3/8''(9.5mm)

The sensor is housed in a copper tube with a slightly curved surface to make intimate thermal contact with the pipe. Temperature changes in the pipe fluid are conducted very quickly through the sensor housing to the element inside. The copper pipe strap fits makes for quick installation on either 1/2 inch or 3/4 inch pipes.
The 3 meter long cable is 24 AWG stranded copper wire with a UL2651 rating and is registered under UL file number UL42323. The thermistor is a 10K type 2 temperature curve compatible with most DDC systems on the market