Differential Pressure Switch to Confirm Blower Air Flow


Thread Starter



I would like to use a differential pressure switch to confirm that a blower is moving air. Please correct me if I am going about this the wrong way, but my plan would be to pipe the positive port on the DP switch into the duct at the blower outlet and to pipe the negative port on the DP switch into the duct at the blower inlet. I'd like to do whatever I can to get the most reliable operation, even at low air flows.

With this in mind, I was wondering what type of static pressure tips or sensors I should mount in my ducts. Looking in a Dwyer catalog, they have what they call "Static Pressure Tips" (part numbers A-489, A-491, & A-493) that look basically like a straight piece of metal tubing that when inserted into the duct would be perpendicular to the air flow direction. (From the picture on Dwyer's website, it appears that the end of these "Static Pressure Tips" is closed, and I don't see any openings along its length, but I cannot guarantee this to be true). Dwyer also has what they call "Static Pressure Sensors" (part numbers A-301, A-302, & A-303) that are shaped in a right angle that would allow me to "aim" the sensor either into or away from the direction of my air flow. These Static Pressure Sensors also have radially drilled sensing holes. I was wondering if these features (or anything else) make these "Static Pressure Sensors" better for my application than the straight "Static Pressure Tip" I described earlier.

If anyone can make a recommendation as to what type of tip or sensor would work best in my application, and if you recommend right-angle style sensors, if you could also recommend how I should aim them, I would greatly appreciate it. I do not have to use Dwyer components if something else will work better - Dwyer just happened to be the first place I looked.

Thanks and best regards,
I don't think it really matters what shape the tips are. Measuring from blower inlet to outlet, the DP will be quite significant. I have often seen just a fitting in the duct wall. If you were trying to measure the flow at one point in a duct, that's when you need the special probes pointing upstream / downstream.

Make sure the tubes don't get plugged by dust buildup (slope tube down towards the duct with the switch at the high point).

To get a feel for how well it will work make a simple U tube manometer from plastic hose with some water.

Hope this helps.

Curt Wuollet

Pitot tubes are generally best at significant airflows. For low airflows, the various heating devices, thermistors, etc. offer good sensitivity. Just hooking a DP sensor from output to input on a blower won't tell you much directly about airflow, there is pressure regardless of flow if the blower is running. Google pitot tube for the ways they are used. This does seem to be a gap in modern instrumentation. We have excellent sensors for pressure and most other physical quantities, but airflow by it's nature, is still hard to measure. And the assumptions you must make don't bode well for accuracy. For simple flow confirmation. I like sail switches, crude but reasonably reliable.

Curt said "Just hooking a DP sensor from output to input on a blower won't tell you much directly about airflow, there is pressure regardless of flow if the blower is running."

Curt brings up a good point, in a situation where there is a back-pressure such as blowing air into a tank filled with water pressure is no guarantee of flow. for a simple application such as a blower providing motor cooling where pressure creates flow though it's pretty reliable.

One simple idea I have used to prove a low flow rate is to place a small AC cooling fan in the air duct, the air flow spins the fan creating an AC waveform with frequency proportional to flow rate. With a F/I converter into a PLC this makes a fail safe switch.

I don't have a lot of faith in flow switches where they are closed 24/7, they tend to stick. If the blower is not running logic should alarm if the switch is not open.


Hi Roy and Curt,

Thanks for your responses. I use a supply blower to pull air from outside of the building. After my supply blowers, the air moves through an electric heater followed by two filters before it is ducted to a supply plenum in a dryer. The supply plenum has adjustable air nozzles that allow the air to be directed at the product we are drying. I use an exhaust blower to remove air from the dryer and send it back outside.

The main things I would like to "catch" with my method of air flow confirmation are the following:

(1) If a blower motor was running but a broken belt results in the blower itself not turning (thus no air flow)

(2) If damper is closed upstream of my supply blower, making my supply blower unable to pull air through my heaters

(3) If a damper is closed downstream of my exhaust blower, blocking the exit path for my exhaust.

I'd be curious if you guys thought a DP switch piped between blower inlet and outlet would be able to catch these situations. Roy - your suggestion to add a fan and monitor the frequency sounds like a really good idea. Is there a typical fan that you use that is relatively friendly for mounting into a duct in an industrial situation? I have tried a Dwyer Sail Switch before, but its range only went down to 200 FPM and I am afraid some of my customers might be running less than that.

