When it comes to low temperature requirements,
what are the current limits of environmental test chambers?
20°C above ambient air — this often the lowest
achievable temperature in a chamber that has no built-in refrigeration
system. This is not a very reliable specification because the chamber's
components (such as the motor of the re-circulating fan) can generate
additional heat. This low temperature is usually specified for environmental
5 to -20°C — the lowest achievable temperature
using standard R-134A refrigerant. This is the same stuff used in new
air conditioning systems and refrigerators. Because of its limited range,
it's not widely used in test chambers.
-35 to –40°C — the lowest range achiveable
in chambers using R-404A or R-507 refrigerants. The system that uses
these refrigerants is sometimes called "single stage" (i.e.,
-65 to –73°C — this low-temperature range is
achievable via "cascade" refrigeration: a "high-stage"
refrigeration circuit is used to cool the "low-stage" circuit.
Cascade cooling is not very efficient, so the actual "cooling"
samples being tested is slower. Because the air is much cooler, it may
appear that the chamber is working fat. In actuality, it takes
the samples longer to catch up.
-85 to -150°C — these temperatures are achieved
using a triple-cascade refrigeration system. This is not common in test
chambers, although it's frequently used in ultra-cool freezers.
-170°C — achieved using use liquid nitrogen.
Nitrogen is an inexpensive method for cooling a chamber; however, its
long-term use of is expensive. Liquid-nitrogen refigeration systes are
best for: Small or not-often-used chambers; Chambers that specifically
need this low range; and Chambers that need temperature change rates
greater than 30°C/min.