Hot and Cold Thermistor water sensor assemblies for THX Solar Differential switch
Looking down the pipe the sensor tip is clearly in the flow, such intimate contact is much preffered over a dry contact arrangement
Order as 15mm or 22mm Sensor Compression fitting kit
other sizes available, enquire.
Kit includes Compression 'T' with sensor fitted and 4m lead set.  
15mm Shown , insertion depth may vary with fitting size, for hottest detection orientate sensor vertically upright, sensor is shown vertically down.
15mm Compression fitting can be easily  'cut in'