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I should first point out that the system I'm referring to will be a self build DIY project. Fan and Filters being the only items purchased complete, And I'm still in the design phase but I'm not a HVAC expert at all. USA has forced air more commonly than here in the UK so I'm hoping you can help me out.

This is a room air filter, It will go under the stairs where I can hopefully sound proof it.

2 or 3 stage filter setup depending on pressure drop to clean the air for a 45m3 room, intending to get 4 air changes per hour.

Fan I have selected thus far is an ERK 250 which will blow into the filters.

Each filter has an initial Pa drop of lets say 50Pa new with a replacement final pressure drop of say 200Pa. My intention is to design a circuit that will have a differential pressure sensor to measure the pressure before and after the filter to calculate the pressure drop caused by that one filter thereby allowing me to calibrate it to indicate when said filter should be changed. The same for the other filters that follow.

Will the first filter effect the reading for the 2nd filter? I can't think of a reason why it would but then air flow dynamics is a black art to me.

Will it work? Are there any commercial filter change sensors out there and how do they work?

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Is the fan before or after the filters in the plenum? Are you drawing a vacuum on the filters, or forcing air into them? – JeffG Mar 17 '12 at 0:48
Hi, forcing air into them. So Fan is before the filters. – dannix Mar 17 '12 at 0:49
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When I was doing HVAC SCADA work, compound filter systems were always treated as a single filter, i.e. the pressure drop was measured over the entire filter system as a single unit, rather than each filter in series. This might have been to keep costs down, but I imagine it was done this way, because it doesn't matter if the first filter was dirty, or the second, because in either case, an HVAC tech would have to go out to the site to replace one (and most likely both) filters. When your TV remote batteries get weak, to you only replace the worst battery or go ahead and replace both? – JeffG Mar 17 '12 at 0:56
wrt sending field engineers to replace filters I can appreciated that it's hardly cost effective to replace just the one when the other is probably close to needing replacement. And no I wouldn't replace one battery in a remote but then a battery does not cost $40 either. I expect the pre filter to fill up much faster than the fine filter. If both the pre and fine filter each ahvae a final 200Pa drop then should I say replace at 380Pa drop across the two? Is it not possible that the pre filter is heavly clogged whereas the fine not so clogged in which case its premature replacement? – dannix Mar 17 '12 at 0:59
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This technique is used to water filters. It should work fine for air filters. However, there might need to be a delay or dampening circuit to keep from setting it off when someone slams a door or opens a window on a windy day. – xpda Mar 17 '12 at 15:23
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3 Answers

up vote 1 down vote accepted

You are quite correct to note that pressure drop across an element within a dynamic fluid system is related to volumetric flow…this has certain implications.

To return to your original question, “Can I sense filter replacement need based on pressure drop?” the short answer is, not really. This is for the reason you state, that such a differential pressure value is dependent upon the volumetric flow, and if the flow is indeterminate or unknown, the value of the pressure differential does not reveal anything quantitatively as to the state of the filter element.

By way of example, a brand new filter element could indicate need of replacement if the fan speed were increased, resulting in a pressure differential increase to whatever level deemed to constitute the service limit of the filter.

The volumetric ‘nameplate’ capacity of the fan or blower purchased cannot be assumed to prevail within the target system, as the impeller speed and volumetric flow will depend upon many variables, not least those of the filters employed.

In commercial and industrial scale systems incorporating multiple elements, pressure differential measurements prevailing across individual elements may be monitored in addition to volumetric flow, which is controlled at specified process levels, as well motor energy required to sustain process levels, and these parameters and other cost factors integrated in calculations to arrive at maintenance schedules yielding least cost operation.

In a domestic DIY situation, the first and second filters could be changed alternately to determine which one, or whether both require replacement to restore desired system performance.

‘Filter capacity’ has been reached when the filter(s) in whatever combination do no longer yield the desired performance (basically CFM/min) which can be quantitatively determined. I hope this explanation will be of assistance.

