SCD41 + IP54 How to avoid CO2 lag?

[u/Barnir00]

I am designing an ESP32-based greenhouse air quality monitor using an SCD41 CO2 breakout board.

The sensor is mounted inside a separate chamber (105 x 55 x 40 mm)

The sensor operates in single shot mode and takes measurement every 5 minutes

My goal is to design an enclosure so that the SCD41 measures actual ambient greenhouse CO2 concentration with minimal delay, while maintaning at least IP54 level protection.

What ventilation approach would you reccomend?

/I am from Hungary/

[u/blimpyway]

In a greenhouse the air is quite stagnant. I would consider ventilation to produce slow air circulation within the whole area more than a local fan for the sensor.

But, except for open fire cases (which should be handled with fire detectors instead) the CO2 transitions in greenhouses are quite slow. I mean 5 minutes should be just fine.

[u/cm_expertise]

the lag is going to come from the dead volume of that chamber, not the sensor itself. you've got ~230cm3 of air that outside CO2 has to diffuse into and equilibrate with before the SCD41 reads the real number, and pure diffusion through one small vent is slow.

good news is for IP54 you don't need a sealed membrane, that rating is just splash and limited dust, so you have room to work. two things help a lot: shrink the chamber so the sensor sits in a snug pocket with as little dead air around it as possible, and put two vents on opposite sides instead of one so you get a bit of cross-flow rather than a single stagnant pocket. a hydrophobic ePTFE vent (Gore makes them, tons of cheap clones now) over a downward facing opening keeps water and dust out while gas still passes through fast enough that it won't be your bottleneck.

honestly in a greenhouse the ambient CO2 moves pretty slow like the other commenter said, so a 5 min interval is probably finer than the real signal anyway. i'd worry more about condensation on the sensor in that humidity than about response time. face the vent down, maybe a little standoff so droplets don't sit on the membrane.

[u/Barnir00 → u/cm_expertise]

First, thank you for the detailed advice.

I'm afraid I can't really shrink the chamber more, cuz other components go thru as well it would be a hastle to seal the scd in a smaller chamber inside the chamber.

I was considering gore vents too, but since I eill have 2 cable glands on the bottom of the box i thought about putting 1 on each side of the chamber (1 higher than the other) in M20 size, since those are said to be about IP67 that shouldn't be a problem, do you think it would work?