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Re: How do Shunts work?
A shunt is simply a precision resistor, of a low resistance value (i.e. one ohm) that has been carefully calibrated so that the voltage drop across it will be a precise number of millivolts per amp of current. (If that's all greek, look up Ohm's Law and voltage drops in resistors.)
Non-techie bottom line: By carefully machining bits of specific metals, you wind up with a shunt that has a voltage drop of 50 millivolts per amp (of the current passing through it) or sometimes 75 millivolts per amp, or another number. Let's say for simplicity that the shunt had a drop of ONE millivolt per amp.
In that case, you could simply hook up a voltmeter across the shunt, read the number of millivolts that were being dropped in the shunt, and read that as amps. Since the typical shunt is dropping 50mV per amp, you now need to add a voltage divider to the voltmeter, so that when the voltmeter sees 50mV, it says "one". 100mV, it says "two".
A voltage divider can be really trivial, so it doesn't really affect construction. The point is that the meter and the shunt must be matched, so they are counting the same quantities as "one" amp.
Some of the things you might not consider: The metals used in the shunt shouldn't oxidize or rust. They should be dimensionally stable as the ambient temperature changes, and their resistance also shouldn't change within the designed load range. If the shunt heats up and changes resistance--all the readings will be off.
So it is really a trivial bit of engineering and machining, but only trivial if the guys who are building it know what all the rules are. And if it is properly made, it works "forever" with zero maintenance or calibration required.