I have a temperature sensor in a -20C freezer. The probe is going into a bottle filled with propylene glycol solution. I want to seal the hole where the probe is inserted into the bottle.
What's a good sealant for this?
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Bonding to HDPE is very difficult. A few adhesives developed fairly recently do ok in some applications, but that kind of cooling will likely cause them to fail.
Instead, I'd recommend a mechanical seal. I would (and often have for this exact purpose) use a bulkhead fitting to seal against the wall and an instrument fitting, aka compression tube fitting to seal against the temperature sensor.
Try this stuff. It's about as good as it gets off the shelf.
I suggest buying extra caps/stoppers and drill a close-fitting hole. If the caps (and your probe/wires can take it), perhaps use some hot-melt glue for a mechanical seal.
You may actually get your best bet by encasing the entire thing in shrink wrap (depending upon the size of the bottle).
Shrink wrap is typically made of LDPE. Due to the low density, the LDPE has many sites where it can still cross-link upon itself. Heating the wrap begins this reaction, and as the material cross-links, it shrinks to form HDPE. This shrinking action forms a good seal. In the area of the hole, additional heating could be applied until the LDPE and HDPE entered a plastic state, welding the two materials together.
Other methods, such as acrylics, work well, but this would insure similar chemical resistance inside and outside the freezer.
In the end, I decided to use a bottle filled with glass beads. This provided the necessary thermal stabilization and it has the added benefit of working below -50C.