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Old 02-07-13, 12:17 AM   #23
theworldtrekker
Lurking Renovator
 
Join Date: Oct 2012
Location: Corvallis, Oregon
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Quote:
Originally Posted by AC_Hacker View Post
I've recorded CO2 today and humidity, and RH is not moving very much... nothing like CO2. The CO2 readings I'm getting do seem to be reasonable, and when two people are here, the readings seem to make sense.
Looking over the plot again I'm still not convinced you're measuring what you think you are, or else I don't understand what you're measuring. Assuming that you are in a constant volume room, exhaling a reasonably steady flow of CO2 (steady source), then I'd assume at these concentrations to see a approx linear line of increasing CO2 over time. What I see however, is a variety of different rates and unexplained drops.

Why, if you were in the house all night do you start out at ~440ppm yet end the day at 1050ppm? Why is the rate increase different between data point 2-3 and 5-6 vs 3-5 and 6-9? Why the drastic drop while you're gone until 1330, but not the same slope of drop when you go to the neighbor with soup? And finally, why when there are two people in the house is the slope of increase less than between data points 2-3 and 5-6 but the same as when you got back at 1300 until 1930?

Here are a few hypothesizes that come to mind:
1) You're measuring CO2, but CO2 in your house isn't driven by human sources. Instead it is driven by construction materials still curing. I know this plagued Biosphere II for a number of years with curing concrete (if my memory serves me).

2) You're sensor is temperature dependent. Its optical in nature and either the LED or more likely the sensing cell is temperature dependent. You've ruled out humidity it sounds like, but I would have guessed temperature is more likely to dominate anyway. Could also be some other factor that is affecting the sensor (supply voltage, saturation, location, etc).

Need more data points... Its not too hard to hook up a SD card to an arduino (SPI interface) and have it log every ~5 min. Either buy a SD shield or OSH Park ~ Welcome (from dorkbotpdx)has a nice cheap PCB service 'local' to us.

Quote:
Originally Posted by AC_Hacker View Post
Do you know how to do this?
...
I do think that the digital approach is more accurate, but I don't have the savvy or the pre-made Arduino library to make it happen.
Go with the analog approach then. I would think this type of discussion would exceed the limit of this type of forum. There is a big difference between getting the led (pin 13) to blink vs creating libraries. I know various different places have tutorials on how to write arduino libraries, but unless you're comfortable with C I'd recommend avoiding that path.

I don't know why you'd look at a PID unless you're trying to track a target value. A bang-bang controller should work for this type of scenario and be much easier to implement. The logic would be, if > y1 then fan speed = high; if < y0 then fan speed = low, else fan speed = medium. Or you could implement a proportional controller if you want it a little more continuous in fan speed variation, but leave out the integral and derivative part of the controller.

I would guess, as stevehull pointed out and I've observed in my house, that humidity will be a greater issue than CO2. So even with a CO2 sensor feed on the controller, you'll also want a humidity sensor feed as well. I'd worry more about the logic of the two combined sensors than trying to accurately track one value. If the house and sensors don't dampen the system enough, it'd be pretty easy to program some basic dampening logic into the fan controller.
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