Home' Afloat : AFLOAT March 2015 Contents 48 AFLOAT.com.au March 2015
cool or warm white
• G4 back or sidepin:
8 SMD $15.00
10 SMD $20.00
15 SMD $25.00
• Festoon 37mm or 42mm
6 SMD $15.00
MR11- 6 SMD $15.00
MR16-15 SMD $25.00
• BAY15D Tower
18 SMD $35.00
For full details see our website
11 Babbage Rd, E-Roseville NSW 2069
Tel: 02 9417 8455
Fax 02 9417 8423
by Kurt Küpper*
* Kurt Küpper is director of Aquavolt
Electric Boat Parts. Tel: 02 9417 8455
TANK LEVEL MONITORING
Most boats have one or more tanks
that hold vital liquids such as
fuel, fresh water or grey or black
wastewater. Clearly it is important to know
how much or how little of these liquids is
in these tanks.
By far the most common system is by
a resistive gauge connected to a sender
unit in the tank. For fuel tanks, a sender
with a float on a lever arm was traditionally
used. These are adjustable for tanks of
A wire lever arm with a float at its
end is attached at its pivot point to a
rheostat type slider. This transmits a
signal of varying resistance to the gauge.
The pivot assembly is mounted on a fixed,
vertical support that can be adjusted to
be positioned at the middle height of the
tank. The length of the moving lever arm is
also adjustable to suit various tank depths.
Most such senders are attached to the
tank by means of a flange with dimensions
standardised by the SAE (Society of
Automotive Engineers). These flanges
have five holes on a pitch circle diameter
of 54 mm. It is important to note that the
holes are deliberately not positioned in a
regular angular pattern.
This is done so that the sender can only
be fitted in one position, to prevent it being
accidentally fitted in an orientation that
may result in the lever arm touching any
side of the tank and thus possibly giving
an error reading. Because the rheostat
is immersed in the liquid, these senders
cannot be used in tanks containing water.
These are cheap senders and very
convenient, because they can be adjusted
to suit a multitude of tank depths. However,
some water gets into most fuel tanks,
leading to corrosion of the rheostat that
soon causes it to fail.
Much better are the senders that
have a series of reed switches in a sealed,
vertical stainless steel tube. Because
the electronic circuitry is fully sealed in
the tube, they are also usable on tanks
containing clean or dirty water.
A bobbin float containing magnets
rides up and down the tube, triggering
circuits of varying resistance at different
heights, thus sending a signal to a gauge.
These are very reliable, reasonably priced
and thus highly recommended.
They are not height adjustable, so the
tank depth must be determined accurately
and the next shorter length of sensor
available specified. Available lengths
range from 150mm to 1500mm.
Remember that the tops of most tanks
can flex when the boat hits waves, so some
clearance of the sensor to the bottom of
the tank is necessary to prevent it being
pounded against bottom. These sensors
also have the SAE 5 hole flanges used on
the lever arm type sensors.
Fitting tank sensors with the five-hole
SAE flange is done in one of two ways.
One can cut a central hole large enough
to allow through the parts of the sender
that need to be inside the tank, and then
drill five smaller holes for the screws. The
latter have to be positioned very accurately.
Then self-tapping screws are used to
secure the sender.
An alternative is to buy a threaded
counterflange that is fitted on the
underside of the tank top. The top wall of
the tank is thus effectively sandwiched
between the sender and counterflanges.
In order to be able to get the
counterflange into the tank in situations
where there is no access to the inside of
the tank, the ring has a gap cut out of it,
giving it a C-shape.
Only one hole needs to be cut into
the tank, with a diameter that is just large
enough for the five mounting screws to
fit inside. At least two retaining screws
are fitted through the sender flange, top
sealing gasket and into the corresponding
holes of the counterflange. Then the
assembly is inserted into the tank with
a twisting action and the five screws
tightened fully, so that the tank topped
is firmly clamped between the flanges.
The reed switch sensors are also
available with a base with a 1.25”BSP
thread, which is a useful alternative to
the SAE flange when a new tank is being
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