Hi Romulo -

Personally, I wouldn't recommend a thermal buffer in any controlled storage. I think measuring air temperature is a better practice because we see the worst case condition. If the reason for wanting the thermal buffer is to avoid nuisance alarms, there are better ways to do this. For instance, use an alarm delay on your monitoring system to filter out the short-term excursions caused by door openings.

I know many do not agree with me. That's fine, and it is always good to have many opinions to choose from as we determine the course that is right for our applications. Personally, I think thermal buffering obstructs our view of the data. I'd rather see the true data, and control the symptom (ex: nuisance alarms) through other pathways.

If you want to do thermal buffering in ultra-low freezers, I recommend an aluminum block. This is what we supply with our Vaisala data loggers for customers who want to use a thermal buffer. The aluminum block acts as a heat sink, and holds the RTD in a specifically drilled hole in the block. The size of the block is sufficient to simulate the thermal response of a 40mL bottle of glycol.