Question from a SW Engineer again.

I've got a device which is mains powered and outputs a 4-20mA current depending on the flow rate in a pipe. I'd like to measure that flow rate with a microcontroller.

On the face of it it's easy. My microcontroller is 3v3 so max current should produce max voltage of 3v3. So R = 3v3/20mA = 165 Ohm. If I pass the current through 165R and measure the voltage across the resistor it's all good.

My problem is I'm unsure if I can actually do that. The uC will measure a voltage relative to it's ground connection so I have to connect the uC Gnd to the negative side of the 165 ohm Resistor. Is that safe?

I was then looking at current mirror circuits but they all share a Gnd plane between the two sides of the mirror so it's no better then the simple 165R.

Is there a "correct" way to isolate this sensor or is it a case of just connecting the ground. My uC will be powered by a Mains - 12v DC power converter so maybe ground will be the same on both sides?