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First, thanks for the detailed answers! Basically I could have a sensor setup as described above across each filter only in addition to a volumetric flow sensor. Once flow is determined the loaded max Pa drop of the filter can be programmed in based on that flow rate. This does allow me to move onto fan speed control later without modification. That being said I'm getting the impression you certainly know more than most but feel it's overkill. I have doubts whether to bother with the whole thing now :-( +1 – dannix Apr 4 '12 at 21:13
up vote 5 down vote

Since the purpose of replacing filters in the first place is to restore airflow which has been diminished by reduced filter porosity, it would be a better approach to use a differential sensor to monitor airflow, rather than a differential across the filter(s)

In a ‘passive’ system with no blower control, the filters are simply replaced when the flow has reached some minimum acceptable level.

In an active system, energy to the blower is regulated in a control loop. The desired operating volumetric flow is first established, and this forms the reference for two control set points for the loop. When the flow reaches the lower set point energy to the motor is increased to restore specified airflow, and decreased when the upper set point is reached. Filter replacement is indicated when the blower can no longer sustain the required flow.

An example of how a differential sensor is applied in deriving volumetric flow can be found here: http://www.bapihvac.com/CatalogPDFs/I_App_Notes/Determining_Air_Flow_CFM.pdf

I have just completed a design applying this principal in extending the service life of HEPA filters in a forensic laboratory environment.

I also used the Freescale MPX2010DP, coupled to a PIC16F818, which has just enough pins to drive the 4 seven segment display digits, scan three buttons (START/STOP, and set point UP, and set point DOWN) and implement PWM control on the motor. The sensor requires to feed into an instrumentation amplifier (fabricated from the ubiquitous LM324) as the full scale voltage output differential is only 0.4V, and the ADC on the microprocessor requires an order of magnitude greater than this to realise adequate resolution.

The two air duct probes, one for total pressure and one for static pressure, which connect to the pressure and vacuum sides of the sensor respectively could easily be fabricated DIY from thin copper, aluminium or other soft metal tubing, or they may be purchased online for very little.

The cost of materials for this type of project would be trivial, the question is whether one would enjoy and derive satisfaction from the substantial effort it will require.

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+1 for the last sentence! – TomG Apr 2 '12 at 1:58
Monitoring airflow alone wouldn't indicate whether the pre filter needs replacement prior to the fine filter. I'd come across that PDF myself while researching. It seems that you monitored air flow in your application, whereas I'm monitoring pressure drop directly. Are they not proportionate to each other? ie as pressure difference increases, so does air flow. I wish to determine not when the system is at capacity but when each individual filter is. Re effort required; This isn't the first project with a µp arduino.cc/forum/index.php/topic,71701.msg567538.html#msg567538 – dannix Apr 4 '12 at 0:34
up vote 3 down vote

You can use a pressure differential sensor like this one. Perhaps install a vacuum guage between each filter stage to measure the restriction of each filter.

If you need to adjust the diferential and your skill level permits, you can build a circuit around this sensor. I figure a differential amplifier with a relay driver to a spst relay giving dry contacts for an output. You could use two circuits, one between filter stages, the other on the output to determine filter condition.

I see from your profile you are a programmer. It would be a trivial task to program a microprocessor to do this task. Maybe an Arduino platform. It has several ADC's, and if you need an output control you can add a relay driver shield. If you go Netduino you can network the whole thing. By using a µp you can change the program as you go, set warning levels, etc...

See these Other sensors

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If you go Arduiuno, there are some knowlegeable folks over at electronics.stackexchange.com – TomG Mar 17 '12 at 14:02
I ordered some sample Freescale MPX2010DP 0 - 10KPa and I do have Arduino already which I was intending to use for this and later some kind of Indoor Air Quality sensor but one thing at a time! I'm a web programmer really. I was an Apprentice trained Elec Tech so my electronics is limited. But I like to bite a big chuck of the challenge. I'd +1 but I can't yet – dannix Mar 17 '12 at 22:07
@TomG added to my account, I'll need that later - gotta love StackExchange!! – dannix Mar 17 '12 at 22:19

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