Allowable exposure to toxic gases in nearly all cases involves time-weighted averages (TWA). Generally, these are calculated over an eight hour period. For Short Term Exposure Limits (STEL), this time period becomes 15 minutes.

OK. So, if you are doing continuous area monitoring. a data acquisition system, such as Interscan’s Arc-Max® will take data from a gas detector (sampling as frequently as three times per second; storing the one-minute averages) and calculate the various time-weighted averages.

But, what if you have a gas chromatography based gas monitoring system, using stream-switching, whereby you are sampling, say, 16 points, switching streams every minute. That means that 94% of the time, your so-called data acquisition functionality is NOT monitoring a given point. How can the requisite TWAs be calculated? There is almost no data!

One suggested scheme has the data logger store an average of the one minute data sampling results of that channel for the “off-time.” Or, you can store the highest obtained reading for the off-time. Either way, your TWAs are based on 94% off-time. What garbage!

How is this nonsense any different from me suggesting that you only operate your gas monitor 6% of the time?

Junk science comes to toxic gas data logging, and most people–including the regulators–don’t even notice.

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