By throttling the gauge manifold, I have always just been able to get a feel for how much liquid/gas I am getting into the suction line/service port. I see no need for these devices. But I do know that was designed to be hooked up to the bottle, and the service hose hooked to it. That allows for a lot of flashing to take place before it gets out of the service gauges/hoses.
Bad information is worse than no information at all.
There are three kinds of people in this world. Those who can count and those who can't!