A simple site build rig that measures the rate of evaporation. Some pointers to help you set this right.
The coin shows the key step to the evaporation control. It is measuring the water level in this shower flood test.
Anything can help set the control. A baking dish, an old tool box - something water tight. I like to tip the stack of coins the same way in the evaporation control and then teach the clients how to read this test.
Above you can see one of my drywall mud boxes with some scrap tile and the coins.