A warehouse looks uniform from the doorway. Racks, a concrete floor, an HVAC unit humming somewhere above. The temperature inside it is anything but uniform. The top shelf under a metal roof in May, the pallet next to the loading dock, the corner the air handling never quite reaches: these are the places where product actually sits outside its label conditions, and a mapping study exists to find them.
Most of the work in a good mapping study happens before a single logger is switched on.
The guidance most people use
WHO Technical Report Series 961, Annex 9, Supplement 8, on temperature mapping of storage areas, is the most practical reference and is widely used beyond WHO-prequalified products. National GDP guidelines say much the same in fewer words. The recommendations below follow it.
Laying out the sensors
Start with a scaled drawing of the space, showing racks, doors, windows, HVAC supply and return points, lights and anything that produces heat. Then:
- Place sensors in a grid across the length and width of the area. For large open spaces, spacing of 5 to 10 metres is typical. Smaller rooms need closer spacing.
- In racked areas, place sensors at three heights: near the floor, at mid-height and at the top storage level. Warm air stratifies, and the top shelf is often the problem.
- Add sensors at known risk points: near doors and docks, below roof lights, beside external walls, close to supply air outlets and in dead corners.
- Put at least one sensor outside the area to record ambient conditions, so you can explain what you see inside.
Mark every position on the drawing with a unique number before the study, and photograph each logger in place. It sounds tedious until someone asks, eight months later, where logger 23 was.
How long, and when
The study should run for at least seven consecutive days, so it captures a full weekly cycle, including weekends when doors stay shut and the HVAC may behave differently. Record at a fixed interval, commonly every 5 to 15 minutes.
If the building is affected by outside weather, which most are, map in the hottest and the coldest seasons. A summer study tells you nothing about the north wall in January.
Choosing the loggers
Data loggers should have a calibration traceable to national standards, at points that cover the range being mapped. An accuracy of ±0.5 °C is the usual expectation for controlled room temperature storage. Check the calibration date before the study and verify the loggers after it, so a drifted unit can be identified.
What the report should give you
- Minimum, maximum and mean temperature for every sensor, with the time of each extreme.
- The hot and cold spots, shown on the drawing.
- Any excursions outside the storage limits, with the likely cause.
- Recommendations for where the permanent monitoring probes should go, usually at or near the worst-case points found.
- Any areas that should not be used for storage, or only with restrictions.
When to map again
Remap after changes that could alter the temperature profile: new racking, a change to the HVAC, a new roof, a change in how the space is used. Many sites also remap on a fixed interval, often every two to three years, even without a change, because buildings and equipment age quietly.
The mapping report is not the end product. The end product is a set of monitoring probe positions and storage rules that you can defend, and a warehouse team that knows which shelves to keep the sensitive stock off.
GAMP Services Limited 2026. All Rights Reserved.