There are many different sensors that work with solar thermal control systems, ranging from flat models which bolt onto equipment, to screw-in threaded models, to pointed models designed for immersion in liquids. Most consist of a ¼-inch diameter copper cylinder about an inch long, with two wires sticking out the end.
The wires connect the sensor to the controller: black wires indicate a 10K sensor, white means it's a 5K model. You should use shielded #16 stranded sensor wire to connect things together, attaching the shielding to the ground lug in the controller with the other end of the shield left unattached. Solder the connections or use watertight telecom connectors, sealing everything with heat-shrinkable tubing. Avoid using metals such as wire nuts, as they will corrode and cause inaccuracies in the sensor readings.
Collector array sensor
This sensor is usually mounted on the pipe exiting the array, as close to the collector as possible so that it picks up the hottest temperature reading. Attach the sensor with a stainless steel hose clamp.
The ideal location is actually inside the collector, attached to the top of the top manifold. If the manufacturer doesn't offer this installation location, you will need to add a grommet to the back plate of the collector, lift off the glazing, install the sensor and feed the wires out through the grommet.
This sensor is best installed about a quarter or a third of the way up from the bottom of the tank. Make sure it is in full contact with the side of the tank for a reliable reading. If your tank is insulated on the outside with foam or similar materials, you need to get the sensor inside the padding. You can jam a piece of insulation in between the sensor and the padding to hold it in place.
If you're using an electric water heater as your storage, you could opt for a screw-in or well sensor and install it in place of the lower heating element. This works particularly well on plastic and fiberglass tanks.