The water feed-in will be at the bottom of the tank, while hot water is drawn from an outlet at the top. There is no convective driving force for the layers of hot and cold water in the tank to mix, apart from perhaps a small amounts of turbulence where the water feeds in the bottom of the tank. Over time the temperature of the hot and cold water equalises by conduction up the copper tank wall. From this you can deduce that depending on where you make your measurements you may get very different results.
For example, my immersion heater is located at the bottom of the tank. It heats by convection, so is able to heat all the water in the tank above it. After using some of the hot water in the tank, the bottom of the tank is cold, but I can still draw hot water from the top. Twelve hours later, the heat has been dispersed, and all I have is a tank full of slightly luke-warm water. If I had two or more sensors I would be able to observe this dispersion of heat.