I don't know exactly how the voltmeters measure it.

But if you dig a hole 1000 feet deep, and then put a 1000 foot ladder in it from the bottom up, the ladder will be zero feet above the ground.

By using -1000 as your REFERENCE, and then adding 1000 to it, you will come up with zero.

If the voltmeter calculates the absolute difference, then good for it. I don't know what the voltmeter will measure.

The point is, the water fuel cell needs -voltage and +voltage that are pulsed equally, and if you do this, then you do it right.

If you try zero and +voltage then you do it wrong

It doesn't really matter how you measure it, this is all about understanding the operation so you can build it with the correct goal.