Understanding Fuel Senders

And tips for choosing the right one

BY DENNISON BERWICK

Knowing how much fuel is in the tank(s) is essential for the safety of a vessel— running out of fuel at a critical moment, such as while navigating a tidal passage or entering a busy marina, could put the vessel and crew in immediate danger.

Fortunately, there are many ways to measure the quantity of fuel in tanks, even tanks with custom shapes. Most use a sender inside the tank which measures the fuel level and sends that information electrically to a fuel gauge.

Types Of Electrical Senders

Float Arm Fuel Sender

The rise or fall of a float-arm inside the tank varies the electrical resistance in the circuit, which moves the needle in a fuel gauge as the level of fuel in the tank changes.

Capacitance Fuel Sender

This type of fuel sender uses the physical properties of capacitance to accurately measure the level of fuel in a tank. Capacitance senders usually have an outer tube and a small inner tube and can be supplied up to two metres (6.5 feet) in length. The tubes can be cut to the fit the depth of a fuel tank.

Capacitance is the ability of two metals held close together, but electrically separate, to hold an electrical charge when supplied with electrical current. Immersed in a liquid, such as electrically non-conducting diesel, the capacitance of the two metals changes according to the level of the liquid. This can then be converted by electronics into resistance (ohms) and displayed on a gauge.

Hands On Boater

Magnetic Reed Switch Fuel Sender

A magnetic reed switch closes when a magnet is near. In a fuel sender, many switches are built into a ladder, each wired to a different size of resistor, and a magnet is placed inside a free moving float. When this float rises or falls with the level of fuel, a different switch in the ladder is closed and current flows through a specific resistor. This changes the voltage in the fuel gauge circuit, moving the needle in the fuel gauge. The highest resistance (ohms) shows the tank is empty, the lowest that the tank is full.

Ultrasonic Fuel Sender

An ultrasonic fuel sender works in the same way as a vessel’s depth sounder. A small piezoelectric transducer:

1) Sends out a sound wave (usually at 40 kHz—far above human hearing).

2) Receives the wave’s echo after it bounces off the surface of the liquid.

3) Measures the difference in time between the signal out and the echo return and calculates the distance the sound wave travelled.

4) An electronic unit then calculates the level of fuel in the tank by subtracting the distance travelled from the total depth of the tank.

5) This is then converted into voltage or resistance (ohms) and displayed on a gauge.

Not all ultrasonic transducers (sensors) work with metal tanks. If the sound waves might bounce off the sides of an oddly-shaped tank, a focus tube can be fitted which directs the sound wave downwards.

Hydrostatic Pressure Fuel Sender

This sender works in the same way as an oil pressure sender—pressure on a diaphragm alters the resistance in the gauge’s electrical circuit. However, the sender is not measuring fuel pressure ; instead it is measuring the fuel’s hydrostatic pressure—the weight (or height) of fuel in the tank.

Magnetic Reed Switch Fuel Sender

Reed switch closes when magnet is near Each reed switch in the ladder controls a different size of resistor

Hydrostatic pressure is the “weight” of a liquid due to gravity—ie. pressure increases in deeper water due to the weight of the water above. The more fuel in the tank, the higher the hydrostatic pressure.

TYPES OF NON-ELECTRICAL SENDERS

Calibrated Dipstick

Dipping a stick to the bottom of a tank is the simplest and most reliable way to find the level of fuel in a tank, though less convenient than other methods. The method does require access to the tank and the dipstick needs to be calibrated by adding a known quantity of fuel to an empty tank and marking the dipstick; eg. 1/4 depth of dipstick = 20 litres, 1/2 depth = 40 litres. This method works well with odd-shaped tanks.

Pneumatic Fuel Gauge

A simple gauge uses a manual push-button pump to inject air through a small tube from the gauge to the bottom of the fuel tank. The more fuel in the tank, the greater the resistance caused by the hydrostatic pressure (height of fuel). A calibration card is used to convert the pressure reading on the gauge into the quantity of fuel.

Sight Glass / Liquid Level Gauge

A traditional sight glass (liquid level gauge) is a clear tube installed on the outside of a tank, connected to the bottom of the tank and either connected to the top of the tank or open to the air. Atmospheric pressure ensures the height of diesel is the same in the tube as in the tank. A valve should be fitted at the bottom in case the tube breaks. The tube may be glass or a clear polyurethane fuel line and should be protected.

Selecting A Fuel Sender

Selecting the “best” fuel sender for a specific boat depends on several factors—and maybe some compromise, just like selecting any equipment for a boat. These factors include:

Accuracy / Reliability

Using a dipstick is the most accurate and reliable way to check the fuel level of a tank; there is little to fail. It is also the least convenient and needs enough headroom above the tank for the dipstick. Senders without a float (such as capacitance and ultrasonic) have no moving parts, are more reliable but are considerably more expensive.

Tank Shape

A float arm sender works well in shallow, regular-shaped tanks. The float arm needs to be able to swing down, without interference from a baffle, almost to the bottom of the tank (leaving a small fuel reserve above the fuel pickup). If the shape of the tank is irregular, a sender with a float or a hydrostatic sender will likely give inaccurate readings unless it can be manually calibrated for the specific tank. (Mid-depth in one part of the tank may not be half the tank’s fuel capacity). The walls of a narrow tank or baffles may interfere with the ultrasonic signal.

Convenience

Some senders, such as a pneumatic gauge and a simple dipstick, must be manually calibrated—the level of fuel in the tank marked off against measured quantities of fuel. Though simple, these are not as convenient as electrical senders.

A sender must be matched to its dis- play gauge—typically either 240-33 ohms or 0-180 ohms or adjustable or digital. Check before choosing the sender or replace the gauge.

Cost

Prices vary from the price of a wooden dowel to several hundred dollars. The cost of installation and potentially a new gauge must also be calculated.

Purpose and Preference

The intended purposes of a vessel can also influence which sender to select. A vessel going offshore requires a more robust and more reliable sender than a vessel motoring a few miles to the start of a sail race. Some skippers want low-tech, others prefer high-tech precision.

Dennison Berwick is author of Marine Diesel Basics 1, Maintenance, Lay-up, Winter Protection, Tropical Storage, Spring Recommission and the value-added Maintenance Logbooks for single and twin-engine installations. He sails a 1982 Chevrier 36, currently anchored in east Africa, and intends to sail from Tanzania to South Africa and across the Atlantic to Brazil in spring 2024.