We had a request on one of our sub-redit safety feeds regarding calculation of STELs and TWA and how they work with our instruments. So there is a really good technical note TN 119 from RAE systems. They're obviously now part of Honeywell but the technical note library is fantastic and it is available at our website in the legislation and guidance section. If you can't find it just get in contact with me.
STEL is a rolling 15 minute average, for those not familiar STEL is a Short Term Exposure Limit. We work in the UK to the EH 40 limits or over in the States OSHA standards from NIOSH. The rolling 15 minute average works on the basis of a mean average value from minute 1 two minute 15 you'll have an average level and then from minute 2 to minute 16, minute 3 to minute 17 etc. A rolling 15 minute average value.
Now the TWA calculation works in a slightly different way. This is a cumulative level that adds up as you go along. So the longer you're monitoring, the higher your TWA level will be. The way that works on most instruments (this is true for RAE Systems amd most other instruments) is for every minute of measurement your TWA will increase by that value divided by 480 which is the amount of minutes in an eight hour day that value will add to the TWA constantly increasing. For example if your first minutes average was 480 (that's easy maths) then a value of 1 will be added and then in your second minute another 480 add another 1. You will then get a cumulative level of 2 and that will continue for the duration of the monitoring cycle and the important thing to know is that will continue beyond an eight hour day. So you can have a value where the TWA actually exceeds your highest peak value depending on how long you've been running the instrument for.
The other thing to bear in mind is the TWA assumes that the only gas exposure is while you have had the instrument switched on. So if you've only switched the instrument on to monitor for 2 hours the TWA value that's given is based on that two hours of monitoring and the calculation method of the average 1 minute value divided by 480.
Hopefully this breif note gives you a bit more information but if you'd like a more detailed explanation then feel free to get in contact with us.
Feel free to give us call on 01489 326031 or outside normal working hours 07951 854824.