Pitot Tube Location for Filter Monitoring

[David L Morse]

I tapped the plenum between the blower housing and the filter tube, and installed a 1/8" barbed fitting. The fitting protrudes into the flow about 1/2"

That 1/2" could be a problem. ASHRAE spec requires that it be flush with the wall. IME about an 1/8" protrusion caused around -0.2" wg error at decent flow rates. BTW, that's not a plenum; it's a transition.

Question - Is there any "rule of thumb" out there for what I should be looking for as regards the change in flow with a dirty filter?

Yes, but it's probably not very useful. By the time there's a noticeable change in flow the filter will be a huge pain to clean. Filter pressure is a much more sensitive indicator of filter loading.
fwiw, relative flow is proportional to the squareroot of one minus the ratio of the change in pressure to the total internal fan pressure. So let's say for a give system configuration your filter pressure changes from .3" to 3.3". IRRC, that DC has an internal pressure of about 15" so 1-3/15 = .8 and the squareroot of .8 is about .9, so about a 10% reduction for a 3" increase. You would not likely notice a 10% change in flow when it happens gradually over time.

EDIT: Just saw Thomas Wilson's post. He has a much better location for the pressure port. That, in fact, is the way I do it, so it must be right

[Kris Cook]

One thing to be aware of with measuring the pressure drop across the filter is that the resistance in the rest of the flow network affects flow and thus the measurement. You want to take a reading at some reference condition most likely with all blast gates open. This condition gives the greatest flow and greatest pressure drop across the filter. You take a reading with filter clean (new is good). This should give a reading in the 1-2 inches of H2O range. Keep that reading on file (ie, write it on the filter). You repeat the measurement at the same condition over time as the filter loads up. You are only concerned with how much it increases. If the reading increases by an inch of water or so, the filter loading is pretty high compared to the rest of the system pressure losses and you would gain some system flow by cleaning the filter. If this does not answer your question, let me know. I can talk endlessly on the topic of measurement.

I think this does answer my question. Thank you. I was going to use the single gate open as my reference condition but opening more gates makes sense. I will give that a try.

[Kris Cook]

That 1/2" could be a problem. ASHRAE spec requires that it be flush with the wall. IME about an 1/8" protrusion caused around -0.2" wg error at decent flow rates.

Wondering how that matters is we are talking about a relative change in pressure...

Edit - I backed the fitting out as far as I could and still maintain a seal. Turns out it does matter. The reading went up exactly 0.2". I will come up with a saddle or block to thread through and try to maintain the fitting flush with the wall.

Thank you.