Environmental Test Chambers: Temperature Range: Low End

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 ovens.
  
• 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., one compressor).
  
  
• -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.

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