The Roxer Natator 125 is a Rolex approved water resistance tester for Rolexes. This particular piece of equipment creates a pressurized environment for the watch. The Natator has a stainless steel tank which is filled with water and the watch.
Rolex Natator 125 Operation
The Natator is filled with water then the watch is also placed inside the tank. The lid is closed and tightened. Pressure is increased by turning the hand wheel you see on the side of the equipment in the picture. There is a pressure gauge which indicates the pressure being applied to the watch.
This test will determine if the watch is water resistant or not, does the glass crack, has water gotten into the watch, are the seals and gaskets okay? The equipment applies pressure to the watch while underwater, while another tester named the Diabolic E places air into the watch and then submerges it in an effort to determine where the watch may be leaking.
It is best that your watchmaker determines the water resistance of your watch with equipment as opposed to you finding out it is not water resistant in the pool or diving.