I have done this, hardest part is finding a reasonably priced (ie affordable) waterproof PT100 probe, mine was £80 odd. Can't remember the make, but their are many PT100 waterproof sensors available, mine was strapped to filter inlet pipe.

You just then connect it to the Logo AM2 RTD module and if temperature is below set point you turn heater on and if above set point you turn temperature off. Easy.

I added hysteresis of 0.5C (taken from a Siemens example) so that you don't get heater turning on and off too quickly.

You can set the temperature set point via front panel buttons, but can't see why you would ever really need to change it from in my case 26C.

I also had a switch on the front panel case, if pressed for 3 seconds turned the heater off during maintenance and flashed the display red to warn heating was off.

Also logged temperature every 1/2 hour to an SD card. Again a bit pointless really as temperature varied between 25C when heating and 28C on very hot days.

I set the thermostat on the heater at 28C in case of error in my PLC code.

In the end I am not sure it was worth it, other than for temperature logging, as the heaters built in thermostat did exactly same but much much simpler than my PLC.

Attached is picture of prototype before the AM2 RTD module was added.

You connect the relevant RTD input block to an analogue threshold trigger. In this block you set inputs as PT100/PT1000 and resolution of 0.1.

You then set on to be 255 (25.5C) and off 245 (24.5C). Note the inverter, thus when block output is high heater is off. We actually did the inversion is an external relay so that is the PLC crashed or someone forgot to put it in run mode the heater would be on and relying on its own thermostat to limit temperature.

Thus it will turn heater on at below 25.5C and off at above 24.5C.

We also suffered all types of interference due to the extremely low signal levels present in an RTD probe, so very very careful system design is needed, with programming safe guards to prevent erroneous signals from say jamming the output on.

Thank you all for the help.

Antonio