In my professional experience, pressure tester systems are the best way to compare the pressure in two systems at the same time. I can measure the pressure in the engine block of a car or the engine of a truck and the pressure in the oil pan and the pressure in the cooling system of an engine.
Here’s the thing though; I don’t really like working with pressure tester systems. I’m not sure why. They’re great at gauging the pressure in one system, but they don’t have anything to say about the pressure in the other system. We have to take them out of the equation and just say, “This one is pressure sensitive, this one isn’t.” What we ultimately need to do is find a way to measure the pressure in both systems and then compare them.