Like diat150 I have been in the Oil Measurement Business for a long time (30 years) and haven't seen a flow computer give the option of totalizing off of a 4-20mA signal. I am not as familiar with the Total Flow units as I am with other Flow Computers so they may do things differently.
But what if it was a custody transfer application? Would that make a difference?
To answer your question: In a true Liquid Petroleum Custody Transfer application you will need to use a volume pulse input to the Flow Computer. The reason is that the meter will need to be Calibrated or Proved as it is called in the industry. To do this you will need to use what we call a Prover. This device is highly accurate and has a known volume. The electronics in the Prover will accept the volume pulses and compare them to the known volume of the Prover. It is pretty much all a software function now. In the old days we had to do it manually. A Meter Factor is then generated to be put into the Flow Computer. The Flow Computer will use this factor to correct the volume so that it shows the true volume based on the Proving. Since Volume Pulses were used to calibrate the meter it must be used to totalize the total volume. That way things are Apples to Apples.
I get the response that the Total Flow has some magical algorithm that just makes it right.
Of course the meter is only one part of a Custody Transfer system. You will also need to run the Temperature, Pressure, and Density into the Flow Computer where it will take these measurements and create a true Net Volume. So yes while not magical, the Total Flow, like all Flow Computers, have algorithm's that make all the calculations in order to present the totals in a way that conforms with the API recommendations or Contract requirements.
There are a lot of things I skipped over here to keep things basic but I will say this: I have been designing, building, automating and repairing Petroleum Measurement equipment for a long time and just about every day I learn something new. The field is very vast and complicated, it is full of "this is how we have always done it" and sometimes when I think that there is no way that something is going to work it ends up working and I come away with a new learning experience.
Good luck in your journey.