Saturday 31 December 2011

A Dipstick on the boat!

A project that I had been meaning to get on with is a Diesel Tank Dipstick. Rosie does not have any fuel or water level metering installed. The old way was to poke a length brush handle down the filler until it reached the tank bottom. Pull out the handle and then read the amount of fuel remaining. 

Well I say read the fuel level, it was more of a guesstimate of the fuel level, as the broom handle is not calibrated in any way. The occasional dip was also not the best way to keep the tank clean and there is always the risk of contaminating the tank in some way. Rosie has a 206 litre fuel tank. I know the tank capacity and that the tank is made of stainless steel. This information was gleaned from the boat builder supplied boat owners hand book.

My first pass at making a calibrated dipstick was to get a piece of flat brass stock from B and Q and to give it some sort of removable calibration marks. This I did with a china graph pen. It was based on the amount of fuel being added to the tank during our last cruise. I was able to reasonably calibrate the location of the 25, 50, 75, 100, 125 and 150 litre marks.

I marked the different capacity levels on the brass stock with a hammer and chisel. I was later in the cruise able to predict when filling at a chandlers that the tank would hold an extra 50 litres. It actually held 48.6 litres before reaching the top. So the calibration gives a bit more of an idea than the broom handle guesstimate ever could.

Then I had an idea, and I came up with the Rose of Arden Dipstick Mk II.

I thought, why not include a method of checking for water contamination each time I dip the tank to check the actual fuel level. If you drop a hollow straw into a liquid, the liquid will flow up inside the straw to the same level as the liquid is in the container. When you remove the straw the liquid will flow back out of the straw into the container. However if you use your thumb to seal the top of the straw once the liquid has reached the same level inside the straw. Then a vacuum is created above the liquid, thereby stopping the fluid from draining out of the straw.

If you place your thumb on top of the straw and then push the straw to the bottom of the liquid before releasing your thumb. The liquid entering the straw can only come from the bottom of the container. This is the method I use to ensure that I get a sample from the bottom of our fuel tank.

The dipstick requires the sighting tube to be used just like a laboratory pipette. Pipettes work by creating a tube with a closed end at the top and an open end at the bottom. Dropping the pipette into a tank containing a liquid and then opening the top allows liquid to be draw up from the bottom of the tank. Sealing the top of the pipette then creates a vacuum above the liquid. The vacuum holds the liquid in the sighting glass when the dipstick/pipette is removed from the tank.

So I modified the dipstick so that the fuel level in the tank would register in piece of clear plastic tube. I use the type used for pumping air into a fish tank. I ensured that the tube supports would also coincide with the major fuel capacity points. Then the small sample of fuel draw up from the bottom of the tank (this is where any water or other contamination will collect) can be released into a small container and checked for water or any other type of contamination.

This project could be easily modified to make a Water Tank Dipstick.


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