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  1. #1
    Join Date
    Jun 2008
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    128

    CO2 sensor auto calibration

    I'm looking at CO2 sensors, in particular the Honeywell C7232B1006. All of them seem to have an auto calibration feature the checks against a baseline to detect drift. The honeywell manual specifies that the sensor must get at least one 4 hour unoccupied period per week. The issue would be that I doubt a duct mounted sensor would ever get 4 hours of "outside" air. How does someone accomplish this?? Even if the building was unoccupied for 4 hours, I don't think the CO2 levels would return to 400ppm. This is a tight house. Thanks.

  2. #2
    Join Date
    Aug 2003
    Posts
    906
    I agree, there is a contradiction between the calibration needs, and the purpose of the device.
    The Automatic Background Calibration makes these devices problematic. They need complete venting of CO2 in excess of outside levels once in a while, and there's the rub with the idea of ventilating only when the building is occupied. The most likely outcome is that the calibration will drift in the direction of underestimating the quantity of CO2. Complete venting is unlikely due to the exponential decay, and the decreased ventilation when the building is unoccupied.

    I turned off the ABC feature on my portable meter. Once in a while I run the sensor outside to check how much it has drifted, which I can correct. Having it hard-wired and mounted doesn't allow that. I imagine you'd need to somehow schedule a full ventilation period with a timer of some sort, or keep a basic level of ventilation going at all times so that CO2 concentrations will likely drift back close to 400 ppm at some point. This somewhat defeats the purpose of this system in the first place. I, too, am curious about what other people think and do in practice.
    -If you won't turn it on then nothing else matters.

  3. #3
    Join Date
    Jun 2008
    Posts
    128
    I would be interested to hear from anyone with real world experience with these sensors. How much drift to you find in your portable meter? If we are only talking a few ppm per month, then I can always build up a portable 24v source and take the meter outdoors twice a year to recalibrate.

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,064
    The setting of the on/off is adjustable. Most of the meters I use do not see 450 PPm and read a little high. I use the controls for ventilation when occupied. I set them at 600-700 ppm depending on the size of the space. I have yet to need to calibrate a meter.
    One air change when unoccupied will achive 70% outside air typically.
    Of course when the wind blows +10 mph, you will get an high air change rate. Even tight buildings will change in 2-3 hours. These meters typically on increase or decrease 50 ppm per auto recalibrate.
    CO2 level is a good sensor for occupancy and a lack of natural/mechancial ventilation.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #5
    Join Date
    Jun 2008
    Posts
    128
    Also, I know the Honeywell unit isn't the only sensor on the market... If anyone has used these systems before in residential ventilation, are there any other manufacturors that require a look? Ive read some info on BAPI VOC sensors and they seem to look good. One of the nice things with the Honeywell is the built in SPST relay to trigger the ERV, BAPI doesn't seem to have it.

  6. #6
    Join Date
    Jun 2008
    Posts
    128
    Quote Originally Posted by teddy bear View Post
    The setting of the on/off is adjustable. Most of the meters I use do not see 450 PPm and read a little high. I use the controls for ventilation when occupied. I set them at 600-700 ppm depending on the size of the space. I have yet to need to calibrate a meter.
    One air change when unoccupied will achive 70% outside air typically.
    Of course when the wind blows +10 mph, you will get an high air change rate. Even tight buildings will change in 2-3 hours. These meters typically on increase or decrease 50 ppm per auto recalibrate.
    CO2 level is a good sensor for occupancy and a lack of natural/mechancial ventilation.
    Regards TB
    The honeywell has default set point of 800ppm on, 700 ppm off. I think that would fit most needs.

    What type of controls do you use for ventilation?

  7. #7
    Join Date
    Jun 2008
    Posts
    128
    Response from Honeywell:

    Here is the response to the question from the product manager.

    It looks for 4 straight hours of unoccupied levels (about 400ppm) per 8 days. They could take it outside every couple months for 4 hours, running, and it should maintain very well. Otherwise, it will slowly drift based on whatever the lowest level of CO2 is in the space per week, thinking that is roughly 400ppm. There is a limit to how much it will adjust each week.

  8. #8
    Join Date
    Jun 2008
    Posts
    128
    I did find another sensor out there from Tongdy (Chinese) that looks like it would be perfect. They say the sensor will be fine and does not require outside air to stay calibrated. TKG-CO2-2010C

  9. #9
    Check out SenseAir Co2 sensors as well.

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