How Return Air Sensing Works
In typical refrigeration control:
- Cold air discharges from the evaporator into the refrigerated space
- Air circulates through (or over) the cargo, absorbing heat
- Warmed air returns to the evaporator intake
- The return air sensor measures this temperature
- Controller compares measurement to setpoint and cycles refrigeration accordingly
This approach seems logical—measure the air coming back to determine how much cooling was achieved. However, return air sensing has a fundamental measurement bias.
The “Success Story” Problem
Return air temperature only measures air that successfully completed the circulation loop. This is the air that:
- Flowed from the evaporator discharge
- Traveled through or over the cargo space
- Returned to the evaporator intake
It’s the “success story” air—air that received the cooling effect and circulated as intended.
The air trapped in dead zones never votes. Stagnant air in corners, behind product stacks, or stratified in warm upper layers doesn’t make it back to the return air sensor. The controller never sees those volumes.
Measurement Bias
Because return air sensing only measures successfully circulated air, it’s systematically biased toward the coldest, best-performing zones in the refrigerated space. Research shows that dead zones can run 4-8°C warmer than return air readings while controllers report setpoint maintained.
The controller isn’t malfunctioning—it’s accurately measuring what it can see. The problem is what it cannot see.
Alternatives and Supplements
Multi-point sensing — Adding sensors at multiple locations extends visibility beyond the return air path
Product temperature probes — Measuring actual product temperature rather than air temperature (see
glycol-buffered probes)
Supply air sensing — Measuring discharge air temperature ensures refrigeration system is functioning but doesn’t address distribution
Discharge + Return differential — Comparing supply and return temperatures indicates heat absorption but doesn’t reveal spatial distribution
R638 Compliance Implications
R638 regulations require temperature monitoring records that demonstrate compliance. Records based solely on
return air sensing may show continuous compliance while product in dead zones experiences repeated excursions—creating a documentation gap that becomes apparent only when product quality issues emerge.
See our technical article on
dead zones and temperature measurement.]]>
