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Differential Pressure Switch - Range Calculation

Differential Pressure Switch - Range Calculation

Differential Pressure Switch - Range Calculation

Hi all,

I've been tasked to select a differential pressure switch for an exhaust application that only needs to determine whether there is airflow or not (no airflow signals an alarm). The low pressure input will be in the duct (perpendicular to airflow), the high pressure input will be ambient. Where I'm measuring is within the room the air is being exhausted from and has 250 cfm through an 8"x8" duct about 5' upstream from an exhaust grille.

What I'm having trouble with is what my actual operating differential is in order to select the proper model of switch. So far I have:

Using Bernoulli's equation,
P1 + 1/2pV^2 + pgh = P2 + 1/2pV^2 + pgh
P1 - P2 = 1/2pV^2 = (0.5)*(1.204 kg/m^3)*(3.13 m/s)^2 = 1.88pa = 0.0076” w.c.

The lowest range on the switch is 0.03-0.10" w.c. which is above my calculated differential.

I'm wondering if the negative static pressure across the grille should be added to the calculated pressure differential? The fan upstream of my location is pulling air, the grille is resisting, so am I correct to assume that this will reduce the pressure further in the duct? Or does the fact that the fan is overcoming this static mean I should ignore it? The grille static pressure is 0.059" w.c. which would put me at 0.0666" w.c., and land within the lowest sensor range.

I can't find anything online explicitly helping with this issue so any advice is very much appreciated.

Thanks in advance,


RE: Differential Pressure Switch - Range Calculation

When differential pressure is used as a proxy for flow the measurement is usually done across "something" with a substantial pressure drop. To avoid the problem that you have discovered.

The "something" might be a filter, a coil, a grille, an orifice or the fan.

Alternately, a "sail switch" might be a better choice.

RE: Differential Pressure Switch - Range Calculation

Thanks for the reply Mint.

I didn't work on this project prior to this so I can't say for certain the methodology behind the pressure differential switch selection, considering there is a sail switch downstream of the fan. I will confirm.

But aside from that - do you have any input on the problem I outlined? Mostly for my curiosity and sanity because I'm still not 100% certain of whether or not the grille static would be included and therefore what setting would be chosen if the pressure switch was to be used.

RE: Differential Pressure Switch - Range Calculation

GGriffin, I think you might be using “upstream” incorrectly. But I understand your meaning. For exhaust, closer to the fan from the exhausted space is downstream from the exhausted space, not upstream, in literal terms (not to nitpick).

Your calculation is probably pretty close to reality. Trouble is, it’s a horrible place to put a pressure switch to prove fan operation.

If you need to prove operation for this fan, I would suggest that you relocate the proving switch to the immediate fan suction, rather than a few feet away from your exhaust register. At the fan suction, you might find a sturdy inlet pressure of minus ~.5-1” w.c. versus your current minus ~.008”. At the immediate fan inlet, set its proof to -0.08” with the switch you described and you’ll be in business.

RE: Differential Pressure Switch - Range Calculation


Thanks for the reply. You're right, I meant downstream from the exhaust grille in the original post. It's an inline exhaust fan so downstream from the grille is the proposed pressure sensor, between the exhaust grille and the fan, and downstream from the fan is the proposed sail switch.

I think the location was chosen because the sensor installation details recommend it to be a certain distance from elbows/other disturbances, and the straight duct into the fan is just downstream from another grille and very close to the fan inlet. But I will likely be looking into other options now.

You mention the calculation is likely correct with the pressure being ~-0.008". I'm still wondering why the negative static pressure from the grille isn't included in this. If the duct were completely open then I could see that being the case. But alternatively, if it were completely blocked off (producing no airflow), then I would assume the pressure would read negative of the amount of static the fan is rated for. This makes me believe that because the grille is producing some resistance that this should contribute to the negative pressure sensed within the duct, landing somewhere between the fully open/fully closed conditions, plus the slight reduction in pressure based on the air velocity.

Any clarification on this, whether anecdotal or mathematical, would be very helpful.

Thanks again for the replies guys.


RE: Differential Pressure Switch - Range Calculation

I don't want to go around in circles, but the -.008 includes the exhaust register. That is not important.

You had mentioned pressure switches, not sail switches. There was a prior suggestion about sail switches. However, I’ve heard that sail switches tend to fail. Add dust or moisture leading to rust, etc…

Sail switch argument: airflow near the register will be about equal to airflow at the fan suction, barring a big duct leak. You can put these anywhere along the duct run.

Pressure switch argument: more sturdy and repeatable. But you have to locate these devices near the fan, not the space exhaust registers.

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