In a cold store the single display usually hangs next to the door. Each time the door opens that spot changes, while racks far from the evaporator sit at another temperature and the screen does not show it. Mapping closes this blind spot with measurements.
- Grid: under the WHO guidance loggers go in a grid 5 to 10 metres apart. With a ceiling of 3.6 metres or less, each point gets three loggers stacked at floor level, 1.2 m and 3.0 m. The door area, evaporator outlet and return, and the farthest corner are always included.
- Timing: all are set to the same recording interval (1 to 15 minutes) and the same start time, and positions go on a sketch and a table.
- Duration: 24 to 72 hours in cold and freezer rooms, at least seven consecutive days in a store affected by ambient temperature. With two refrigeration units, each is also measured running alone.
- Log: door openings, loading and dispatch times are written down so spikes can be explained.
The map changes two things. The most sensitive consignment goes to the most stable zone, and alarm thresholds are set on the measured distribution instead of a catalogue figure. Air readings do not stand in for the product's own temperature, so product temperature is taken separately.
A store affected by the seasons may need a summer and a winter study; cold and freezer rooms usually need one. Re-mapping follows a change in rack layout, refrigeration equipment or set point, and unexplained deviation in the records; WHO gives three years as an example of periodic repetition.
