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Old 02-06-13, 07:50 PM   #21
theworldtrekker
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Not sure what sensor you were using for the plot, but I'd be careful to make sure you're not tracking something else like temperature or humidity with the sensor. Reading through the minimal spec sheet with it, it claims to need 14 days to calibrate the ABC system--I would assume uncalibrated from All Electronics by default. This also assumes that the unit sees 400ppm at least 3 times per day for calibration.

Specified accuracy is after re-zeroing process
or 14 days of continuous operation with ABC Logic.


As far as building the system, I'd try to find out the necessary protocal to interface with it over a SPI bus. You have one per arduino chip (assuming not a mega) but can select which device you want to interface with via a cs pin if you're also wanting to drive say an lcd.

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Old 02-06-13, 09:35 PM   #22
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Quote:
Originally Posted by theworldtrekker View Post
Not sure what sensor you were using for the plot, but I'd be careful to make sure you're not tracking something else like temperature or humidity with the sensor.
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.

I think that one thing that makes the reading seem anomalous is that my house is small, only about 700 sq. ft. and I'm mostly living in and only conditioning a part (25%) of that area. So the respiration of one person makes a big difference, to say nothing of more than one person... which is why I want to build this HRV.

Quote:
Originally Posted by theworldtrekker View Post
As far as building the system, I'd try to find out the necessary protocal to interface with it over a SPI bus. You have one per arduino chip (assuming not a mega) but can select which device you want to interface with via a cs pin if you're also wanting to drive say an lcd.
This sounds like a good approach.

Do you know how to do this?

Here is some guy who was able to reverse engineer the digital interface to the sensor. He was going to build an Arduino library for this particular sensor, but he says that his computer crashed and he lost his work.



...bummer.

I don't have a logic analyzer or any experience in using one, even if I did have one.

Pin 4 of the CO2 sensor is analog out, 0 volts to 4 volts, with 0v. = 0 ppm and 4v. = 2000 ppm, linear. I thought I'd run that into an A-to-D in on the Arduino. So, I think I can figure my way through this approach.

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.

I already got a couple of fans that are low current, quiet, permanent magnet ECM fans. They should do the job nicely. They are 12 V and only draw 650 ma each.


Seems to me that it will be pretty straight forward to use the data from the Arduino's A-to-D and use it to control the duty cycle of a PWM going through a transistor to the fans.

I'll have to do some experimenting when it's all together to adjust the rate.

This might even be a job for PID.

So, I know a bit about Arduino programming, actually a very little bit.

If you have any suggestions, I'd like to hear.

Oh, and here's the HRV core I got:


And I even kind of have plans for the box:


So right now, I'm in the CO2 controller design stage of the project.

I tried to buy a controller from the company in Ireland that makes these things, but their controller only works on 220v 50Hz.


Best,

-AC
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Old 02-07-13, 01:17 AM   #23
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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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Old 02-07-13, 02:32 AM   #24
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Quote:
Originally Posted by theworldtrekker View Post
What I see however, is a variety of different rates and unexplained drops.
I think that people assume that I live like they live. I don't.

I am only conditioning one room.

As I said previously, it is not a large room.

The kitchen is not heated and not conditioned and not subject to CO2 monitoring.

The bathroom is not heated and not conditioned and not subject to CO2 monitoring.

The room I sleep in is not heated and not conditioned and not subject to CO2 monitoring.

The big drop was because I left my house to run some errands. There was no one home to generate CO2. The CO2 started climbing higher when I returned... The smaller drop was when I went next door to visit a neighbor. CO2 climbed much higher when a friend dropped by.

I was pretty sure that I had explained all that before.

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Originally Posted by theworldtrekker View Post
Why, if you were in the house all night do you start out at ~440ppm yet end the day at 1050ppm?
See above... ("...not heated and not conditioned and not subject to CO2 monitoring...")

Quote:
Originally Posted by theworldtrekker View Post
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?
I'm logging by hand, I log when I'm home.

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Originally Posted by theworldtrekker View Post
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).
In the space that is conditioned, my construction materials have been curing for 32 years. Would that be long enough?

Quote:
Originally Posted by theworldtrekker View Post
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). [/QUOTE]

Nope, it's been 67 degrees all the time... dead on.

Quote:
Originally Posted by theworldtrekker View Post
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.
Great! I have an Arduino, I have an Ethernet shield, write me the code and I'll run it.

Quote:
Originally Posted by theworldtrekker View Post
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.
I tracked humidity all day, and it has been within 1% of 43% all day.

