I was thinking this would be more common but I'm not coming up with much.
I want to monitor some sensors (mainly all temperature for now) from an existing system. For example, some 3 wire thermistors that the system needs to still monitor for its own protection and knowledge. They are red/white/black and seem to have 5v reference. I would like to 'piggyback' off the terminals on the circuit board into an Arduino of some sort so I can publish the data to my home automation.
Another sensor is a simple 2 wire 10k resistor which seems to be fed 2.120vdc from the thermostat it's connected to.
I realize it's quite obvious I'm fresh to this game, but I'm hoping someone can help me out.
As implied the problem is whether the existing system is affected , there may also be isolation safety problems - this is a bit unknown. Also if they are thermistors , knowing the output voltage verses temperature characteristics will be a problem.
In any case if you want to measure the performance of a system , using seperate sensors will always give the best result .
Have a look at DS1820 temperature sensors , cheap, easy to use and accurate.
Yes, the existing board cannot get the data in any form that it isn't now. I didn't know if I could simply feed the data to the arduino in parallel and just monitor it.
It's a Geothermal system and everything is quite integrated where it'd be nice to make use of factory mounted accurate sensors that are custom fitted to plumbing and what not.
I for sure have the data sheet for the 2 wire buffer tank thermistor, but I'll have to disassemble some of it to track down the 3 wire sensors.
Some of these are buried, or in the case of the buffer tank, custom to get an accurate reading from the tank. Its threaded into the top and at a certain length to get consistent, stable readings.
I don't know enough to ask the right questions, I guess for now I'm curious as to why it's not possible.
A thermistor changes its resistance with temperature. To know the temperature, you must know the detaiiled response characteristics of the particular thermistor and you must measure its resistance.
To measure resistance, you need to give it a known current and measure the voltage or (more common) give it a known voltage and measure the current. For the best accuracy, you must be able to eliminate the resistance of the wires leading to the thermistor because that may also vary with temperature. The best way to do that is a 4-wire or "Kelvin" connection to the thermistor. By sending voltage down alternate wires, the thermistor amplifier can measure and subtract the resistance of the wires.
So now imagine Doofus comes along with his multimeter and tries to measure the resistance of the thermistor. Measuring voltage is easy - just put the probes onto the two ends of the thermistor. But Doofus doesn't know how much current is passing through the thermistor, so he can't calculate resistance. Worse, some of the current that the original device was sending is now passing through the multimeter. Two resistances in parallel give a lower resistance, so it sees the wrong value on the thermistor.