Grey Water Level Sensors for Campervans — Ultrasonic, Pressure & Float Switch Guide

Knowing how full your grey water tank is before it overflows through the sink and onto your kitchen floor is the difference between a comfortable van life day and a wet, soapy mess. A grey water level sensor gives you this information — either as a percentage on a display panel or as a warning light when the tank is nearly full.

This guide covers the three main types of grey water level sensors used in UK campervan builds, how they work, what fits which tank material, and how to install them.

Why Grey Water Sensors Are Tricky

Grey water tanks present a unique sensing problem compared to fresh water tanks:

• The water contains soap, grease, food particles, and hair. These coat sensor surfaces and cause false readings.
• The tank is often mounted under the van, exposed to road spray, mud, and vibration. The sensor must be weatherproof.
• Many grey water tanks are odd shapes — long and shallow, or tapered — to fit in the under-body space. A sensor that works in a box-shaped tank may not work in a tapered tank.
• Foam on the surface of the grey water (from soap) can confuse some sensor types, making them read "full" when the tank is half-empty.

Sensor Types

Ultrasonic Sensors (SeeLevel II)

Ultrasonic sensors measure the distance from the top of the tank to the water surface by sending an ultrasonic pulse and timing the reflection. The sensor is mounted on the outside of the tank wall (no contact with the water). This is the most reliable type for grey water.

How it works: A sensor pad (approximately 60mm × 40mm) is bonded to the outside of the tank wall with silicone adhesive. The sensor sends an ultrasonic signal through the tank wall, reflects off the water surface, and returns. The time taken indicates the distance to the water surface, which is converted to a percentage fill reading.

Pros:

• No contact with grey water — the sensor never gets coated with soap, grease, or hair
• Works through PVC, polypropylene, ABS, and aluminium tank walls
• Accurate to ±3% regardless of tank shape (sensor can be calibrated for tapered tanks)
• No moving parts — no mechanical failure

Cons:

• Does not work through stainless steel tanks (the ultrasonic signal cannot penetrate stainless steel) or through tanks with thick internal insulation (more than 5mm)
• Requires a minimum tank depth of 100mm (shallow tanks cannot be measured)
• The sensor must be positioned where the tank top and bottom are parallel (the signal reflects off the water surface perpendicularly)
• More expensive than other options

SeeLevel II specific:

• Price: £60-£80 per sensor pad
• Display: sold separately (the 709 Series display panel is £50-£70 and shows all tank levels — fresh, grey, and battery)
• Installation: bond the sensor pad to the tank wall with the included silicone. Connect the two wires to the display panel. The sensor is powered by the display panel (no separate battery connection needed).
• Calibration: the display panel learns the empty and full levels automatically (press the "calibrate" button when the tank is empty, fill it, press again when full).

Best for: Full-time van builds with PVC or aluminium grey water tanks. The SeeLevel system is the most reliable option and the only one that works consistently with soapy grey water.

Pressure Sensor (Whale / TankSure)

A pressure sensor measures the hydrostatic pressure at the bottom of the tank — the weight of the water above the sensor. The reading is converted to a fill percentage.

How it works: A pressure transducer is mounted at the lowest point of the tank (or in the drain pipe near the tank outlet). The transducer measures the pressure of the water column above it. As the water level rises, the pressure increases proportionally.

Pros:

• Works with any tank material (stainless steel, PVC, aluminium, polyethylene)
• No moving parts
• The sensor is mounted outside the tank (or in the drain line) so it can be replaced without draining the tank
• Accurate to ±5% in cylindrical and rectangular tanks

Cons:

• Less accurate in shallow or non-uniform tanks (the pressure reading is proportional to depth, not volume — a tank with a sloping bottom gives a non-linear reading)
• The sensor port can clog with grey water debris (install a debris filter or mount the sensor in a clean-out port)
• Requires a minimum water depth of 30mm below the sensor to get a reading (the last 30mm of water in the tank is not measured)
• The transducer is sensitive to temperature: readings drift by 1-2% over the range of UK winter to summer temperatures

Whale TankSure system:

• Price: £40-£60 for the sensor module
• Display: Whale TankSure display panel (£60-£80) or can be wired to a generic 0-5V display
• Installation: drill a 22mm hole in the tank wall near the bottom, install the supplied bulkhead fitting with the pressure transducer inside. Wire the sensor to the display (3 wires: +12V, ground, signal). The display shows the tank level as a percentage.
• The Whale system includes a "re-learn" function that recalibrates to the tank shape. Run the calibrate cycle with the tank empty and again when full.

