How to identify hot and cold spots

A hot spot is the location that runs warmest, a cold spot the location that runs coldest. That sounds obvious, but 'warmest' can mean highest single reading, highest mean over the study, or the location that spends the longest time near the upper limit — and each definition can point at a different sensor.

Decide the criterion first

Write into the protocol which statistic identifies the spot — most protocols use the per-sensor maximum for the hot spot and per-sensor minimum for the cold spot, with the mean used to confirm that the location is consistently at the extreme rather than affected by a single event. Recording both prevents an argument after the fact.

Check the data before trusting the extremes

• A flat-lined sensor cannot be the cold spot.
• A single abnormal jump is more likely a sensor fault or a door event than a stable gradient.
• Sampling gaps near the extreme value weaken the evidence.
• Duplicated or non-monotonic timestamps usually mean an export or merge problem.

Then look at time

Plot the extreme sensors against time and against a reference sensor. A hot spot that only appears during defrost cycles or deliveries needs a different response from one that is warm all day. Excursions — periods outside the limits — should be listed with start, end, duration and peak, per sensor.

ValiTrac's analysis reports per-sensor minimum, maximum, mean, median and standard deviation, identifies hot and cold spots from the configured criterion, and lists excursions with duration and magnitude. Where sensor coordinates are not provided it says so rather than inventing a spatial map.

Highest maximum names S07. Highest mean names S11. Longest time near the limit names S11 by a wide margin. All three are defensible criteria and they do not agree, which is exactly why the criterion belongs in the protocol.

The physical story behind those two columns is also different, and worth reading. S07 spikes to 8.4 °C for fourteen minutes and is otherwise unremarkable — a door event or a delivery. S11 never quite reaches 8.0 °C but sits just under it for three hours a day, which is a genuine hot spot in the airflow. For most products, S11 is the one you would rather know about; for a product with a hard upper limit and no excursion allowance, S07 is.

Recommended practice

Report the maximum as the primary criterion, because it is what acceptance is assessed against, and report the mean alongside it as confirmation. A location whose maximum is extreme but whose mean sits with the pack has been visited by an event; a location extreme in both is a stable feature of the space. Those two findings lead to different actions — a procedural control against a layout or airflow change — so the report should distinguish them rather than reduce both to one label.

Where the spot goes next

• The hot and cold spots become the positions for permanent monitoring, which is the whole reason the study identified them.
• If an existing monitoring probe is not at or near one of them, the report has to say so and recommend the move — and somebody has to make it.
• The spots become the reference positions for every periodic review. If the hot spot holds the same offset from the room average for two years, the distribution has not shifted.
• Keep the same positions instrumented in any re-mapping, so the two studies can be compared position by position.
• A cold spot below the lower limit in a 2–8 °C room is a freezing risk, not a curiosity: the answer is an exclusion zone marked physically, or a deflector — not a wider criterion.

Check the data before trusting an extreme. A flat-lined sensor cannot be the cold spot, a single impossible jump is a fault rather than a gradient, and a gap next to the extreme value weakens exactly the evidence you are relying on.