Katerina Swanson
The measurement of fuel in a tank depends on the type of tank and the vehicle in which it is installed. For instance, in the case of vehicles like trucks, the fuel level is measured using a fuel gauge, or a depth measurement with a stick. In the case of airliners, the amount of fuel is measured as mass (kg or lbs) instead of volume, as mass is important for weight and balance. The fuel volume is measured using capacitance probes and then converted to mass by measuring the fuel density.
How fuel is measured in a tank
| Characteristics | Values |
|---|---|
| Fuel gauge | Installed in the tank to measure the amount of fuel |
| Measuring stick | A stick is lowered into the tank to measure the fuel level in inches |
| Tank volume calculator | Used to calculate the volume of fuel in the tank by inputting tank dimensions and fuel measured in inches |
| Rule of thumb for round tanks | When the gauge hits 1/4, the tank should be considered empty |
| Depth measurement | In round tanks with built-in steps, depth measurement with a stick is necessary to dial in a fuel sender |
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What You'll Learn
Fuel volume in round tanks is hard to measure accurately
The volume of fuel in a tank is calculated based on the tank's geometry. While calculations are straightforward for tanks with simple shapes like rectangular prisms, cylinders, circles, and spheres, tanks used in vehicles are rarely perfect geometric shapes. Therefore, calculations for fuel volume are often only estimates.
For example, the total volume of a cylinder-shaped tank is given by the area of the circular end times the length. The area of the circular end can be calculated using the formula A = πr^2, where r is the radius. However, actual fuel tanks may not be perfect cylinders, and there may be other features that are not accounted for in these calculations.
Oval tanks present a similar challenge. While the volume can be calculated by finding the area of the end and multiplying it by the length, oval tanks used in vehicles may not conform exactly to the geometric definition of an oval.
In addition, the shape of the tank can make it difficult to determine the volume of fuel in a partially filled tank. For example, in a horizontal oval tank, the volume of fuel in the tank depends on whether the tank is filled in the two halves formed by a horizontal cross-section.
Due to these challenges, measuring the volume of fuel in a round tank accurately can be difficult, and calculations often only provide estimates.
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Fuel mass is computed from volume and other factors
Fuel mass is not measured directly; instead, it is computed from volume and other factors. For instance, the mass of the fuel in a car's tank can be calculated by knowing the number of gallons/litres in it and the specific gravity of the fuel.
In aviation, the fuel volume is measured using capacitance probes, which calculate the fuel quantity based on the level of fuel in the tank. This volume is then converted to mass by measuring the fuel density. The Airbus A320 family of aircraft uses such a system, where the Fuel Quantity Indication (FQI) system calculates the fuel quantity. The pilots require the information in terms of weight, which is why the volume is converted to mass.
In larger aircraft, the fuel quantity is displayed in kg or lbs, as mass is crucial for weight and balance considerations. It is also important because the energy stored in the fuel is proportional to its mass. When the engine is set for cruise parameters, it will burn roughly the same mass per unit of time, making mass a more useful parameter for range calculations.
Volume, on the other hand, can be measured easily with a simple float in the tank. It is expressed in litres or gallons, which are convenient units for measurement. However, volume is not as accurate as mass because it changes with temperature. For instance, 40°C avgas has 10.5% more volume than -40°C avgas of the same mass.
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Fuel volume in airliner tanks is measured via capacitance probes
Fuel volume in airliner tanks is a crucial parameter for pilots to monitor, and this is achieved through capacitance probes. These probes are a part of an aircraft's fuel quantity indicator system, which provides information on the fuel level in the tanks. The Airbus A320 family of aircraft, for example, employs capacitance probes to measure fuel volume.
Capacitance probes are electronic sensors that are installed inside the fuel tanks. These probes consist of two concentric plates separated by a fixed distance and are open at the top and bottom, allowing the fuel to fill the space between the plates. The key principle behind their operation is that the capacitance of a capacitor depends on the dielectric constant of the material between its plates. In this case, the material between the plates is either fuel, air, or a combination of both, depending on the fuel level in the tank.
As the fuel level in the tank changes, the amount of fuel between the plates of the capacitance probes varies, resulting in a change in the capacitance. This change in capacitance is measured and used to determine the fuel volume in the tank. The probes are installed in multiple locations within the tank to ensure accurate readings, even when the aircraft manoeuvres and the fuel shifts due to the attitude of the aircraft.
