When you measure the outlet (top) water temperature of a typical domestic hot water cylinder it's not a lot of use from a control perspective because it pretty much reads the same value until the tank is completely empty of hot water and then it suddenly reads close to the inlet water temperature!
This is because the water experiences a density change as its temperature changes, and the water at the top is always the hottest water, the incomming cold water simply sits below it. Slowly over time, you find that top temp will fall as it looses heat to the colder water below it, but this isn't really enough to put in place much of a decent control system.
For example, my system pretty much sits at a top (outlet) water temp of between 55 and 50 degC, yet the 1/3rd point (1/3rd of the distance from the bottom) and the 2/3rds point experience much greater swings, and you can use them to estimate how much water is in the tank of any given temperature, ie the remaining capacity of the tank, and control heating as necessary.
![Passing 2 wires into a hot water cylinder 2151649-0](https://www.eevblog.com/forum/index.php?action=dlattach;topic=425527.0;attach=2151652;image)
The lower temperatures clearly show when hot water is drawn from the system, as the "slug" of cold water comes in, immediately drops the temp on the lower sensors, and then slowly warms up over the next few hours as the top temp drops very very slightly. During heating, the lower temp jumps right up, by more than 20 degC, yet the top temp onyl changes a couple of degrees