In most cases, gas monitoring applications focus on a single gas. There may well be interest in detecting this compound at several points, but the end-user has determined that it has one particular target analyte. Sometimes, though, there will be two gases of interest. Common examples are hydrazine and nitrogen tetroxide; and carbon monoxide and nitrogen dioxide. (More details on NO2 versus N2O4.)
If a mixture of toxics will be present, it is necessary to consider the possibility of cross-interferences. That is, will gas “a” also give a response on the gas “b” sensor—and vice versa? If there is such an interference, how significant is it, and can it somehow be dealt with?
There is also the matter of calculating the TLV-TWA or PEL of this mixture.
For a mixture…
The TLV-TWA = c1/t1 + c2/t2 + …cn/tn
Where c1, c2…. cn are concentrations of different substances in the mixture, averaged over an 8-hour time period and
t1, t2…. tn are TLV-TWAs of the corresponding substances.
Note that if the value is > 1, the mixture TLV-TWA has been exceeded.
In the case of OSHA PELs, the same calculations apply. Some authorities caution that the mixture formula should be used only with substances that have similar chemical and toxicological properties.
Bear in mind that performing such calculations in real time would require a suitable data acquisition system, provided with functionality whereby an alarm would sound, based on the calculated TLV or PEL. While this computation in itself presents no great technical difficulty, the chemical/toxicological similarity element can be a stumbling block.
Thus, end-users are more likely to monitor and alarm for the compounds, and focus their actions on the most toxic substance in the particular environment. As such, interest in TLVs or PELs of mixtures is not widely seen.
Please contact us for further help on multi-gas monitoring, or any other gas detection application.