The capacitance transmitted by all the probes in a tank is totalled and compared in a bridge circuit by a microchip computer in the tank's digital fuel quantity indicator. This system compensates for temperature variations, which can affect the dielectric constant of the fuel and, consequently, the capacitance readings. By accounting for temperature fluctuations, the system provides a steady indication of the fuel volume, regardless of the aircraft's attitude or fuel temperature.
Additionally, the information obtained from the capacitance probes can be used to calculate the fuel mass by measuring the fuel density. This conversion from volume to mass is performed by the Fuel Quantity Indication and Conversion (FQIC) system, which utilises the fuel density values measured by specific devices in each wing tank. The resulting fuel mass information is then displayed to the pilots.
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Fuel gauges in trucks are not always reliable
However, there are several reasons why this system may not always provide an accurate reading. Firstly, the shape of the fuel tank can affect the accuracy of the measurement. If the tank is not a perfect cylinder but has an irregular shape, the fuel level may only vaguely correlate to the actual fuel volume. Additionally, the fuel sending unit itself can malfunction, causing the fuel gauge to read incorrectly.
Another factor that can affect the accuracy of the fuel gauge is the presence of extra fuel in the tank that is not accounted for by the gauge. For example, when the tank is full, the float may be completely submerged, and it takes some time for the float to start moving as the fuel is used, resulting in extra "small ticks" on the gauge that indicate fuel usage without actually moving the needle. Conversely, when the tank is running low, the float may hit the bottom of the tank before it is completely empty, allowing you to continue driving even after the needle has hit the 'E'.
In modern vehicles, electrical gauges are generally considered to be as accurate as their mechanical counterparts, provided they are purchased from reliable manufacturers. However, it is still possible for electrical gauges to malfunction, and in some cases, they may be intentionally designed to hover around the "normal" range rather than displaying actual temperature variations. Additionally, aftermarket gauges, such as boost gauges and exhaust temperature gauges, are gaining popularity among diesel truck owners who are modifying their vehicles for performance. While these gauges can provide valuable information, their accuracy depends on the quality of the parts and the installation process.
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Calculators can help determine the volume of a cylinder-shaped tank
Calculators are a useful tool to determine the volume of a cylinder-shaped tank, which is a common shape for fuel tanks. The volume of a cylinder is calculated by multiplying the area of the circular end by the length of the cylinder. The formula for the area of the circle is π x radius x radius, or π x r x r. So, the volume of a cylinder is calculated as: volume = π x r x r x l, or π x radius^2 x length.
Online calculators can be used to find the volume of a cylinder-shaped tank, as well as other shapes, including rectangular, oval, capsule, and elliptical. These calculators are versatile and can be used for water, oil, and fuel tanks. The user simply inputs the shape of the tank, the dimensions, and the calculator will output the volume. The volume can be given in various units, including cubic meters, cubic feet, litres, US gallons, UK gallons, and BBL (US oil barrels).
It is important to note that these calculations are estimates, as real-world tanks may not be perfect geometric shapes. The inner dimensions of the tank should be used, and if these are not available, the thickness of the tank walls should be accounted for. The volume of a cylinder-shaped tank can also be calculated by hand, and then converted to the required units, such as cubic feet to gallons, or cubic meters to litres.
For more complex tank shapes, such as dome tops, cone tops, oblique cone bottoms, and ovals, the volume can be calculated by summing the volumes of simpler shapes. For example, the volume of an oval tank can be calculated by assuming it is two halves of a cylinder separated by a rectangular tank. Similarly, the volume of a capsule-shaped tank is the sum of the volume of a sphere and a cylinder.
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Frequently asked questions
You can install a fuel gauge, but these are not always reliable. You can also use a dipstick to manually measure the depth of the fuel. The owner's manual should also provide an estimate of your vehicle's fuel tank capacity.
In aircraft, fuel volume is often measured using capacitance probes, which measure the level of fuel in the tank. This volume is then converted to mass by measuring the fuel density.
The total volume of a cylinder-shaped tank is given by the formula V(tank) = πr^2l, where r is the radius and l is the length of the tank. The filled volume of a horizontal cylinder tank can be calculated by multiplying the area of a circular segment by the length.
