There isn't one "best" tank sender, but rather a suitable one for every combination of medium, tank shape, and installation situation. If you start with the measurement principle instead of the product, you can make the right choice in four steps.
The four measurement principles
Floats. A body floats on the liquid and moves a signal via a lever or slider. Simple, inexpensive, common – and with moving parts in the medium. In wastewater and faeces, this is the weak point: what needs to move gets dirty and blocks.
Hydrostatic pressure. A probe at the bottom of the tank measures the pressure of the liquid column above it. The pressure only depends on the height, not on the tank shape – which makes this principle independent of geometry. No moving parts.
Ultrasonic. A sensor sends an impulse from above and measures the time it takes to reach the surface. Also without moving parts in the medium. Sensitive to tilting, sloshing and foam, and with a dead zone directly below the sensor.
From the outside through the tank wall. The sensor is glued to the outside and measures through the wall. No intervention in the tank, no leakage risk – but dependent on wall material and thickness.
Step one: the medium
The medium excludes options before anything else is considered.
- Wastewater and faeces. Mechanical float senders are practically ruled out here – they get dirty. Ultrasonic from above is the typical solution for this, and hydrostatic probes are also suitable.
- Water. Almost anything is possible; tank shape and installation situation are decisive here.
- Diesel. Hydrostatic probes are available in a version approved for diesel – the material question is not a minor one here.
- Petrol, kerosene, petroleum and explosion-hazardous areas. The usual immersion probes are expressly not approved for these. Here, it must be checked what is permissible before anything is ordered.
WarningMaterial approval for the medium is not a recommendation, but a condition. A probe in a medium for which it is not approved can become leaky – and with flammable liquids, this is no longer a measurement problem.
Step two: the tank shape
A rectangular tank is the simple case. Tanks that follow the hull or furniture construction are not: half height there does not mean half volume.
All height measurement methods – float, hydrostatics, ultrasound – measure height. The conversion to volume is done by the display or the interface using a characteristic curve. In the case of irregularly shaped tanks, compensation is therefore not optional fine-tuning, but a prerequisite for a usable display.
The question of tank shape also influences the installation length: probes and ultrasonic sensors are available in several lengths, and the correct one depends on the tank height, not the volume.
Step three: the installation situation
Three questions are decisive here:
- Can I access the tank from above? Ultrasonic and immersion probes require a feed-through in the tank top.
- Am I allowed to open the tank? For built-in or hard-to-reach tanks, this is often the exclusion point – then only external measurement remains.
- Does the vehicle or ship move significantly? Tilting and sloshing affect ultrasonic the most and hydrostatics the least.
Step four: the output signal
Only at the very end does the question of the signal arise – and it is the easiest to solve, because there are interfaces that translate between them.
- Resistance 10–180 Ohm or 240–33 Ohm. The two common characteristic curves for analog round instruments.
- Current signal 4–20 mA. Insensitive to long cables, typical for immersion probes.
- Voltage signal 0.5–2.5 V. Typical for ultrasonic sensors.
If the sensor's signal does not match the existing display, there is no reason to change the sensor: a universal tank interface converts between the formats and can be calibrated to the geometry using an adjustable tank height. In digital systems, the tank interface takes on this task and understands all of the signal types mentioned simultaneously.
When a limit value is sufficient
Not every question requires a fill level. Sometimes it is just: full or not full, empty or not empty. A float switch is sufficient for this – two states instead of a curve.
Typical cases are a warning for the black water tank, dry-run protection for a pump or bilge monitoring. A limit value sensor is not the cheaper solution here, but the appropriate one.
Typical mistakes in selection
- Started with the product instead of the medium. The medium rules out options – everything else is chosen after that.
- Ultrasound in fuel or water tanks. In case of tilting, swell, and a full tank, the measurement becomes unreliable.
- Forgot the characteristic curve for the tank shape. The most common reason for a display that no one believes.
- Sensor chosen according to the existing display. There are interfaces for that – the sensor should fit the tank, not the instrument.
- Installation length determined by volume. The tank height is decisive.
