The method is simple; stating the conditions is the hard part
With an empty chamber, multiple thermocouples are placed inside — typically nine points, denser for larger volumes. The unit is brought to target and stabilised, then point-to-point deviation is recorded. That is uniformity.
The difficulty is that the same furnace yields materially different numbers depending on loading, atmosphere and stabilisation time. A "±3 °C" figure without its conditions cannot be compared and cannot be accepted against a contract.
Our approach is to record the measurement conditions in the test report that ships with the equipment, so the customer can reproduce the test under the same conditions. A nominal figure is worthless in a contract; a figure a third party can reproduce is not.
What the delivery report records
Every unit is heat-tested with an empty chamber before dispatch and the report ships with it. The preconditions it records are: furnace model, ambient temperature, auto-tune temperature, PID values, controller parameters (PAF / SPr / OPH / FILt) and power limit.
The run record covers start temperature, time, target temperature, maximum overshoot and profile tracking.
Publishing these fields is deliberate: it makes the word "measured" checkable. If all you receive is "uniformity ±3 °C" without them, the figure cannot be reproduced and therefore cannot be enforced.
Empty-chamber and full-load figures are not interchangeable
An empty-chamber test reflects the equipment itself — its heating and control capability — and is a genuine equipment figure. It does not directly predict behaviour under your load.
Loading changes the result for concrete reasons: the workpiece and fixturing have thermal mass and absorb heat, slowing local rise; setters and racks form conduction paths and radiation shadows; and the workpiece itself develops an internal gradient.
The proposal therefore has to be calculated against your actual load and fixturing. Quoting empty-chamber uniformity without asking about loading guarantees a mismatch at acceptance. This is also why we insist on a workpiece drawing and the loading arrangement.