Thursday, January 21, 2010

CO2 Sensor Placement

EMAIL: I've been curious for a while about the discrepancy between the recommendations for sampling locations expressed in ASTM D6245 and the ASHRAE and US GBC requirement for being in the breathing zone. After reviewing some of monitoring data for a nearby school, I'm finding that there are significant problems associated with this sampling location requirement, so I thought I'd pass it along.

RESPONSE: Thanks for the paper (indicating temporary CO2 spikes of over 1,000 ppm due to people exhaling near the sensor). I think CO2 sensor placement is like occupancy sensor or thermostat placement in that it must be located intelligently. For an occupancy sensor, it has to sense the people coming in but not be falsely triggered by people in the halls or miss people behind obstacles. For thermostats, they need to be close enough to the occupants to represent their conditions w/o getting direct sunlight to artificially read hot. For CO2 sensors, they need to be in the breathing zone to represent the air that people are actually breathing but not so close to people as to get artificially high readings from nearby exhalation. You always hear about stratification of air and this is why I’d want the CO2 sensor in the breathing zone “strata”. In a well mixed room, I guess it wouldn’t matter, but I don’t know how many rooms are well mixed or how you’d be sure yours was one. I’ve seen a number of engineers place them in a return air duct which is fed from a ceiling mounted grate, but in these same rooms the warm supply air is provided from ceiling diffusers and it seems like fresh warm air could stay along the ceiling and go straight to the exhaust, giving the CO2 sensor an artificial low reading. In the scenario I just described, ASHRAE 62 gives that style of room a 0.8 zone air distribution factor and assumes the room isn’t adequately mixed. In those rooms I’d much rather see it in the breathing zone. We’ve worked on a couple of day cares and talked about demand control ventilation for children and felt the CO2 sensor should be even lower to get the air quality where the kids were actually breathing.

Of course, I’m not the IAQ expert, so I’d defer to you as to the likelihood of adequate room air mixing. Maybe a similar study with CO2 sensors at different elevations would help, though most building owners aren’t going to want to pay for this.

1 comment:

  1. 快樂與滿足的秘訣,就在全心全意投注於現在的每一分,每一秒上 ....................................................