Multi-Conductor DC Clamp Meter
Can't remember the name but its the line integral of H dot dl around a closed loop = current enclosed, and since you (should) have the same magnitude current, but in opposite directions, the current enclosed is zero. That is on the conductor.

But ... I have had a problem at work with inaccuracies measuring DC current; when the DC clamp meter got too close to the coil of a 24VDC relay (designed to carry 400A), the reading on the meter would change. And then if I lined it up directly with the coil of the relay, I could get even bigger offset. It shouldn't because it has a metal shroud over the coil, but there it was. As I recall, the coil side was 1 &lt; x &lt; 3A. 

Maybe you can poke around the area of the coil of the known working solenoid valve with your DC Clamp meter you are trying to analyze, assuming it is safe, and you have access to that point, see what happens.