Alternative methods for monitoring tank levelsBoat Electrics
Kurt Kupper*



Alternative methods for monitoring tank levels

Last month we discussed resistive tank level sensors in detail. However, there are other sensors based on alternative operating principles. Some can also be used on fuel tanks, some are suited specifically to fresh, grey or black water tanks. There are too many options to cover in detail, so we will try to briefly touch on most of them just so that readers are aware of the alternatives for possible further research.
As discussed, the resistive senders with a rheostat on a lever arm cannot be used on any tanks containing water. The type with the reed switches in a vertical stainless tube can be used for water and fuel, but when used in black water tanks are subject to malfunction, because solids very soon prevent the free movement of the bobbin float.
A variant of these sensors that has a perforated stainless steel sheath inside which the float moves overcomes this problem. The complete assembly can be unscrewed from the tank and the sheath easily removed to facilitate routine cleaning. These sensors are in our experience a better option than alternatives that purport to be maintenance free.
Capacitive sensors sense the current that flows between two adjacent conductors through an isolating medium. The conductors are two parallel rods or a coaxial rod and tube arrangement that hang vertically in the tank. The current flowing between the conductors depends on the medium separating them. Thus if the rods are submerged in say water, more current will flow between them than if they are separated by air. The capacitance is a nearly linear function of fluid height between empty and full. Most of these are calibrated for a 4mA to 20mA empty to full range. Sensors and gauges are available for petrol, diesel, water and black water. Some brands contain circuitry that converts the output to a varying resistance or voltage signal.
A more recent development is the use of ultrasonic sensors. They fit onto the top of the tank and have no parts hanging into the tank. They can be used in any medium and tank depth over 200mm deep. They are programmable for just about any tank shape, which is ideal for boat tanks which often have sloping sides, as this overcomes the problem experienced with most traditional systems that assumed a regular tank shape and thus a linear relationship between liquid depth and volume.
Cutting a hole into a tank to fit a sensor is not always possible or desired. Various products are available that stick sensors onto the outside of the tank and sense the liquid through the tank wall. The sensors are connected to indicator lights that light up if there is liquid at the level of the sensor. The display would typically indicate empty, ¼, ½ and ¾ full. Some products can only detect water, some can only be fitted to plastic tanks.
A seldomly used principle is the one that utilizes a pressure sensor at the bottom of the tank.
Lastly, we should not forget to mention that there are non-electrical alternatives to measuring tank levels. The most basic of these are by sight (i.e. open the cap and look into the tank!), using a dipstick or fitting sight glasses.
A lot more sophisticated yet still beautifully simple are the systems using air pressure. Essentially these employ a tube that hangs vertically to the full depth of the tank. The tube is connected to a small plunger type air pump and an air pressure gauge. To take a reading, the plunger of the pump is operated once, blowing air down the tube, thus purging any liquid in the tube hanging into the tank.
We now have a column of air inside the tube, surrounded by the liquid in the tank. This exerts a back pressure up the tube, which is indicated on the gauge. The greater the height of liquid surrounding the column of air, the greater the pressure that will be read. An empirical calibration has to be made once to cross reference various air pressures with corresponding tank volumes. One gauge and pump assembly can be used to monitor up to ten individual tanks. The system is just about fail safe.

*Kurt Küpper is director of Aquavolt Electric Boat Parts. Tel: 02 9417 8455 www.aquavolt.com.au