When I need precise temp. control I use a controller. This decides when to send power to the thermometer(s). I t displays the tank temp and allows me to change the temp settings. The controller prevents a heater mishap where the heater gets stuck full on. Although I am measuing mostly in F, my controllers work in C. This gives me a range in F because of the difference in the two scales.
I set the thermometer temp 1 dg C higher than the controller is set to. This way if the controller gets stuck on the thermometer temp setting insures the heat should not get any higher. The digital controller, once set, displays the water temp.
But, being a belt and suspenders type of guy, I also have a suction cup thermometer on the tank as well.
I use the Azoo Micro Controller good for up to 1,000w of heaters.