Empty versus loaded mapping studies

Load changes airflow, thermal mass and recovery time. An empty cold room recovers from a door opening in minutes; a fully racked one may have pockets that take far longer. Product also shields sensors, so a loaded study needs care to keep sensors measuring air where product sits.

Typical sequence

• Empty mapping during qualification to characterise the equipment and detect gross faults.
• Loaded (or simulated-load) mapping under representative conditions before routine use.
• Re-mapping after significant changes to layout or load pattern.

Where simulated load is used (water-filled containers, empty cartons), document it and be honest about the limitation in the report.

What load actually changes

Load is not one variable but four, and they pull in different directions — which is why a loaded result cannot be predicted from an empty one.

Thermal mass is the one that produces misleading reassurance. A fully loaded cold room recovers from a 30-second door opening more smoothly than an empty one, so the door test looks better loaded. The same mass means a compressor failure takes far longer to show up and far longer to recover from.

Simulated load, and being honest about it

Water-filled containers and empty cartons are the usual substitutes when real product cannot be committed, and both are legitimate if the report says what was used. Water has roughly four times the specific heat capacity of typical packaged goods by mass, so it over-represents thermal mass and under-represents the air gaps a palletised load actually contains. Empty cartons do the reverse: they obstruct airflow realistically and add almost no mass.

The practical compromise is to match the geometry with cartons and the mass with a smaller quantity of water, and to write down the limitation rather than implying the simulation was equivalent. A report that says loaded is a claim; one that says loaded to 80 % of capacity with empty cartons arranged in the routine pattern is a record.

Sensor placement under load

• Sensors measure the air product sees, so they go in the gaps at product height — not pressed against a carton, and not buried inside a pallet unless the protocol says that is what is being characterised.
• Keep the sensors in the same positions as the empty study so the two can be compared position by position. That comparison is the evidence for what load did.
• Document the loading pattern with photographs. A year later nobody will remember whether the pallets were double-stacked.
• Load to the worst realistic case. A study run at half capacity places no limit on how full the space may later be packed, and that is exactly the gap an auditor will find.

The usual sequence is empty during qualification, to characterise the equipment and catch gross faults cheaply, then loaded before routine use, and re-mapped after any significant change to layout or load pattern.