The sensor is compatible with any HX and SBDU E61 group head, with an M6 thread inside.

The digital thermometer helps measure the brew water temperature about 1.25" away from the puck in brewing.

It also measures the temperature of the group-head when it's idle. It requires no modification to the espresso machine but can ONLY be installed in E61 group-heads that are fully exposed with a hole as shown in the picture.

The thermometer helps you figure out the best temperature in the setup as the water temperature controlled by PID may vary from that of hitting the puck. The difference can be material for different machines, different brew conditions, and over the course of a typical 23-27 second shot.