Thermoprobes with flat-loop evaporators utilize proprietary internal components within the riser to force the working fluid to travel in one direction within the evaporator. As the working fluid absorbs heat, it vaporizes and expands. The expansion helps to push the working fluid around the loop. The working fluid moves around the loop in two-phase flow, increasing in velocity as it proceeds around the loop. This allows the loop to be placed on a 'relatively' flat plane. The term 'relatively' means that there can be some undulations in the vertical profile of the loop. Typical designs allow for as much as six inches (150mm) amplitude in the undulations with no detrimental effects on the system. The subgrade can be graded to a near flat condition and the loops then placed directly on the graded material. Typically the level of the loops is approximately 1.5 feet to 3 feet (0.5m to 1.0m) below the base of the subgrade insulation. Loop lengths of 500 feet (150m) are not uncommon.