Some additional thoughts around the Dessalator TDS sensor... We have one of the older boats where the sensor actually does control the reject valve. I do not know how to tell which type of circuit board you might have on your boat. And while it works... as Bill Rouse says it is not to be trusted, or at least not without regular verification.
The TDS sensor consists of a rubber plug with two stainless steel pins into the water flow. The resistance between those pins is measured. High resistance is "good" water and low resistance is "bad" water. So far so good--in theory.
The problem is that the system is terribly "un-robust," and almost everything that can go wrong with it makes you think that "bad" water is really "good."
The first problem is that the stainless pins corrode over time. The layer of corrosion on the pins increases the measured resistance making "bad" water look like "good" water to the system. This was my first clue about the corrosion issues with this sensor. Our reject valve began accepting water immediately on starting the pump, which was a change in behavior, and not what I'd expect. If your system is accepting water from the membranes as soon as you turn it on, check the sensor for corrosion.
Second problem: The pins can become disconnected. The system dose not seem to recognize this and just assumes that the infinite resistance it sees means that the water is perfect...
Third problem: The measured resistance will actually vary depending on how much of the pins are exposed in the water. The design of the sensor is literally two 3mm SS pins stuck in a rubber plug. There is nothing that fixes their position. Deeper immersion of the pins means the water would have to have lower TDS to trigger the switch.
So... Operational lesson always check your water with another meter.
Another lesson, If the pins on the probe corrode, the probe (the rubber plug and two pins) can be purchased from Dessalator US distributers for about US$75. You can also reuse the rubber plug, get 3mm OD stainless rod, cut it to length, and make your own new pins for a few pennies each.
Annapolis, MD, USA