Have you ever measured the CO2 of the space you live in, by any chance?

-AC
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Old 02-07-13, 08:02 AM   #25
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worldtrekker,

AC has a good sensor and it doesn't respond to humidity and temperature as do many CO2 sensors. All that said, we need to look at the total volume sampled (VERY small) inside a home that is not room to room ventilated as is in more typical (larger volume) houses. In this very specific scenario, you can have the CO2 values he has measured in the specific room he is in.

AC in in a very unusual house (700 ft2) and we need to keep that in mind.

I really like the Linear LTC1041 8 pin chip for doing exactly as you say. Proportional control for an HRV (or ERV) is clearly overkill. In keeping with ACs request that we focus on controlling the HRV, I would look at the following:

http://cds.linear.com/docs/en/datasheet/1041fa.pdf

VERY easy to hook up and only uA of current to drive it. I also like the ability to sense every few minutes to further reduce current. The inherent hysteresis eliminates the on off on "chatter" than a simple bang bang switch has. The 1041 is also cheap!

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Old 02-07-13, 01:57 PM   #26
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Steve

Thanks for the chip idea, but...

Quote:
Originally Posted by stevehull View Post
...Proportional control for an HRV (or ERV) is clearly overkill...
I'm sorry Steve, but it is not 'clearly overkill' at all. In a small air volume, proportional control would be much more comfortable, and unobtrusive.

About four years ago, I graduated from an on/off gas central forced air heating system to proportional controlled (AKA: inverter technology) mini split heat pump, and the civility and comfort of a proportional system is so unquestioningly superior that I'm just not interested in going back.

Those mini splits really have set a higher bar, not only in comfort but also in economy. It's just amazing how many tiny transistors you can cram into a tiny space nowadays... and what you can do with them.

I already have an Arduino clone (Teensy) that I plan to use, and if I understand it correctly, it can function in the same way as the IC you suggested. It also has analog-to-digital built in and PCM built in, and the capacity to handle another sensor (humidity) or two.

Quote:
Originally Posted by stevehull View Post
...AC has a good sensor and it doesn't respond to humidity and temperature as do many CO2 sensors...
The sensor I'm using in my controller is just capable of measuring CO2. But I have a borrowed sensor that I have been using just to monitor my environment, and it does read temperature and humidity and CO2. The measured levels of temperature and humidity are very stable. I imagine humidity will change once I get the HRV working, and I plan to incorporate a humidity sensor into the design.


Just to help guide your thinking, the HRV core that I have is identical to the one used in THIS UNIT.

In fact I bought mine from a former US affiliate that still had one left in stock.

The favorable economic operation of this unit, and the size of the house that it is meant to service is very attractive to me, because it approximates the size of my own house. I realize that my house is on the small size... but I do plan to expand the conditioned area, but not to the whole 700 sq. ft.

So here is a description of the controller that the Irish engineers came up with... hopefully, a similar controller can be developed. I mean, they can't be that much smarter than us, can they?

Quote:
...the smart controls of the system monitor the temperatures of the 4 air-streams in the unit. The system will always run itself to maintain the highest level of efficiency while ensuring CO2 and humidity levels are regulated using sensors for each. When CO2 reaches 530ppm or humidity reaches 40%, the unit automatically slows down to trickle speed until levels become higher again. At 50m3/hour, the breathing window emits just 25dB of sound, while at high-speeds of up to 150m3/hour this increases to just 40dB. This is normally indistinguishable from normal household background noise. Breathing Window. The future of Passive House ventilation.

Cross ventilation above and beneath doors is crucial in all Passive Houses with ducted HRV, and often under-estimated at the design stage, not so with the Breathing Window. With subtle architrave ventilation detailing, threshold gaps and some ventilation of bedrooms to cross-ventilation zones, we ensure gas equalization throughout the building, with typically 2 units acting as the lungs of the house. One unit running at medium speed (100m3/hr) provides enough fresh air for 4 people (Passive House requirement are 25m3/person.hour).

Up to 15 times better heat transfer efficiency at the heat exchanger when compared to standard flat plate heat exchangers gives the breathing window higher system efficiencies.

In delivered volume comparisons, the breather window has 60% less pressure drop in the system caused by ductwork used by other units. This results in lower fan speeds, greater efficiency and greater savings for you.

Due to the small dimension of our heat exchanger, most moisture leaves the building as saturated vapour, which means less frost occurrence and less defrost cycling compared to standard units of similar efficiency. A FiWi unit tested in Iceland had a 95% lower defrost cycle rate than standard HRV units.

