I have two hot water tanks, one is heated by heating elements, and both are heated by a heat pump. The one with the electric ellemtns flows into the other as the under floor heating takes heat from one and returns cold to the other.
Basically, I want to measure the average temperature of both tanks as one measurement, in order to r=trigger the heat pump. I want to avoid constantly cycling the water through them.
So I came up with the idea of putting 2 pt1000 sensors in parallel at the top outlets and 2 in parallel at the bottom inlets, and then wiring the pairs in series in order to average the temperatures.
For any project, trying to cut corners or trying to outsmart yourself is never a good idea, certainly not at the very start of the data path.
The physical sensors are the start of the path that results in calculating the temperature and, for example, showing it on a display. Treat the sensors as if they are the most important part of the project.
Suppose that a wire of one of the sensors is broken or if there is a short circuit. You can detect that if each individual sensor can be read by the Arduino, but not if you have combined multiple sensors into a single signal.
Try to get the data from each individual sensor as good as possible into your Arduino board. After that you can do in software whatever you want.
Putting two RTD’s in parallel is the same effect as putting any other two identical resistors in parallel. You get half the resistance. So your 1000 ohm RTD’s would show 500 ohms. A very very cold temperature.
But then the parallel pairs are put in series to get back to 1000. The result is rather meaningless since there is no idea what temperatures are contributing to the result.
Yes I have to use a pt1000. It’s an existing heat pump with one pt1000 in a thermo well but it only takes the temperature of one tank, and now I have two. The second tank has no thermowells, and is also fitted with heating ellemnts that heat just that tank.
I was thinking that if I put a sensor at the top of both and at the bottom, and put in series the 2 pairs, they’d average out roughly.
Ie tank one top 70c, bottom, 20c & tank two top 50c and bottom 40c.
70c = 1270
20c= 1077
50c=1194
40c =1155
Maybe if the pairs were grouped in a certain way, ie the most likely hotter pair (tops of tank) and the most likely colder pair (the bottoms).
You can strap ds18b20’s to the outside of the pipe and then add insulation on top, it will be pretty accurate !
And don’t forget water sits in layers and does not mix unless there is a flow , so the average of a top and bottom temperature is not the average temperature of the water , it can be virtually full of hot water and the bottom will still be cold .( which why the water remains hot until the tank has been emptied of hot water and all replaced with cold).
The layout of the tanks is important - do they thermal syphon between them ? Where is the hot water taken from? . Where is the cold inlet ?
How does the pump circuit work ?
Normally I would expect a three way valve to direct the heat pump output to either tank, controlled by thermostats near the base of the tank .
Your existing PRT maybe to control the outlet temperature of the heat pump.
You haven’t said so explicitly, but is the reason you want to do this so you can just plug the parallel arrangement into an existing input of the current control system? Your goal is to not create a different system using an Arduino or other microcontroller. Correct?
Yes that’s it. The heat pump has a NTC as a temperature sensor input. I need to have a rough average of the temperature of both tanks to feed the heat pumps control system.
I am not using an Arduino.
