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The Accuracy of Boat Gas Gauges (How Does It Work)

Light aircraft and boats have something unexpected in common. They are both subject to varying forces as they accelerate, turn, decelerate, and get pushed about by wind currents (aircraft) and waves (boats). The earlier models were fitted with float-type gas sender units, and both are renowned for being inaccurate.

If you have a traditional float type of sending unit fuel gauge installed in your boat’s gas tank, it should only be used as an approximate gauge of the gas tank level. However, if a capacitive liquid level sensor gauge is working properly, the reading will be very accurate.


One of the most common questions in boating groups is whether it is normal to have such an inaccurate gas gauge. If this is the type of unit installed in your boat’s gas tank, it should ever be used as a backup to record the engine hours when filling up the gas tanks and the number of hours run in the interim.

The Accuracy Of A Boats Gas Gauges Depends On The Gauge

Two standard technologies are used to act as boat gas gauges.

The two technologies are

  1. Float type of sending unit
  2. Capacitive Liquid Level Sensors

Float Type Sending Units

Float type sending

The oldest and most common type of gas level sending unit is the float-type device fitted inside the gas tank.

It works like the Cistern Ball Valve in a regular toilet.

The float type sending unit consists of

  1. A float
  2. A metal arm
  3. The sender unit

The float is positioned on the end of a shaped metal arm, which is attached to the gas sender unit.

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As the gas is filled into the petrol tank, the ball floats and moves upwards with the gas level. It causes the metal arm to change the angle, which changes its position on the rheostat in the gas sending unit.

As the position of the arm changes relative to the gas sending unit, the resistance changes, and so the current to the gas gauge is changed.

In principle, this is a foolproof method of recording the level of gas in the tank. However, it does have weaknesses.

The Angle Of A Boat Changes Constantly

Boats are subject to constantly changing forces, including wave action, turning, accelerating, and decelerating.

To make matters worse, when a speed boat accelerates, the sender unit and the floats are subject to inertia. The boat’s longitudinal attitude changes as the bow lift and accelerates onto the plain.

The inertia and the plaining motion force the gas into the back of the tank. It will affect the gas level, which is recorded by the gauge.

Likewise, if the boat experiences turbulent water, it will also impact the gas gauge readings.

It Has Moving Parts

As the gas gauge has several different moving parts, it also has many inherent weaknesses.

  1. The electrical attachments can become damaged or corroded, impacting the resistance measure and, therefore, the gas level indication.
  2. The rod’s actual mechanics and pivot on the sender unit can become compromised, making it stiffer and therefore not as freely moving as it should be.

Each of these issues will affect the quality of the gas level, as shown on the gauge.

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Summary Of Float Type Sending Units

While the gas reading from this system may be correct, it could just as easily be wrong. 

If your boat has a float-type sending unit and you intend to cruise a little further away from where help is not as easily accessed, always back up your gas readings against the engine hours that have run since you last filled the gas tank.

Capacitive Liquid Level Sensor Gas Gauge

Capacitive liquid level sensor

Capacitive Liquid Level Sensors do not have any moving parts. 

This technology uses two electrodes (capacitors) placed in a single tube that has a hole in the bottom and affixed to the inside of the gas tank. A non-electrical conducting material (called a dielectric) is installed between the two capacitors. 

There is an electric field between the capacitors; however, the dielectric material prevents the circuit from being completed when the gas tank is empty. When gas is added, the circuit is completed as gas in the tank conducts the current between the electrodes.

The gas enters through the bottom of the tube, and the capacitors measure the quantity from within the tube.

As the gas levels change, the capacitates change progressively, and the gas level can be very precisely determined.

To measure the gas level, a current is introduced to the electrodes, and the current level is measured. 

There Are No Moving Parts

One of the primary benefits of a capacitive liquid level sensor gas gauge is that there are no moving parts involved. There are, therefore, no mechanical linkages that can break or wear out.

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Because of this, there are fewer potential failure points in the device.

This system is more durable, with a longer estimated life than a traditional float-based gas sender unit.

The Measurement Is Very Accurate

While a float-type gas sender unit can record changes in the gas level, the rheostat resistance measure is not as sensitive as a capacitive liquid level sensor.

A capacitive liquid level sensor can measure minute changes in gas level. Better still, capacitive level sensors provide a continuous, ‘stepless’ output. It is as opposed to float sensors which use a series of reed switches that require a more substantial level change to record the difference. 

While this sensitivity could make the sensor vary its reading when in choppy water or while accelerating or turning, the way the capacitive liquid level sensor is orientated, the instability is reduced.

The tube with the hole in the bottom is sitting at the lowest part of the gas tank.

While not immune to sudden changes, the gas which enters the tube is significantly insulated from the surface level fluctuations in the gas tank, as the submerged holes slow rapid changes in liquid level on the outside.

It makes it less sensitive to changes in the boat’s attitude or the inertia which occurs as the boat accelerates, decelerates, or turns.

Capacitive sensors don’t need to be installed vertically. They work just as well mounted at an angle of 60° without modification to help cover the full depth of an irregular tank shape.

Cost Of Capacitive Liquid Level Sensor Gas Gauges

Capacitive liquid level sensors are more expensive than traditional float-type gas sender units. However, the difference is not as high as when they were first launched, and bearing in mind the benefit they offer, they now represent good value in mind.

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If your boat is older, it is almost certain that the gas tank is fitted with a float-type gas sender unit. While this may indicate the gas level, there is no guarantee that the reading is the correct one.

Capacitive Liquid Level Sensor Gas Gauges offer a very precise gas level reading which is not impacted by the speed or angle of the boat. When these devices were launched, they were very expensive. However, while they cost more than float-type devices, the difference is not as extreme as it used to be.