Best for: Vans with stainless steel tanks (where ultrasonic sensors do not work) or builds where the tank shape is uniform (rectangular or cylindrical). Less suitable for custom-shaped under-slung tanks with non-parallel sides.

Float Switch (The Budget Option)

A float switch is a mechanical switch mounted at a specific height in the tank. When the water reaches that height, the float lifts and closes (or opens) a circuit. This gives a binary reading (full or not full at that point), not a percentage.

How it works: A plastic float on a hinge or vertical rod rises with the water level. At a set height, the float triggers a microswitch inside the sensor body. The switch connects or disconnects a wire, which can illuminate a warning light, trigger an audible alarm, or send a signal to a control panel.

Pros:

• Cheap (£5-£15 per switch)
• Simple — no electronics to fail beyond the switch mechanism
• Easy to install (drill a hole, fit the switch, wire it up)
• Works with any tank material and any liquid (including thick grey water)

Cons:

• Binary only (full or not full). If you want to know the level at 25%, 50%, and 75%, you need three float switches at different heights — each requiring a hole in the tank
• Moving parts: the float mechanism can jam with grey water debris (hair, soap residue). Cleaning requires draining the tank and removing the switch
• The hinge mechanism wears over time (approximately 18-24 months in grey water use). A jammed float gives a false reading
• Not suitable for tanks with internal baffles (the float may get trapped)

Typical installation: One float switch mounted at the 90% full level. Wire it to a red LED on the dashboard. When the LED lights, the tank is 90% full and needs emptying. This is the most common DIY grey water level solution in UK campervan builds.

Best for: Budget builds and weekend van use. The 90% warning switch is adequate for most users. If you want to know the exact percentage, use a SeeLevel or Whale system.

Resistive / Conductive Probe Sensors (Not Recommended)

Some systems use a vertical probe with electrodes at different heights. The water conducts between the probes and the common ground, indicating the level. These are common in caravan fresh water tanks but do not work for grey water.

Why they fail in grey water: The soap and grease in grey water leave a conductive film on the probes, causing a continuous "full" reading within days of installation. The probes require cleaning every 2-3 weeks in grey water use. Avoid this type for grey water sensing.

Installation Considerations

Tank Material and Sensor Compatibility

Sensor Position

Ultrasonic: Mount the sensor pad on the TOP of the tank (the signal goes down to the water surface). The sensor must be level (parallel to the water surface). Position it away from tank edges and corners (minimum 50mm from any side wall).

Pressure: Mount the sensor at the LOWEST point of the tank (the bottom). If the tank has a sump (a low-point outlet), mount the sensor in the sump. The pressure reading is affected by the slope of the tank — mount it at the lowest point the water collects.

Float switch: Mount the float switch on the tank SIDE at the desired trigger height. The float must have clearance to swing freely — check the manufacturer's specification for the required clearance (typically 50mm above and below the switch).

Wiring

• Ultrasonic and pressure sensors use low-voltage (5V or 12V) signals. Wire them with 0.5mm² or 0.75mm² multi-core cable.
• Float switches are simple make-or-break switches. Wire them with 0.75mm² twin-core cable. The switch carries very low current (<1A) — no heavy cable required.
• Always fuse the power supply to the display panel (1A fuse is sufficient for SeeLevel and Whale displays).
• If running cable under the van, use waterproof connectors (Deutsch, AMP Superseal, or heat-shrink butt connectors). Do not use Scotchlok or T-tap connectors — they corrode within months in the underbody environment.

Cost Comparison

The Bottom Line

For full-time van life: The SeeLevel II ultrasonic system is the only choice that works reliably with grey water over years of use. No contact with the water means no soap film, no hair jams, no cleaning. Budget £130 for one grey water sensor + the display panel.

For weekend/casual use: A single float switch at the 90% level is sufficient. Cost: £15. Wire it to a red LED near the sink. When the LED lights, empty the tank. If the float jams (unlikely with occasional use), you will notice before it overflows.

For stainless steel tanks: Ultrasonic does not work. Use the Whale TankSure pressure sensor. Budget £120 for sensor + display. Mount the sensor in a clean-out access point so you can clear debris without draining the tank.

Do not use resistive probe sensors for grey water. They fail within days. Do not use the cheap ultrasonic sensors from eBay (the non-SeelLevel ones) — they are not calibrated for tank shapes and lose accuracy after a few months.

The SeeLevel system is expensive but is a buy-once, fit-and-forget solution. Most full-time van lifers who use anything else end up replacing it with SeeLevel within a year.