Temperature mapping a storage space

A storage room, cold room or refrigerator is usually controlled from a single sensor. The controller reads that point, switches the cooling or heating, and the display shows a steady number. The number is true for the place where the sensor sits. It says little about the top shelf by the light fitting, the corner behind the door, or the space directly under the evaporator fan. Temperature mapping is the study that finds out what the rest of the space is doing.

The idea is simple. Calibrated data loggers are placed across the space in three dimensions, left to record for a defined period, and then read together. The result is a picture of how temperature is distributed: where the space runs warm, where it runs cold, how much it swings, and how quickly it recovers after a disturbance.

Why one sensor is not enough

Air in an enclosed space does not mix evenly. Warm air rises, so upper shelves often run warmer than lower ones. Walls that face the sun or an unheated corridor pull the nearby air with them. Doors let in outside air every time they open. Cooling units create a cold zone near the coil and the outlet, and a warm zone wherever airflow is blocked by stock or shelving. Lighting and other equipment add heat locally.

None of this shows on a controller display. A room can hold its set point perfectly at the sensor while a pallet in the far corner sits outside the acceptable range for hours each afternoon. Guidance on good distribution practice addresses exactly this. The EU guidelines on good distribution practice expect an initial mapping exercise before a storage area is used, under representative conditions, with monitoring devices then positioned where the mapping found the extremes. The World Health Organization publishes a technical supplement devoted to mapping storage areas, and ISPE publishes good practice guidance on the same subject.

Planning the study

A mapping study starts with a written protocol. It states the purpose, the acceptable temperature range, the number and position of loggers, the recording interval, the duration and the conditions under which the space will be tested. It also states, before any data exists, what counts as a pass.

Conditions matter as much as positions. A room studied empty behaves differently from the same room fully loaded, because stock blocks airflow and adds thermal mass. A room that depends on the building's ambient temperature behaves differently in July and in January, so studies of such spaces are often run in both seasons. Many protocols also include deliberate disturbances: a door held open for a set time, or the cooling switched off, to measure how far the temperature drifts and how long the space takes to recover.

Placing the loggers

Loggers are typically arranged in a grid across the width, depth and height of the space, with extra positions where trouble is expected. A common plan covers:

• the corners and the center at low, middle and high levels;
• positions near doors, windows and external walls;
• positions near the cooling outlet, the return and any heat source;
• a position beside the controller's own sensor, so the two can be compared.

Every logger should carry a current calibration certificate, traceable to national standards and covering the range being studied. A logger that reads half a degree high will move a hot spot on the map, so the calibration records are part of the study's evidence. The recording interval should be short enough to catch the cycling of the cooling unit and the effect of a door opening. The duration should be long enough to include a normal working pattern, usually several days and a weekend.

Reading the results

When the loggers are collected, their data is downloaded and compared. The analysis usually reports the minimum, maximum and mean at each position, identifies the warmest and coldest points, and states whether every position stayed within the acceptable range for the whole study. It also reports how the space behaved during the planned disturbances: how high the temperature rose with the door open, and how long it took to return to range.

A study can pass overall and still show that part of the space is unsuitable. A shelf that drifts out of range during each defrost cycle, or a corner that runs cold enough to approach freezing, can be marked as a zone where material is not stored. That decision belongs in the report, with the data that supports it.

What the map decides

The map has three practical uses. It tells the facility where to put the permanent monitoring probes: at the points the study found to be most extreme, not at the most convenient spot on the wall. It tells staff where material may and may not be stored. And it becomes the baseline against which later changes are judged.

Mapping is not done once and forgotten. A new cooling unit, rearranged shelving, a change in how the room is loaded or a repeated pattern of alarms all change the conditions the original study measured. The usual practice is to repeat the study when such changes occur, and otherwise at an interval set by a documented risk assessment. Each report is kept with its protocol, the logger calibration certificates and the raw data, so that anyone reviewing the space later can see how its current layout was decided.