We have two Nuaire incubators, a dual-chamber NU-8700 and a single chamber NU-4950 both set to 5% CO2. From the age and schematics they appear to use the same model CO2 sensor. Both incubators are well maintained.
When I open & close either door in the NU-8700 the CO2 falls as low as 1% but only as low as about 3% on the NU-4950. Recovery is a little faster on the NU-8700 despite the greater apparent loss. Since the NU-4950 has a larger door and fewer shelves I would have expected a greater exchange of air with that incubator rather than the NU-8700.
Can anyone explain why I'm seeing such an apparent precipitous drop? Is there accuracy in what I'm seeing?
It is possible that there is a draught affecting the NU-8700 but not the NU-4950, that you are not aware of. Is the NU-8700 nearer the door than the NU-4950?
Thank you for your suggestions. Unfortunately the sensors aren't readily interchangeable since they are built into the casing. The incubators are about equidistant from the door and set at right-angles to each other; since the NU-4950's door would block direct flow between the culture room door and the inside of the chamber that is a potential cause. I'll try it with the culture room fully sealed from the rest of the lab. (It gets hot quickly if the door is kept closed.)
I did notice the shelving in the NU-8700 is level with the sensor, while there is clear spacing in the NU-8700. Lowering a shelf in one chamber of the NU-8700 restored behaviour to near normal. The drop is only to 2.5-3% but it now takes a full 5 min to recover rather than 2 min. Possibly with a smaller drop on the sensor there is initially less CO2 being injected into the chamber. I don't know what the aerodynamics are that would make this the cause especially since there is a constant flow from the chamber to the sensor; I would have expected partial recovery before replacement CO2 was injected.
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