Almost certainly something to do with optimal phase transition behaviour in common refrigerants, I'd say. I would imagine that -80C is more attractive than -40/-60 also because it freezes water rapidly enough to skip a few of the more damaging ice crystal types, but ultimately I suspect it's just an economy thing. As well as compatibility with the temperature of dry ice, which is either used for transiting frozen goods or as a backup/band-aid for failing freezers.
In some places one can just buy dry ice easily enough, which presents another possible route to accessing mid-range temperatures if combined with different bath fluids - when combined with acetone it can go down to nearly -80C but a combination of different fluids, or minor electronic contrivances to control airflow around a bath, could allow control between -80 and -20. Probably.
Just watch out for condensing ice build-up with custom-built rigs. This is a problem even at -20C and it's why most modern freezers have an antifreeze cycle (which is problematic for home labs because it exposes enzymes and cultures to regular temperature spikes). I've never built a refrigerator but I imagine this is one of the harder engineering problems to solve without relying on regular defrost cycles. Perhaps a silly-but-practical route would be to build two of the same rig and just alternate them to permit regular defrosts.
13 Apr 2021, 00:02 by dkotes@gmail.com:
No idea, but would honestly love to know out of all the temperaturesthat could have been chosen, why -80C and -20C are the standards.On Mon, Apr 12, 2021 at 5:54 PM Tom De Medts <tdmedts@gmail.com> wrote:Dear all,Is it possible via DIY engineering to design and builda freezer that is colder than -20C, but warmer than -80C.-20C freezers are relatively inexpensiveand-80C freezers are way more expensive, also large in size, requiring maintenance plansI want to know whether there is something in between....Thanks!---- You received this message because you are subscribed to the Google Groups DIYbio group. To post to this group, send email to diybio@googlegroups.com. To unsubscribe from this group, send email to diybio+unsubscribe@googlegroups.com. For more options, visit this group at https://groups.google.com/d/forum/diybio?hl=enLearn more at www.diybio.org---You received this message because you are subscribed to the Google Groups "DIYbio" group.To unsubscribe from this group and stop receiving emails from it, send an email to diybio+unsubscribe@googlegroups.com.To view this discussion on the web visit https://groups.google.com/d/msgid/diybio/CANpz%3DfEL1ppS%3DK9qtgb3egpgO_c8uqaEmouy2QJaK1Ms6__VCQ%40mail.gmail.com.---- You received this message because you are subscribed to the Google Groups DIYbio group. To post to this group, send email to diybio@googlegroups.com. To unsubscribe from this group, send email to diybio+unsubscribe@googlegroups.com. For more options, visit this group at https://groups.google.com/d/forum/diybio?hl=enLearn more at www.diybio.org---You received this message because you are subscribed to the Google Groups "DIYbio" group.To unsubscribe from this group and stop receiving emails from it, send an email to diybio+unsubscribe@googlegroups.com.To view this discussion on the web visit https://groups.google.com/d/msgid/diybio/CAGdeWmQ%3DW%3D75VdqcGFaF3iAFFkHzOSEFiF5BSMTuSM-91tQPrg%40mail.gmail.com.
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