range of
concentration.  The tubes generally fall into 2 categories:  organic and
inorganic substances.  But these are only used for "spot checks" in a
specific location, and are a one-shot deal.  I believe you are considering
the capability to monitor a fairly wide geographic area, and for it to a a
continuous sampling.

There are also articulated pump type samplers that will draw air for
extended periods of time.  But since they also use the single-use detector
tube, are not much use in this scenario.

Now, there are instruments intended for continuous sampling, but other than
flammable/combustible vapors such as benzene or other aromatics, are
generally targeted at a specific gas, such as H2SO4 (hydrogen sulfide) or C0
(carbon monoxide).  But again, few instruments have data outputs.  Draeger
and MSA are common manufacturers of these instruments as well.

What does a portable meter like this cost?  Typically around $2500.  But the
periodic maintenance will often exceed many times that over the life of the
instrument, as individual sensors cost between $200-$500 each and usually
require replacement every 2-3 years.

Another big wrench is calibration.  All of these type instruments require
routine calibration against a known standard, otherwise their accuracy is
useless.

But I do have some good news, or advise.  There are a few very unique
portable meters made for first responders, called "sniffers".  There are
targeted mostly for HazMat teams, and often used by railroads to detect
leaks in tank cars.  Thus, the types of substances they detect are mainly
geared at stuff shipped by railroad car or freight truck.  They aren't super
accurate, as they're intended as a preliminary detector of the presense of a
hazardous gas.  For greater accuracy you take additional samples with your
Draeger tube.  I **believe** some sniffers may include a data port for
downloading results to PC's in the field.  This would likely be the closest
thing to what you might need.  
http://www.montec.com/htm/mtdetectortubes.htm
(Draeger hand pumps costs around $500 and a 10-pack of detector
tubes usually sell for between $20-$50).

So I guess what we need to look at more closely is whether the list of
potential contaminants you want to test for falls within the capabilities of
currently available meters.

I'll see if I can find some links to sniffers.
But I do question the fundamental purpose of linking the 
Actual detecting sensors to APRS.  I will absolutely NEVER
Recommend APRS as a source of warning or dissemination of life threatening
information, especially to the general public.  There's just too
Many things that can go wrong, especially with a volunteer network of
generally untrained people, and equipment thrown together with duct tape and
bailing wire.  Hell, we can't get most of the folks to even understand the
difference between true and magnetic north!

-----Original Message-----
From: aprssig-bounces at lists.tapr.org [mailto:aprssig-bounces at lists.tapr.org]
On Behalf Of Russ Chadwick
Sent: Thursday, October 07, 2004 19:43
To: TAPR APRS Special Interest Group
Subject: [aprssig] Chemical sensors

Does anyone know of an inexpensive sensor that can measure the concentration
of a given gas in the atmosphere?  For example, something that could give
the concentration of poison gas as might be used in a terrorist chemical
attack.  A device that could monitor the concentration of this gas might be
a good addition to APRS weather stations situated in large cities.  I'd
appreciate it if you could send me any URLs off line.

Thanks.

Russ Chadwick    KB0TVJ
Boulder, Colorado, USA
http://www.wxqa.com



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