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Step 2 - Hook up pressure sensors

A project log for When should I replace my A/C air filter?

How can I spend >$100 and >100 hours to save $1 a year on air filters? They say to replace them every month, but is that really true?

Tim RightnourTim Rightnour 08/14/2019 at 01:510 Comments

Step 2 is the hardware portion of our experiment.  We need 2 pressure sensors, set up on either side of the filter. 

For this, I selected a PI Zero (because it was easy to program), and a pair of BME680 sensors.  They claimed really good accuracy on the barometric side, and to boot, they had these neat "air quality" probes, which, I figured might be interesting at least.

The Adafruit ones are nice, because they have the ability to change the i2c address by grounding one of the contacts.  This means I can run both on one Pi.


Essentially, without needing a ton of details here, I hooked both sensors up to the pi, with about 8" leads on them, and powered the whole thing up.  Then it was a matter of writing the software.

The code to read the sensors is really easy to write in python. Just install the bme680 library from pip, and follow the simple example.  Where it all gets kinda fiddly is the gas sensor.

Basically, the gas sensor reads some kind of VOC data.  If you put a sharpie near it, it immediately registers.  But it's super unclear what it's actually picking up.  The sensor itself reads in Ohms.  They include a library that will convert it into some kind of magical "Air Quality" value.  However, looking at how it calculates this, part of the formula involves the humidity.  They consider "dry" or "wet" to be bad air quality.  This might be true for comfort, but for my use, this is not helpful at all. Therefore, I plan to just read the raw ohms values and store those.

For the curious, I didn't just write a simple gather and store program.  For many years, I've been writing my own Home Automation software.  It's called "Gnhast" (Garbled's Nasty Home Automation Scripting Tools).  I integrated these sensors right into the network of stuff feeding my data.  I also feed all the data from my HA software into Influx, so I can make pretty graphs with Grafana.  (I just upgraded to that from RRD, which I used for many years).

So not only do I have things like the Venstar Thermostat wired in and feeding data, but I also have a Brultech GEM that feeds watt usage data for every circuit in the house into this thing.  That means I can pull all kinds of data together, and try to really figure this problem out.

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