The trick is to get it there. The test is a lot more reliable if you use deaerated water to remove any small amount of bubbles. Rotary pumps are often selected to move liquids with a low vapor pressure point, or fluids with a lot of entrained bubbles. The Hydraulic Institute establishes the point at the first indication of any of the following. Cavitation noise is heard.
Rotary pumps present a few advantages over their centrifugal cousins.
You can change the flow without upsetting the pump's efficiency. The pump can handle high viscosity fluids efficiently. The pump is self priming You get a smooth pulse free flow of the liquid into the system. You can get the desirable high head low flow combination that is need in many high pressure applications. In summary, PD pumps are great pumps and we would use a lot more of them if they could produce the volume of fluid most of our process applications require.