Water level metering and oxidized copper wires

Artemis has two 500 liter water tanks. These are integral with the hull and on each side of the keel. Just where you want one ton extra of ballast.  There is a water level gauge on the main utility panel. Since Artemis became ours, they always showed full regardless of actual level. An annoyance and not the highest of priorities but when I had time, I began tracing the problem. 

In old school electrical systems, typically these gauge circuits are very simple and look the same in both maritime and automotive worlds.

Essentially the sender simply is a resistor that grounds the instrument. One side of sender is earth (ground) the other comes from the instrument labeled "sender". The other end of the instrument itself, labeled "ignition", is the positive (+) supply that completes the circuit.

One thing that stood out was that both tanks hade a very fancy (advanced) non mechanical gauge from the renowned German brand of VDO. And indeed did it have two wires coming out of it and where joined with the ships wiring. The connection between the sender and ship wiring was of the cheap automotive kind that use isolated spade connectors. They where clearly corroded on the inside and sure enough, some gentle tugging and the wires came off with the telltale whiteish powder. 

As you can see from picture the of the VDO sender, the two connector and wires. No moving parts.

I proceeded to cut off some of the corroded cable as the corrosion of the copper seemed to go into the wire  (not tinned) on the ships side it. It looked like the cables in picture below, no matter how much I cut of, corrosion hade crept up and soon I was going to run out of wiring

It is a tedious process of scraping off this copper oxide and it thins out the strands why the crimping of connector gets compromised. I looked in the ships store of bottles and cans to see if I could find a chemical that could take this off in a neater way. 

I was in luck.

The active compound here is oxalic acid, a mild non toxic and organic acid. Put some in an egg cup with the exposed wire and let it do the job in a few minutes. 

Was pretty confident things would work after this fix. What can go wrong?

 It is rarely easy.

How could such a simple thing be so enigmatic? I took a closer look at the grey cable from the VDO sender and discovered it had not two but four wires..? Time to Google and sure enough the circuit diagram  for this sender is quite different. First of all it is an active device meaning that it needs to have power supplied to it. So, in the way I found it connected, it could never have worked! Both senders where wired identically.

What is more obvious looking at its schematic is that it needs to be connected to a matched instrument...

Time to do it right from the start. A new instrument and two senders where purchased. The sender is mechanical with a float sliding along a rod. It is a Chinese job but with decent quality and in all stainless steel. Replacing the gauge required taking the entire panel apart, of course.

It fit, threaded, directly into the existing flange on the tank with same threads. Thankyou!

This time proper sealed connectors where used. They are completely watertight at both the cable entry point and in the connection. These will never cause a problem again. 

The yellow grommet slides into the mating plug and provides a watertight seal

The reward after the level gauge rebuild is functioning level reporting. A switch selects which tank to monitor. 

Does this experience have a moral to it? 

Perhaps, one is NEVER use bare copper on a ship. All cables must be tinned. Many manufacturers sin with this and I think a trademark of quality in an electrical system is the use of tinned cables. In this case the ships wiring was not particularly exposed to water or salt. Yet, 16 years did its work regardless. Water enters between the copper strands and insulation by way of capillary action. This allows water to climb upwards.

 I will probably never know how it was possible for somenone to install these very expensive VDO senders in the wrong way and walk away from the job when it obviously couldnt have worked.