We use a SHT41-AD1B-R2 moisture sensor from Sensirion to monitor RH inside a sealed package. After drying the package at 120C for 24 hours, we expect the RH would decrease due to moisture removal from drying. However, the RH reading increased significantly. What would be the reasons for the RH increase after drying? The package material outgassed during drying. Does SHT41-AD1B-R2 RH responds to other gases besides moisture as well?
A truly sealed package that is heated will not end up with less moisture (absolute humidity) because there is no where for the moisture to go. So as soon as the sealed package returns to normal temperature it should have the same level of moisture.
IME, the SHT41 and similar type %RH sensors can be sensitive to, and/or damaged by, exposure to outgassing from other materials (e.g. silicone caulk). They really only work well in air that you are happy breathing.
it is not a hermitic sealed package. we did see weight loss after drying, but the weight loss could be the outgassing include moisture.
To verify if the sensor was damaged during drying, we drilled a hole on the plastic lid of the package after drying and placed the package with hole into 30°C/60% RH chamber. The sensor reading did match the chamber reading.
Perhaps this expectation is incorrect; if you’re driving moisture out of an object into the environment within a semi-permeable enclosure, the drying occurs by moisture exiting the object, into the atmosphere within the enclosure, then diffusing through the enclosure wall into the outside atmosphere. In order for that diffusion to occur, the humidity inside must be greater than that that outside.
Imagine hanging wet laundry to air-dry on the curtain rod in one’s bathroom, and closing the door. When you come back a couple hours later, would you expect the humidity within the bathroom to be higher or lower than before?
It’s possible that there are other chemical response factors involved, but I’d consider the first principles first. The little silica gel packets that tempt you to eat them by instructing you not to are made for this very sort of of situation…