Hi now I understand how to use the mosfet. Whel I will try like dc42 said...
Hi everyone, as promised I am back reporting on what I've done with the results.
By the way, I'm not sure why you were told to get a MOSFET involved.. That's not necessary.
So, basically what I have done is take the +5VDC and GND from the arduino board to a dc to dc converter. I used a Murata NME0505SC.
Then it's quite simple, take two 6N137 optocouplers and wire one to the RX and the other to TX. You'll need to use two resistors for each one, I just used 430R and then you need to put a .01 microfarad cap between pin 5 and 8 for each one per the datasheet. Obviously each of them will be on the separate ground/potentials.
Anyway, everything works perfectly now!! Yay! I have my readings stable and accurate with the probe circuit electrically isolated.
Can you post photos and the schematics of your design?
I will be really helpful, in case the mosfet don't work I can try this way!
Thanks you guys 