There are several different stone types. We recommend olivine diabase or vulcanite stones. Small stones 5-10cm are good for electrical saun aheaters, and the 10-15cm stones are good for wood-burning sauna stoves.
It really depends on the usage. If sauna is used 1-2 times per week, it would be good to change them once a year. Try picking up two stones and knock them on each other; if they crack, they should definitely be replaced.
According standard 60364-7-703 (Electrical installations of buildings) part 703.412.5, the sauna heater shall not be connected via residual current protective device (RCDs). Only a certified electrician is allowed to connect your electrical sauna heater.
Controller shows the temperature of the area where the thermostat is place. It is most likely hotter above the heater than in the other parts of the sauna. For safety reasons, the thermostat must monitor the hottest part of the sauna.