Recently it occured to me that this 20KRPM brushless turbine i have that i can buy rather cheap too, incredibly efficiently consumes 300W of power. On paper, a 12″ x 24″ (~30x60cm) H14 Hepa can operate above the required 0.5m/s airflow with a mere 50-80W fan. One of the downsides of induction fans, the most common high power fans, is you cant change their speed unless they were designed with that functionality in mind. Ones that are, usually are incredibly expensive, offsetting their value. Usually a flowhood motor costs more than it needs to because it needs to work unregulated matched perfectly to your filter, producing more than 0.5m/s airflow while also not exceeding the pressure rating of the filter.

This fan here is sold sometimes for its intended purpose, as a jet-plane turbine, but moreso sold as a turbo charger fan, and an ineffective one at that. But its incredibly cheap, if you shop around, i got mine for around $20 i think, its way cheaper if you buy the $2 signal gen and $4 ESC seperate.

Anyway, this is a high power fan you can speed control. Long story short the objective i have in mind will be to do the following;

1. Measure the static pressure
2. Aproximately measure the zero pressure max flow rate, or at least confirm how accurate the advertised flow rate is
3. using at least 3 measurement points, applying a linearly increasing load, use an air flow meter (the meter is cheap and cant be trusted for specific readouts since its not calibrated but its offset is fixed so, difference measurements are fine) to determine the nature of the fans curve.
4. using values 1 2 and 3, i can determine the maximum flow rate, the static pressure, and formulate a curve based on how the flow rate changes when i vary the back pressure.

The specific methodology is that i will be using a fluid manometer

A fluid manometer will push down/lift a liquid of known density proportionately to the pressure. i can use this to determine the fans static pressure, and then, if i set one up diagonally (height determines pressure so diagnonally you increase displacement for more precise readout) i can use it to set up precise known pressure resistances. Ultimately that amounts to stuffing wool or a cloth into a pipe and doing, really anything, until the pressure reads what i want it to. then, i use my air flow meter to check the flow rate. A cheap air flow meter is unreliable for getting a precise readout, however, using it to tell the difference between multiple wind speeds works out just fine, i will apply multiple steps of resistance and just need to determine the difference. This will allow me to determine the inflection of the curve, as well as vaguely tell me its shape. For example, with the curve pointing up, convex, from a range of 0 – 200pascals, at 10 and 30 pascals, the difference between the two in wind speed should be small, wheras 170 and 190pa should see the difference between almost zero airflow and 1/3rd of the total airflow.
Fans also tend to produce somewhat specific pressure/speed graph shapes so, moreso this will help me aproximate the type. if i had a proper decent flow meter it would be a different story but, this incomplete picture is still better than nothing at all, and if nothing else, i can always do a full-load-chart too noting that the flow meter is not reliable and to assume an unreasonable error like 20%. again, not accurate, but better than nothing.

Of course, for the flowhood itself this hardly should matter, the fan is only viable if it can operate at twice the capacity required, furthermore, it will be tuned by the user using a fluid manometer, each fan has a known flow rate at a known pressure so, whatever you blow at it, be it from a pedestal fan or an air compressor, when theres say, 120Pa behind the filter, you know its at least whatever, 300M3/H. divide that by the cross section of the size of the filter and now you have your wind speed (volume per time / area = depth per time, since area x depth = volume, where depth is speed in this case)

Since i own the blog and all i might also insert a calculator when im done to help people out.

And on a final note, ill just mention the point of this is that i believe its possible to make a 12×24″ flowhood using a decent H14, of maybe 3-5″ thickness, for a total cost of less than $200AUD, i know the filter is attainable for $90, less if you buy in bulk, the box is simply enough to DIY for $10-20, a fan is $20, controls, $6, power supply matching the fan, $20-35, that all comes around to $146-171. the box would cost $20-45 if you have it made at carpentry/cabinetmakers workshop, the benefit there is you can make them construct it to fit the filter snug with a tolerance down to the millimeter. I had my first flowhood made this way and it didnt even need to be sealed up, just a single screw to clamp the filter in place.