Demand Control Ventilation using CO2 Sensors

[Grizzly68]

I'd like to get some feedback from folks on the use of CO2 sensors to increase outside air delivery to a building. I know the theory is romantic but is the application effective? I keep hearing negative comments about the sensors going out of calibration, long response times, etc. What feedback do you guys have?
Thanks
Grizzly

 

[ChasBean1]

Grizzly,

Most CO2 sensors use non-dispersive ifrared (NDIR) detection. From my own experience, these sensors are significantly affected by barometric pressure. NDIR sensors that I calibrated in Reno at 1,000 ppm all read 1,300-1,400 ppm when we got to Stockton, some reduced (3,000-4,000 ft) altitude later. Imagine what a 35,000 ft altitude plane ride does.

They need to be zeroed and spanned after installation - forget the factory calibration.

From there, I would recommend running a zero and approx. 1,000 span gas at least monthly through the sensors, adjusting the zero and span setscrews as needed.

Assuming you can maintain this type of regimen, CO2 sensor use can be effective for areas with intermittent, heavy occupancy. This includes classrooms, theaters, cafeterias, gyms, and auditoriums. I would not use CO2 control in high air exchange buildings such as labs and hospitals, because the levels wouldn't typically exceed 200-300 above ambient.

I hope some of this helps. Best regards, -CB

 

[DRWeig]

Hi Guys,

Chasbean identified a common problem. Calibration is very dependent on pressure (altitude). The lower the pressure, the fewer atoms of CO2 in the optical chamber, thus the ppm reading will be lower at higher altitudes and higher at lower altitudes. The more feature-packed units have a simple altitude adjustment nowadays, though, so it's easier to compensate.

Most manufacturers are offering three-to-five year calibration guarantees.

If you have a variable occupancy, I'd go for it.

DRWeig

 

[imok2]

As all of the comments are basically true there seems to be a solution to the problem of calibration and aging. The "duel beam" on NDIR is one method of compensating for changes to the sensor optics resulting from aging and partical buildup. As far as drift goes an approach is to have the sensor calibrate itself on a nightly basis when the space is unoccupied, and inside levels drop to base line outside levels. Studies have determined over 3 years of of operation of 3 sensors using a nightly automativ baseline calibration that sensor accuracy remained well within 50ppmover the duration of the test.ASHRAE JOURNAL, 2/2001 Mike Schell, Dan Int-Hout.

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