Tank Liquid Level Measurement ~ Phase 1 Complete January 2016

Knowing the amount of fuel or water you have on board is never really important for coastal cruising, it is however for offshore sailing. Rua Fiola was fitted with dipsticks that require you to lift some floor hatches an unscrew the dipstick, look at some wetness and decide whether you have enough or need nore. This while simple is a bit touchy, and we notice many cruising boats have spare cans lashed to the safety railings. Is it for extra capacity or in case they run out? You decide. Another problem is that multiple tanks are interconnected and have valves to control flows. It is quite easy to run out and request help only to find that another tank had a good supply but was turned off.

We wanted a definitive answer - how many gallons do we have and how many gallons will it take to fill things up. Is there a safety cushion with zero on the gauge but a few gallons still in the bottom of the tank. We llked a Blue Seas systems liquid level monitoring and the parts cost for 4 tanks with ultrasonic senders and the display was over $1,000. We could go to typical resistive swing arm senders and gauges from Moeller but that seemed crude. We liked WEMA Reed Switch ~ Magnetic sensors. They have 9 reed switches so your readings are every 11% +/-. They work for all fuels, water and even holding tanks. When you buy the senders you specify the lenth you want to fit the tank. We deliberately set the lengths a little shorter than the suction tube so that we would read empty before we started to draw air. The holding tank measurements we will defer to a later date.

Most fuel senders give readings between 32 and 240 ohms. You can apply a known voltage across the sender and read the current flow with an ammeter which will be proportional to the resistance. We decided on making a sensor one leg of a voltage divider, then we could read the voltage change as the resistance changes. Votages are easy things to work with in Arduino circuits. What we needed was a method of switching between the 4 sender voltage divider circuits. Crack the books and re-read using a transistor as a switch. Final step is to use a keypad to tell the transistor ladder to read another voltage and thenfind out how to display this on a 2 x 20 LCD display. When finished we had used up every port on the old Arduino Mini. If we want to include holding tanks and make some other things on the boat under program control we could use an Arduino Mega 2560.

Tank Liquid Level Measurement ~ Phase 2 In-Process

We now have some fiddly electronics which is barely better than a breadboard prototype. Where to put it and how to protect it? At this stage you wonder why you did not go with a Blue Seas solution. Ego often overcomes reason. We needed to control some other functions so we put a capabilities list together for a sub panel.

The image shows the CAD drawinf for the sub Panel that we used for programming the plasma cutter. It is 20.75" tall and 5" wide, made from 1/8" aluminium sheet. The RHS shows the red lines that are the external dimensions of the items in there respective locations. At one time we were going to put Rhule Bilge pump AUTO/OFF/MANUAL control switches, but decided not as our experience with the always on strategy wa s working well.

When you power up the board with it regulated power supply switch and measure Fuel or Water levels, the readout tells you how many gallons to add to fill the tank. As we are only drawing from one fuel tank at a time, when we change over tanks we open the second tank valve first wait a short time before closing the first tank valve. As all tanks are nominally 50 gallons a little mental arithmetic tells you the current status. We have a decision to make at this time. The circuits work well but will unboubtedly fail due to salt corrosion and we also need to start measuring holding tanks. We have never designed our own printed circuit boards, but we will probably need to do this if we want the liquid level system to be reliable for the next 10+ years.