An averaging temperature sensor (ATS) is used to maintain the design mixed air temperature of an airside economizer system. These devices have sensing elements from 5 to 50 ft (1.5 to 15 m) long. To provide acceptable performance, they must be installed so that they sense the average ambient air temperature and the average temperature of the mass flow of air. Because sensing the average temperature of the mass flow of air is the criterion for satisfactory control, the installation of the sensor is critical. However, design documents rarely detail the sensor installation.