Posted By Descarte Looking at EH40 exposure limits firstly for toluene and xylene exposure limit (8hrTWA) is: Toluene 50ppm or 191mg/m3 Xylene 50ppm or 220mg/m3 Not ever really liking chemistry can you confirm that the difference in the mg/m3 even though the ppm is the same is due to the difference in atomic mass of the aforementioned compounds xylene being - C8 H10 Toluene being - C7 H8 Or am I going off on the wrong track Yours being special Des

Posted By Descarte So I get an A-, ok thanks.

Posted By John Webster ppm - parts per million, is a very non-specific term which is open to misinterpretation unless the bases of those parts is clearly specified. Normally we make assumptions based on the phase of the substance - so for soluble solid contaminants in water, ppm probably means wt/vol, but liquid contaminants might be wt/vol or vol/vol - and we all know what assumption leads to! Gasses are the worst of all. Even something as specific as mg/m3 could be far wrong - by orders of magnitude - if the gas pressure & temperature are not specified. Concentration expressed as pph (%), ppm, ppb, ppt etc cannot be relied on unless the basis is also specified eg %w/w, ppm w/v.

Posted By Richard Chalkley Gary - another ex-chemist?? I was beginning to think I was the only one to be in H&S.... All these construction types all over the place (tongue firmly in cheek) Miss the Laboratory? Richard.

Posted By Merv Newman ppm and mg m3 should be specified at ntp. Normal Temperature and Pressure : 1 bar and 21 degrees C Otherwise you can't make a decent cup of tea on the summit of mount everest. Lost me T-shirt. Merv

Posted By Merv Newman Smurfer, if you have done some chemistry, like wot I have, you would know about NTP. Water boils at 100°C if ambient temperature and, especially, atmospheric pressure is "normal" : 21°C and 1 bar. If atmospheric pressure is lower than "normal" then water will boil at a relatively lower temperature. Cannot confirm, but memory says that, at the top of Everest, for example, your kettle will boil at 50°C. So your cup of tea will be cold and very weak. The equilibrum of a chemical reaction is defined as occuring at NTP. Higher or lower temperatures or pressures will produce different end points as the higher they are will tend to drive reactions more towards the "completion" point. Which is why many plastic synthesysing plants (polyethylene, PVC, polypropylene) operate at very high T&P. OK ? It's now the "Wheatabix moment" See you later. Merv

Posted By Adrian Watson Dear All, For WELs, STP is defined as 20 C & 101.325 KPa.(HSE 2005) For TLVs, STP is defined as 25 C & 760 Torr (ACGIH 2006) PPM is vol/vol for occupational hygiene purposes. you also need a 15 c temperature difference from STP or a 5.000 KPa to make a 5% difference in the results. Regards Adrian Watson

Posted By Smurfer spot on, adrian. can i just add 0oC 101.325kPa is ref conditions for sampling in ducts.

Posted By Descarte So in normal day to day UK Circumstances where temperature does not deviate much from 21 +/- 15 Deg C in average working conditions and under average atmospheric pressure Occ hy monitoring results should not differ greatly by more than 5%, which is probably about a average standard of error? Obviously for things like stack monitoring or if in Scotland where temperatures can very to extremes (well we had a few cold days up 'ere) these would need inclusion in to your calculations. That and when monitoring you should be taking 3 or preferably 5 samples on a variety of days if possible you should be able to reduce your error margins, or at least be able to remove spurious results Hmmm seem to have deviated from the original question and into inaccuracies and variabilities whilst undertaking occy hy monitoring. Goes to show the variety of knowledge and flexibility of these forums and the people on them. Des

Posted By Richard Chalkley Whereas, the pedant that I am, would hand back the data and ask for it to be properly annotated. As to the discussion about NTP, is nothing quoted at STP any more? After all the Ideal Gas Equation is not that difficult... Richard.

Posted By Robert K Lewis Table 1: Standard reference conditions in current use °C / kPa / % RH 0 / 100.000 0 / 101.325 15 / 101.325 / 0 20 / 101.325 25 / 101.325 25 / 100.000 20 / 100.000 / 0 15 / 100.000 °F / psia / % RH 60 / 14.696 60 / 14.730 59 / 14.503 / 78 59 / 14.696 / 60 Just to be a boring old/ex chemical engineer as engineers always have to have a choice of methods in order to achieve a close fit to the answer wot we have got. Bob