Thanks again for your help,

Curt Wuollet

Those conditions could probably be detected with a pressure transducer. 1 would of course give little or no pressure. 2 might fool you using DP, but a gauge pressure setup on the output would read very low for that one. And 3 would read higher than normal. So I'd just reference to ambient and set high and low limits above and below your normal static pressure range at the outlet. By gauge pressure, I mean open the low port to the room.

In this specific situation, where there is normally always some flow, pressure could work.

You might be able to just watch the air temp near the heaters, low flow would produce an increase in temp.



Hi Curt,

Thanks for your response.

You mentioned that if the damper upstream of my supply blower was closed, this would result in a low gauge pressure at the blower output. If my drive belt was broken (so the motor was running but the blower was not), I believe this would also have a low gauge pressure at the blower output. This sounds good so far - if I monitor gauge pressure (with low port open to room), I should be able to catch both situations.

The only problem is, I have an existing machine where I am using a Dwyer 1910-00 differential pressure switch (range 0.07 - 0.15 in wc) with the high port plumbed to the blower output and the low port open to the room. This has actually been my standard method for air flow confirmation for many years and it has worked very well for me. On this particular machine, however, my customer has been having problems due to the switch de-activating even though the blower is running, the damper is open, and the switch activation setpoint is set as low as possible (0.07 in wc).

My only guesses as to why this customer is having difficulty where my other customers have not (considering all of our machines are basically the same) are the following:

(1) They are running their supply blower slower

(2) The are running their exhaust blower faster in relation to my supply blower.

(3) Less than ideal duct-work construction/layout allows air from outside of the building affect the pressure in our duct work. For example, on one particularly windy day, even with our blower not running, I measured 0.16 in wc between the blower outlet and the room even when our blower was not running. As this is greater than the high end of my switch range, I was not able to get the switch to de-activate on this day. I am not sure if it is possible for outside air/wind to have an effect when my blower *is* running, and if so, if it would be possible for outside air/wind to lower the pressure at my blower output, but this has crossed my mind.

Anyway, it is the issues with this particular customer that made me consider plumbing the low port of the DP switch to the blower inlet. My reasoning is that this should increase my measured DP (compared to when I had the low port open to the room), thus making it easier for me to get above the low range of my DP switch (0.07 in wc). Also, I hope this might at least partially cancel the effects of outside air/wind as I would think the effect of outside air/wind would be similar at the blower inlet and outlet (whereas outside air/wind would have no effect on the room).

If you think that monitoring DP between inlet/outlet might make it difficult for me to detect a situation where the damper was closed, I may have to consider an alternate method of air flow confirmation.

Roy's idea of mounting a fan an AC cooling fan in the duct work sounds intriguing, but depending on where I mount it (before or after my heaters), I would have to make sure it can withstand the temperatures I will expose it to. If I use this method for my exhaust, I will also have to make sure it can withstand the solvent vapors that I will expose it to. If that creates any headaches, I have found an FCI air flow switch that uses thermal technology that seems like it can handle my temperatures, solvents, etc. It is more expensive and it requires power (whereas my air pressure switch does not), but if it can do the job reliably, it might be the way to go.

Thanks again for your help - I really appreciate it.



Thanks Joe,

I do monitor DP across the filters so that I can generate an alarm when the filters become clogged. Unfortunately, I can't use this for air flow confirmation because I am required to confirm air flow even if the filters are not installed.

I appreciate your help.

Best regards,
If you want to prove air flow in any situation you will need to measure the flow directly.

Thermal flowmeters work well provided there is no entrained droplets as a tiny drop will seem like a huge flow of air.

The sort of fan I have used is the inexpensive AC powered saucer fan as used in cabinet ventilation, they require very little air flow to make them spin, without being powered up they should last a very long time (no heat from the windings). From memory a 120 VAC fan puts out a nice sinewave about 10 V P-P. You can measure the sinewave with a frequency to Voltage transducer (frequency proportional to flow rate) or perhaps just an AC Voltage relay. Flow is in series so the fan can be in any part of the circuit, blower suction for example.

Have you thought about monitoring the heater temperature, after all that's how a simple thermal flowmeter works, flow cools down the heater element, restrict the flow and the heater temperature will rise.
One way to solve that might be to make a restriction that keeps the blower outlet pressure over .07 in WC.

But if the building pressure varies a lot or is way out of whack, it might be hard to use ambient. This leaves the classic, use a restriction and connect your DP sensor to the upstream and the downstream of the restriction. That DP _is_ proportional to flow and immune to ambient pressure changes. I like when things go in circles. This is more often used with liquids.