Standard HRV systems suffer from high thermal resistance at the exchanger surface due to the boundary air layer. As heat is exchanged by the copper wire in our FiWi exchanger, the boundary layer has little or no effect on heat exchange, increasing efficiency. Our competitors often rely on heat loss from inefficient fans to boost their efficiency figures. Our high-quality fans lose very little heat, and yet we have still measured 85% heat-exchange efficiency in an installed unit. At -10 degrees outside, a breathing window has been measured supplying fresh air of 17.5 degrees to a building at 20 degrees. Most of our competitors only achieve such results in the laboratory! We can increase this to 92% by weaving the wires closer together but the pressure drop through the heat exchanger increases canceling the gains.
So, this is the direction I am going in...

Best,

-AC
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Old 02-08-13, 07:57 PM   #27
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I have decided to go with the Teensy++ 2.0 because:
  • They are well made
  • They are small
  • They are inexpensive ($24)
  • They use the Arduino Development Environment
  • They are compatible with the world of Arduino libraries
  • They have an Analog-to-Digital input (actually eight)
  • They have a PWM output (actually six)
  • I know the guy who makes them (PJ), so I can ask him in person for help
  • I already have one in my Geek Box

So, the Teensy comes just as in the photo above. Wires can be soldered right to the board, for experimenting, but there is much more flexibility if header pins or sockets are added to the board, which will then fit perfectly into an experimenter's bread board. I have already tried an actual bread board to align the pins and I melted the plastic (sigh).

Here is a row of header pins. These can be purchased from any electronics supply house, sometimes even Radio Shack.



It's necessary to use just a little bit of care when soldering the pins, else they won't line up perfectly with the bread board. If you use a piece of experimenter's perf board, to align the pins, they will be in good shape.

Here's the Teensy with the header pins perfectly soldered to their thru-holes.


I just happen to have a microscope lying about, so I examined my soldering job.

Upon inspection, they're not all so good...


The flaws don't look so very bad, but they are indicative of insufficient heat and bad flow. Could cause hard-to-find problems down the road. A couple of touches with the soldering iron, and re-inspection, and I'm ready for the next step. [NOTE: if you'd prefer to avoid soldering on the pins, the Teensy is also available with pins already soldered on]

The next step is to go to the Arduino Download Page and download the latest stable (no betas for me, thank you) IDE (AKA: Integrated Development Environment).

For me that would be the one for Windows...


The Arduino IDE is about 91 Mega Bytes, so depending on your bandwidth, it could take a while...

This gets installed before the Teensy programs. It doesn't seem to spread itself out all over your computer like other programs, so wherre you install it is wherre it will run from. Remember where it is installed, because you'll need to find this folder again when the Teensyduino add-on files get installed.

Also required is the Teensyduino, which is an add-on to the Arduino IDE.
So I download the Teensyduino add-on... again, in my case, it is for Windows.


...this file is about 22 Mega Bytes, so it'll take a while, also. Install this after the Arduino IDE has been installed. You'll need to install them in the same folder that contains your Arduino IDE.

Next to get is the Teensy loader, which gives you a convenient way to load and try different programs.


... this one is pretty small and is quick to download. It just runs from it's location... no formal install.

Last, is a small file, LED Blink, Both Slow & Fast that actually contains two hex files, "blink_fast.hex" and "blink_slow.hext" that are test files that you can load and run to verify that your Teensy & the Integrated Development Environment is all working as it should be.


So that concludes acquiring and setting up the Development Environment for not only regular Arduino du jour but also the nifty thrifty Teensy.


Now the programming begins, and this is the part I may need some help on.

Best,

-AC
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Old 02-09-13, 12:42 PM   #28
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So I'm trying to get my Arduino chops together, and have been looking at many various tutorials.


THIS ONE that I came across is the best, most detailed, most thorough I have found.

-AC
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Old 02-10-13, 01:32 AM   #29
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Great mini-tutorial. This should get me rolling on some of my deferred projects. Just needed a bit of a kick-start.
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Old 02-10-13, 11:18 AM   #30
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Great mini-tutorial. This should get me rolling on some of my deferred projects. Just needed a bit of a kick-start.
Thanks! If you see that anything is missing or in error, please send me an EcoRenovator private message and I'll edit in the corrections/additions.

We all need a kick-start from time to time.

BTW, the guy that developed the Teensy (PJ) goes to the bi-weekly DorkbotPDX meetings, and there you will find a happy, supportive community of accomplished tech-heads, including PJ. And also, since it is Portland, a variety of great micro-brews.

Best,

-AC

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