Dear Chris and all

I had mentioned to Chris outside the forum (sorry should have copied all in) that it might be useful carrying out a tracer gas decay test. As many of you note the hot wire anenometer will not sense direction and the use of pitot tubes or a flow grid may not be suitable for such low pressure ranges. This is the reason that I suggest a decay method may be the best way to analyse the overall flow rate through the room. This is the way I measured flow rates in both model environments and test rooms when I did my PhD. The main reason for this is the unsteady nature of ventilation flows themselves. Ventilation by its nature is not unidirectional across an opening it is an unsteady I.e. varying and fluctuating pressure across the opening that if you measure it with a transducer will have positive and negative amplitude all the time. It is the differences in the combination of buoyancy and wind driven pressures around the building that cause a variance in the mean pressure differences that drive the flow in one or other direction overall. Obviously this combines with other factors such as opening position, size, etc. The tracer gas test will get around this unsteady issue as you are then looking at a time decay method that will give you the decay rate and therefore a direct relationship to the flow rate into a room. Fairly easy to test, a relevant gas sensor, datalogger (though many sensors now have them built in) and a can of trace gas like SF6 or preferably something less ozone depleting but easy to test for.

I recommend that anyone studying ventilation flow reads the book on ventilation theory and measurement by David Etheridge and Matts Sandberg. It is an invaluable resource and will answer many questions people may have regarding these sorts of studies.

Regards

Dr Paul Carey