Hi there!
I (will) have a water temperature controller for our pool this summer. I am starting to plan an Arduino project to interface with a couple functions of this controller, which is mostly just reading digital outputs. So far so good.
What I also would like to do is to read the same temperature sensor which the controller reads. The sensor is a NTC thermistor. I generally know how to measure and calculate the temperature out of a NTC thermistor with a voltage divider setup.
However, here is the catch: The controller already reads this NTC thermistor. If I would just hook it up "normally" to my Arduino, I would interfere with the controllers' readout. Also, as the controller most likely uses 12V, it would damage my Arduino.
Do you guys have an idea how I could get this value without disturbing the other controller? I was thinking that using an "external" voltage sensor and reference the output to the 12V. However, I fear such a sensor could also interfere with the measurement of the other controller.
Read the voltage across the thermistor, with a high impedance input. You should take some voltage measurements there first, to determine the sensor voltage range. Post the results of that here.
Thanks for your response. Well I don't have the controller yet, it is coming in the next months. So I can't measure yet. But I do have a table showing the correlation of resistance with temperature from the manufacturer. It is a 6.8kOhm thermistor.
The "other controller" is a mass manufactured product, I don't know for certain what voltage it works at. But I certainly can find out once I have it. I know that other outputs rely on 12V, so I assume it is the same for the thermistor.
thanks for the support. Would you mind sharing what the options would be, depending on what I will measure? I'd like to understand it, not just ask for a solution and implement it without learning something from it.
I understand that measuring voltage with a high-impedance input makes sense, as it should only minimally impact the voltage measured by the controller. Would there be other ways to do it?
I would use a separate sensor , probably a DS18b20.
If you fix the sensor to the outside of a pipe ( metal) or some fitting carrying the water and then insulate around it , you will get a pretty good result , and you can correct any difference ,
This is the safest option as it doesn’t effect that controller you’ve bought . Also by having a separate sensor you can confirm the controller is doing the right thing ( if there is one sensor and it’s say 10years out , you won’t know )
There is also the issue of electrical safety when it comes to pools and you need the right isolation in your power supply . If you don’t understand don’t do it !
Thanks everybody! I do have a manual, but it is not available in English and it does not contain any technical info that would help.
A second thermistor is a possible idea. I did not consider it at first because I thought the physical installation could be complicated and reusing the one that is already in place would ensure I measure at the best possible location (i.e. where the pool technician places it). I don't think there will be any metallic pipes to mount it outside of the water.
However, I like the idea of having a second sensor to have something to compare the values to each other.