For my day job I design engineering plant systems, mainly consisting of copper pipework. This tool has been designed so that i can insert a tube of copper and quickly be able to identify what size it is.

The sizes on the gauge are the imperial diameters of copper tube that my company use, up to 2 1/8" (I've also added the metric equivalents). However, the principle can be carried over for whatever pipe sizes you need.

The triangular peep holes lie flush with the bottom face of each interior step, and each peep hole will have a corresponding pipe size. When you insert the pipe, it'll drop down into the smallest step it can fit in, and you'll be able to see the copper through the hole (or the light passing through the holes underneath), indicating what size pipe you have. Each step diameter is the width of the pipe + 0.5mm for a bit of clearance.