Differential Pressure Sensor

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I'm just thinking about a project and looking for ideas, I want to add a DPS across my furnace filter to set up an alarm in my home automation system when it needs replacing, currently my thermostat uses a fixed time schedule for filter replacement. 

If anyone has done this and has suggestions on a sensor I'm all ears. 

TIA

Jim

 

 

 

 

 

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There are issues doing it that way, since pressure is complex related to airflow.  Instead, they more often monitor the reduction in airflow at the output, compared to a baseline (clean filter) flow level.   Whe airflow drops below a certain CFM rate, you know the filter is clogged up.

 

http://2avrmz2nom8p47cc28p2743e-wpengine.netdna-ssl.com/wp-content/uploads/2010/07/Determining-Air-Flow-in-CFM.pd

 

I had a friend who had bought & lived in a 50's era for a few years.  He was complaining that the house was cold & the furnace barely put out any heat & it had been that way each winter.   I asked if he changed his filter & he said he looked & their furnace did not have one, probably since the house was fairly old.  I told him that made no sense, my grandparents house from the 1920's even had a furnace filter.  A week or so later he called & said he noticed a "panel" in the hallway ceiling that was painted over.  He pried it open and there was a filter with a foot (or more!) of dirt accumulated over years and years since whoever painted over it !!       

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. Apr 14, 2020 - 05:52 PM
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One possible solution: https://www.amazon.com/Breakout-...

Still looking for others, would like to find a digital interface (I2C, SPI).

Diff pressure is how it's done in commercial buildings to monitor filters, so should work for me, but I'll have to experiment to find a proper alarm level, and to complicate things, the furnace has a variable speed fan!

Never said it would be easy...

Jim

 

 

 

 

 

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Diff pressure is how it's done in commercial buildings to monitor filters,

Are you sure?...that wasn't exactly what I was finding, but I'm half guessing.  I know that someone sold a filter whistle that would sound when the filter got clogged (haunted vents!).

 

 

https://www.amazon.com/FILTERSCAN-Furnace-Filter-Monitor-Alerts/dp/B013EVUESE

Although the CleanAlert FILTERSCAN Air Filter Monitor may be installed upstream, downstream, or differentially across the air filter (using an available tubing kit - see below), the best results come from installing downstream of the air filter (that is, between the air filter and the system blower fan, usually located at the return duct of the HVAC system).

  

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Yes, I'm sure, see diagram above from a commercial air handler, the first element above is the filter with diff pressure sensor across it.

 

 

 

 

 

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Funny you should ask.

 

My most recent project uses a Bosch BME280 Absolute Pressure, Temperature, and Relative Humidity sensor.

(Not to be confused with the BMP280, don't ask me how I now know that!)

 

Although it is an absolute pressure sensor, not a differential pressure sensor, you could easily place one on both the high side and the low side of the filter instead of routing pipes from each side to a single sensor.

 

You would, as a bonus, get your HVAC output Temp and humidity, as well as the values for the return air supply.

 

Know that it is a 3V (Max) sensor, but there are many breakout PCBs in several variants.

Many have a dinky tiny 3V regulator on the PCB, as well as I2C 5V-3V level converters.

Some are 4-Pin, some are 6-Pin, (and include a few extra signals).

 

Photo is of a purple BME280, 5V, 4-Pin breakout PCB installed on a Nano breakout PCB, (Mega328P, 16 MHz, 5V).

(The Nano and an external (unnecessary) 7805 are on the back side of the PCB).

 

If you go this route, get the breakouts with the 6-Pins, so that you can set the I2C address of the two sensors to different addresses.

 

The BME280 is the little square silver chip in the upper right hand corner of the little purple breakout PCB.

(Not something I even wanted to consider hand soldering...)

 

JC

 

Edit: Typo

 

 

 

 

Last Edited: Tue. Apr 14, 2020 - 08:09 PM
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Hi Jay.

 

You got me! I went looking on eBay for the " Nano breakout PCB " and could not find anything like yours so decided to blow up the image to see if there were any identifying marks/text. Yep... there it was in full view... "Carter". Your own design!

 

Cheers,

 

Ross

Ross McKenzie ValuSoft Melbourne Australia

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http://www.automatedbuildings.com/news/mar04/articles/pati/pati.htm

 

The commonly accepted industry "rule of thumb" is that a filter is ready for replacement once it has reached twice the initial pressure drop across the filter bank. In practice, achieving a doubling of the differential pressure is problematical. The statement is true when the air velocity remains constant for both the clean and the dirty filter measurements. However, in constant volume system (constant motor speed) the fan often begins to push less air as the filter becomes impeded. As the obstruction increases there is no commensurate increase in pressure. Thus in the worst case, achieving a recommended doubling of the pressure many not be reached until the system becomes excessively impeded/obstructed resulting in system not functioning as designed. Similarly, in a variable speed fan system using differential pressure to monitor filter impedance can lead to inaccurate results. As the velocities are changed the pressure also changes resulting in erroneous measurement of the filter status.

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!