Good summary, Don.  I would like to add that the most important feature of any gas detection  instrument is the person attached to it.  That person needs to know what they are measuring, why they are measuring it, the interpretation criteria, how to interpret same, possible interferences and confounding factors, the detection limits of the instrument and, last but not least, how to react to the readings.

W.

Wayne Wood | Associate Director, University Safety (EHS) Ð Directeur Adjoint, Direction de la prŽvention (SSE), Services universitaires | McGill University | 3610 McTavish Street, 4th floor | Montreal, Quebec, Canada, H3A 1Y2 | Tel: (514) 398-2391 | Fax: (514) 398-8047

From: DCHAS-L Discussion List [mailto:dchas-l**At_Symbol_Here**MED.CORNELL.EDU] On Behalf Of Don Abramowitz
Sent: Monday, October 31, 2011 10:20 AM
To: DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU
Subject: Re: [DCHAS-L] Portable Gas Detectors

A four gas detector as others have described is a useful tool, but it's important to keep in mind its limitations, especially when investigating unknown odors.  The combustible gas detector is designed to detect flammable levels of gas, not toxic levels.  A reading of 1% LEL, the lowest reading displayed on many instruments, reflects a concentration of 500 ppm for a substance whose LEL is 5% in air.  That's quite a snoot-full for many substances.  The percent oxygen scale is even less sensitive.  20.9 % oxygen is the same as 209,000 ppm.  A change in reading to 20.8% is a change of 1,000 ppm.  And to displace a given volume of oxygen from air, you need to also displace about four volumes of nitrogen to account for the other ~80% of the air, so a drop in oxygen from 20.9% to 20.8% indicates that about 5,000 ppm of something else has displaced the oxygen.  A reading of 19.9% would be considered within the OSHA critieria for confined space entry, but it could also mean there are 50.000 ppm of an unidentified contaminant present.

CO and H2S are fairly straightforward and are sensitive in a health-relevant range, but the sensors are subject to interference, so a "hit" on a CO sensor may be caused by another substance in the air.  I have noticed, for example, that isopropyl alcohol makes the CO sensor on my meter go crazy.

I have been in situations where there was clearly something amiss, where someone had waved a four-gas multimeter, observed no change in the readings from ambient, and pronounced the air "safe," as if the device was a Star Trek Tricorder.  (If anyone knows where I can get one of those, please let me know.)

                                                                          Don
Donald Abramowitz, CIH
Environmental Health & Safety Officer
Bryn Mawr College
Bryn Mawr, PA

________________________________
Hello Everyone,

I recently became the first CHO at my institution and joined the ACS DCHAS.  The LISTSERV has already provided me with valuable information.  Our institution is small and Chemistry is a "service" area to the rest of the College (including a small Biology Program).  Last week we had a report from a staff member of a "strong and ugly odor" coming from a chemistry lab.  The staff member called Public Safety and they called me. It turns out that the lab tech had just prepared reagents containing cyclohexane.  After the incident was determined to be "minor", the Public Safety Chief told me that his office used to have a Portable Gas Detector, but that said detector was lost. Can any of you recommend such a device? Brand? Detectors?

Thanks in advance for any assistance,

Mario

Mario G. Garcia-Rios, Ph.D.
Associate Professor, Biology and Chemistry
Chemical Hygiene Officer
Mount Ida College
777 Dedham Street
Newton, MA 02459
(617) 928-4061

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