Thanks for the info Roy,

The only thing a little tricky for me for using a cabinet ventilation type fan is that my duct work is usually round. My blower inlets are round so I don't have a flat spot where I could mount that type of fan on the way in.

My blower outlets are rectangular, so that gives me a chance. For my supply blowers, my blower outlet goes to a transition piece (with flat surfaces) that increases the rectangle size to match the size of my electric heater(s). On my exhaust blower outlet, I have a short transition piece that that goes from rectangle to round. I'll have to check whether this piece has a large enough flat surface for me to mount a fan. My only concern is that my exhaust air flow is hot (possibly up to 150 deg C) and can contain solvent vapors. I would be concerned about either damaging the fan allowing solvent from my exhaust to enter the room through the fan instead of going where it is supposed to go.

I do monitor the temperature just after my heaters for the purpose of over temperature alarms, and you are right, blocked air flow does cause this temperature to rise and trigger my over temp alarm. If I can get it to work reliably, I would like to find a method of air flow confirmation that is independent of my over temperature alarms.

Thanks again for your help,
Hi Curt,

Thanks for your response. If I wanted to create a restriction in a 12 inch diameter round duct, do you think I would I have to decrease the diameter of the whole duct? Or do you think it is possible for me to put some sort of "insert" in the duct with an opening of say 2 inches on one side that drops down to one inch on the other side? If I did this, most of my air flow would go around the restriction, but hopefully enough would go through the restriction to satisfy my air flow confirmation needs. If this could work (especially for existing machines where the duct work is already built and installed), I think creating an insert would be easier than stepping down the whole duct.

Thanks again for your help,

If you have an elbow, you might be able to use a differential pressure switch across the elbow. You might also use a Dwyer Series DS-300 annubar (averaging pitot tube) with a differential pressure switch (see link below). You may not meet the straight run requirements for accurate flow measurement but for a flow/no flow indication, you should be able get a repeatable value for flow (assuming it is relatively constant), which you could calibrate your D/P flow switch to for a flow/no flow indication.



William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.
> The only thing a little tricky for me for using a cabinet
> ventilation type fan is that my duct work is usually round.
> My blower inlets are round so I don't have a flat spot where
> I could mount that type of fan on the way in.

The fan wouldn't have to completely fill or "block" the duct. As long as it's in the air stream, it should work as a flow confirmation. If you were trying to measure the actual air flow, it would matter, but probably not here. You could put it well away from the heat and solvents.

> If I can get it to work
> reliably, I would like to find a method of air flow
> confirmation that is independent of my over temperature alarms.

I agree with this point as well. It's good to get other confirmation, especially since other things also affect the temperature.
Thanks Bill,

Looking at the bulletin for the series DS-300 Flow Sensors, I'd like to get a feel for what is the minimum air flow that would allow me to reliably generate enough DP to make my 1910-00 DP switch (with a range of 0.07- 0.15 in wc) change states. The bulletin has a formula for calculating DP for a particular flow if you know the flow coefficient for your pipe size, your temperature, the specific gravity for the gas you are moving, the diameter of your pipe, and the static pressure in your pipe.

The only piece of information I don't really know is the static pressure. Do you know if there is a way I can estimate this without actually measuring it? Considering I am using a variable speed drive with my supply blower to feed air through heaters, filters, ductwork, and then adjustable air nozzles into a drying chamber, and I am using a variable speed drive with my exhaust blower to pull air out of this drying chamber, I imagine the actual static pressure at any given location would be the result of a complex interaction of the exhaust blower and supply blower and all the ductwork, filters, restrictions in between them.

The reason I am concerned is I think my customers operate at very low flow rates (possibly less than 200 feet per minute. I am just afraid that this might not create enough DP across the DS-300 to make my DP switch change states.

Thanks again for your help.

best regards,
Thanks for your response Joe,

I was envisioning mounting the fan on the side of the duct (just as you would mount it if it was on the side of a cabinet). This would make the fan orientation parallel to the air flow and air, dust, etc might be able to pass through the fan between the room and the duct (and vice versa).

Do you envision putting the fan inside the duct, perpendicular to the air flow?

I'd be curious what Roy says as well, being he was the first one to mention using a fan like this for blower confirmation.

Thanks again,
I had envisioned his suggestion to be mounting the fan inside the duct as if it were going to blow air through the duct and just measuring the voltage at the leads. He may have had something else in mind, though.