Temperature Logging Project from External Sensor

Projects General Guidance
1

Hello,
What I thought was going to be a simple project is either turning out to be quite difficult or I am missing something very obvious

I have a project where I am trying to record temperatures from a Thermocouple using an LED display with an analog voltage output. I am able to successfully test this with my multimeter. A reading of 95F outputs 95mV to my meter. I figured hooking this up to my arduino on two different analog pins and measuring the voltage between the pins was pretty straight forward but no....

I have an SD Data logging shield for an Arduino Uno, but I cant get it to read the voltage. Both the thermometer and the Arduino are going to be on non-common grounds (cant be avoided) so thats part of the challenge here.

The difference in voltage is very small but this is also over a very small 25 mA signal strength. I believe this is what I did not foresee.

My next thoughts are to get an op-amp setup to amplify the signal. While my multimeter can read it I dont think the arduino can. Looking for an off the shelf accesory board if I can find it. Would appreciate any help / thoughts / advice / widsom. Thank you.

2

An ADC input of the Arduino is designed to measure a voltage with respect the Arduino GND, with the Arduino GND connected to the input signal GND.

With default settings, the Arduino Uno R3 can measure voltages between 0 and 5V, in 5 mV steps, so 95 mV would correspond to a very small signal (19 on a scale of 0 to 1023).

You can improve that by using the 1.1V internal ADC reference voltage instead, resulting in steps of about 1 mV.

3

The thermocouple Type unknown has a very low output voltage it needs to be amplified. Also the voltage is generated by disimilar metels touch each other so your leads need to be the same material. When it switches to the electronics you need to add what is called cold junction compensation. A nice part that does that is the MAX675.