The insulation figure on the certificate, the K coefficient, is a laboratory result; wall thickness is only one of its inputs. The roadside inspection guide of Turkey's Ministry of Transport and Infrastructure describes the rig: the equipment goes into a closed test tunnel, the tunnel is set to 7.5 °C, and heaters raise the inside of the body to 32.5 °C. The heating power spent holding that 25-degree gap is measured, and K follows from it. Per the guide, 12 sensors are placed inside and 12 outside, spread according to the shape of the body; one thermometer would miss the thermal bridge at a corner or a leak around the door.
| Stability condition | Value in the guide |
|---|---|
| Steady state | at least 12 hours, variation no more than ±0.3 K |
| Preceding 6 hours | variation no more than ±1.0 K |
| Power comparison | two 3-hour periods at least 6 hours apart, difference under 3% |
| K calculation | 6-hour period, mean difference between start and end no more than 0.2 K |
| Total duration | 2-3 days |
These conditions secure a balanced state in which the heating power reflects only the heat escaping through the walls; a measurement taken before balance is reached makes K look worse than it is.
The same measurement logic settles the question of refrigerated vehicle or insulated container: the comparison is made from reports, not from claims.
The report is issued to a manufacturer and a type, not to one vehicle. Article 11 of Turkey's regulation on special equipment for perishable foodstuffs requires the type test to be repeated on one reference unit for every batch of 250 units of the same type; if production passes 250 within 2 years, the test is repeated at the end of year 2, and if 250 is not reached within 6 years it is repeated anyway. The class comes from here too: 0.4 < K ≤ 0.7 W/m²K is normal and K ≤ 0.4 is heavy insulation, shown by the second letter in codes such as FRC or IR.
