The U-value (formerly k-value) is the most important value used to rate the energy efficiency of building components. With the new testo 635 measuring this value has never been easier. Three temperature values are needed to determine the U-value: outer temperature, surface temperature of inner wall as well as indoor air temperature. Using the new wireless probes, the outer temperature can be quickly and easily measured with the window closed. The probe is simply positioned outside and transmits readings by radio to the measuring instrument in the room. With the new patented U-value probe the two other temperatures required are measured using one probe. To measure surface temperature, three wires from the U-value probe are attached to the inner wall using modelling clay. The air temperature is measured by a sensor on the probe plug. The three temperatures needed are determined by the connected temperature probes and transferred to the testo 435. The instrument calculates the U-value from them and shows it directly in the display. Temperature probe to determine U-value, triple sensor system for measuring wall temperature, modelling clay included