I am fairly sure that you are getting air into the system. 

There are a lot of ways, but the most common way I see owners getting air into the system is when they change filters. When changing filters, you should always fill the filter canister completely full, then the trick I use is: before tightening down the canister, I slightly turn ON the flushing valve until water spurts out the top of the filter cannister, then tighten it down.

If you feel that you have not allowed air in this way, maybe you should spray some soapy water on the suction side of your watermaker hoses beginning with the hose from the saltwater manifold. Hoses get brittle when they get old and will sometimes leak on suction without leaking on pressure.

I hope this helps you find how air is getting into your watermaker.

Regarding sampling, I modified the output of the Dessalator D160 watermaker to go to a 3-way valve. In one position the valve allows water into the tank, in the other position, water goes to a tap at the galley sink for testing and discard. It is not an easy project. I copied what a Dessalator Rep did on another SM in Martinique. I believe that there might be an easier way that what the Dessalator Rep did and I copied, because he drilled through the engine room/galley bulkhead to run two hoses...it is all properly sealed, but it is difficult to make sure that you are drilling in the correct position from one side to the other...and that bulkhead is double-walled.

You might also check the inline electronic monitoring that Gary Silver did on his SM with a D160. It is in the photo section. It is really nice because it is a continuous TDS monitoring system.

I hope this helps you.

Bill Rouse

BeBe #387

St John, USVI