How Does a Fuel Sender Work?

A level sender is the sensor that sits inside your tank, measures how much fluid is in it, and sends that reading to a gauge or display. "Sender" and "sensor" mean the same thing here - a device that senses level (or temperature, or pressure) and passes it on. Most people meet them as a fuel sender, but the same device reads water, oil, AdBlue and more.

The clever part is how it turns a moving fluid level into a steady, accurate signal - with a free-floating magnet, no mechanical linkage and no electrical contacts to wear. Here's what's going on inside the tube.

01 / BASICSWhat a level sender is

Wema level senders are built from stainless steel with an NBR float, so they suit most fluids - petrol, diesel, water, oil, even fertiliser. If you're unsure whether one is right for your application, just ask and we'll advise.

They're also double insulated: the internal electronics are isolated from both the sender body and the contents of the tank. That matters anywhere fuel and stray electrical current must never meet.

02 / MECHANISMHow it actually works

Inside the tube is a PCB carrying a row of magnetic reed switches. The float - a ring containing a magnet - wraps around the outside of the tube and moves up and down with the fluid. As the magnet draws level with each reed switch, it trips the switch as it passes, and the sender reports the level.

This is the real difference between a Wema sender and an old swing-arm sender. There's no arm, no hinge and no wiper dragging across a resistance coil - the parts that wear out and corrode. The float touches nothing; it simply drifts on the surface, magnetically coupled to the switches through the tube wall. It's why we regularly see Wema senders still working 25 years or more after they left the factory. Browse the range in level senders.

03 / OUTPUTSOutputs and resistance ranges

Senders come with several outputs: resistance, 0–5 V, 0–10 V, 4–20 mA, and dedicated digital outputs for NMEA 2000 or CAN bus J1939. For a traditional analogue gauge you want a resistance sender; voltage and digital outputs are for digital systems and displays.

Resistance senders come in two ranges, commonly called European (0–190 Ω) and American (240–30 Ω). The names are historical - both are used worldwide today, and there's no performance advantage to either.

As a rule of thumb, a two-wire sender is resistance and a three-wire sender is a voltage output - though senders with high or low level alarms can have three, four or five wires. The output is always stamped on the ID plate. And one important warning: never connect a resistance sender directly to 12 V - it's passive, drawing only a few milliamps from the gauge, and mains voltage will burn it out.

04 / FITTINGFitting it to your tank

A sender is bolted to the top of the tank, screwed into a boss or flange, or - on larger truck senders like the TX range - secured with a bayonet fit. Flanges come in stainless steel for strength and corrosion resistance. On an aluminium tank, fitting the stainless sender through a nylon flange keeps the two dissimilar metals apart and prevents galvanic corrosion.

You can also fit an inspection hatch with a 1.25" BSP thread, letting you install a sender without cutting more than one hole. Retrofitting, and the old fitting was different? A converter adapts the hole - we make them in nylon, brass and stainless steel.

05 / RESOLUTIONA word on resolution

A standard-resolution sender has a switching point approximately every 20 mm; a high-resolution sender approximately every 10 mm. The shorter the tank, the more this matters: for example, in a 400 mm tank 10 mm of movement is about 2.5% of the contents, while in a 1500 mm tank it's just 0.6%. That's why our high-resolution range focuses on shorter senders, where the extra precision counts most.

06 / WHY WEMAWhat sets a Wema sender apart

Two senders can look identical in a photo and be worlds apart in the things you can't see. The market is now full of cheap look-alikes that copy the shape but not the engineering - unproven components that fail early, no real testing, no support when something goes wrong, and none of the development that goes into a design refined over decades.

A Wema sender is the original. Forty years of development, marine-grade stainless steel, and a non-contact magnetic design with nothing to wear out - backed by people who actually make and understand the product. Most carry a 3-year guarantee, because we expect them to keep working in the harshest environments for far longer than that.

07 / SHOPFind the right sender and parts

08 / FAQCommon questions

Not sure which sender fits your tank? Use our replacement-sender guide, or get in touch - we're happy to advise on the right sender, gauge and fitting for your application.