Delta T
The temperature difference between two measurement points, most often supply and return air, or an equipment inlet and its exhaust.
Also called: ΔT, Temperature difference
In practice
Air carries heat in proportion to how much of it moves and how much its temperature rises along the way. Delta T is the second half of that relationship, which makes it one of the few numbers in a data hall that says something about what the air is actually doing rather than how the room feels.
The most useful reading is across the cooling units: supply temperature against return temperature. A healthy delta T means the air being returned has genuinely picked up heat from equipment. A small delta T alongside plenty of fan energy is the signature of bypass — conditioned air finding its way back to the unit without passing through anything that needed cooling. Nothing alarms, the room looks fine, and the fans and plant work harder than the load requires.
The same measurement at the rack tells a different story. Equipment has a characteristic temperature rise across it, so comparing inlet and exhaust says whether a cabinet is getting the airflow it expects. A rise larger than expected usually means the cabinet is short of air and its fans are recycling what they can find.
Delta T is a diagnostic, not a target to optimise on its own. Raising it by starving equipment of air is not an improvement, and a room can have a respectable delta T while individual cabinets are in trouble. Read it beside inlet temperature at the rack face, which is the measurement that determines whether the equipment is being treated correctly.
Scope of this definition
This is an educational summary of how the term is used in data center practice. It is not a standard, a specification or engineering advice, and where a real decision depends on it, the current adopted standards, verified site information and qualified professional review are the correct sources.