Every chilled unit goes out of range several times a day, because people open doors. A monitoring system that treats each of those as a failure produces an alarm nobody believes within about a week.
Key takeaways
A single reading out of range is not an event. Duration is.
Consecutive time out of range triggers the alarm; cumulative time triggers a review.
Different units get different rules; a walk-in is not a display fridge.
The alarm carries the last few hours, because trajectory is the diagnosis.
Fewer, louder alarms beat more, softer ones. Every time.
Two clocks
Fig 1. The two duration rules and their different consequences. Separating them is what lets the system catch a door left ajar all afternoon without alarming at three in the morning.
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Why cumulative matters too
A door propped open for two minutes every ten minutes all afternoon never triggers a consecutive rule and does real damage. It is a common and genuinely bad pattern — usually somebody restocking with the door wedged — and only a cumulative measure catches it.
It is a review flag rather than an alarm because the right response is a conversation the next morning, not a call at four in the afternoon. Matching the urgency of the response to the urgency of the problem is what keeps both channels credible.
Different units, different rules
Unit
Range
Breach after
Why
Walk-in freezer
−22 to −16 °C
20 minutes
Large thermal mass; slow to change either way
Under-counter fridge
1 to 5 °C
10 minutes
Small; warms quickly
Open display chiller
1 to 7 °C
20 minutes
Constantly disturbed; a tight rule is unworkable
Vaccine fridge
2 to 8 °C
5 minutes
The stock is not replaceable
Delivery vehicle
−20 to −15 °C
30 minutes
Doors open at every drop
The fourth row is the useful contrast. The rule is not about the physics of the unit; it is about what is in it and what happens if you are wrong. A vaccine fridge gets a tight rule because the consequence of a missed breach is severe and irreversible.
Every one of these numbers is a judgement that belongs to whoever is responsible for the stock, written in configuration where they can see it, not embedded in code. The system applies rules; it does not choose them.
Fewer, louder
Fig 2. A month of alarms under two rule designs. The three that matter are identical; what differs is whether anybody is still listening when they fire.
This chart is the entire argument of the post. Alarm fatigue is not a failure of discipline by the person on call; it is the predictable result of a system that cried wolf two hundred times and then once did not.
The alarm carries history
A message saying “freezer 2 is at minus nine” is much less useful than one showing the last six hours. Was it minus eighteen an hour ago and rising steadily, which is a compressor? Or has it been hovering around minus fifteen all evening, which is a door seal?
The trajectory determines what somebody does when they get out of bed, and including it in the alarm itself — a small chart, not a link to a dashboard behind a login — is the difference between a useful alarm and one that requires a laptop.