Mr. Henry Ehrens, President of Supco, developed what he called a “balance tester” for testing the flow capacity of a cap tube. It was manufactured and sold as the Supco Compressor & Capillary Tube Tester. As simple as it is to use, it was never truly understood, so few ever made it to the field. The following info was donated by MechAcc from HVAC-Talk, who was fortunate enough to have found two balance testers.
Here’s a PDF slide show reproduction I made of the original user’s manual and a diagram showing how it’s set up for testing: