The choice of tank sender depends on four questions: What is in the tank, how deep is it, how can the sender be mounted, and on which device should the values appear? Those who clarify these four questions beforehand will order correctly the first time. Those who skip them will have the sender in hand and find that the opening does not fit.
An overview of the methods
Float senders TGW and TGT
The most proven technology: A float follows the liquid level in the dip tube and transmits its position as a resistance signal. The output signal is between 10 and 180 ohms – the internationally common resistance range for tank instruments, which most monitors can handle.
Installation is from above, requiring an opening in the tank lid. Various installation lengths cover different tank depths. TGT is the version for fuel, TGW for water.
Suitable for fresh water, fuel, and wastewater. Not suitable for blackwater tanks.
Pressure sensor TDS and TDN 200
The sensor is lowered from above to the bottom of the tank and fixed there; it measures the hydrostatic pressure of the liquid column above it, i.e., from bottom to top. This makes it independent of the tank shape and the most versatile sender in the range. The TDT screw-in sensor version is fixed in the tank instead of being suspended.
Can be used for tank depths up to approximately 2 meters. Suitable for fresh water, fuel, wastewater, blackwater, and other liquids.
NoteA pressure sensor must know what medium it is measuring. Diesel is significantly lighter than water, so the same fill level generates lower pressure. A water characteristic curve in a diesel tank will constantly show too little.
Ultrasonic sensor UTV
Sits on top of the tank and measures the distance to the liquid surface without contact by measuring the transit time of a sound pulse – without contact with the medium and without moving parts. Very precise, as long as the sound path is clear and the surface is calm.
There is a dead zone of 2.5 centimeters below the sensor that cannot be detected. A spacer ring creates space for this, and a sound guide tube calms the surface in case of inclination and wave formation.
Not suitable for tanks where liquid can directly reach the sensor, and for heavily foaming media.
Gobius C — Radar from the outside
Measures without contact from the outside through the tank wall. For plastic tanks, no drilling is necessary; the sensor is mounted externally – making it the most retrofit-friendly solution because there is no intervention in the tank and no risk of leakage. For metal tanks, drilling is required because radar waves cannot penetrate metal.
The dead zone is approximately 5 centimeters in the bottom area. Setup and display are via an app; a version with an NMEA 2000 connection integrates the values directly into the instrument network and onto compatible chart plotters.
Gobius Pro — Level switch
Not a fill level sensor, but a threshold detector: glued to the outside of the tank, without drilling, it reports whether the fill level is above or below the mounting point. The display is binary; there is no percentage value. Mounted at the top, it is a full indicator; at the bottom, it is an empty indicator.
Flow sensor DFS 24
Measures not the fill level, but the actual flow in the pipe, accurate to the liter and in real time. Exclusively for fresh water. As a supplement to the tank sensor, it is the only way to know daily consumption – and thus the basis for any supply planning over long distances.
Direct comparison
| Sensor | Principle | Fresh water | Fuel | Wastewater | Blackwater | Installation | Dead zone |
|---|---|---|---|---|---|---|---|
| TGW / TGT | Float, 10–180 Ohm | yes | yes | yes | no | from above | none |
| TDS / TDN 200, TDT | Hydrostatic pressure | yes | yes | yes | yes | from above to bottom | none |
| UTV | Ultrasound | yes | yes | conditionally | no | from above | 2.5 cm |
| Gobius C | Radar, non-contact | yes | yes | yes | conditionally | from outside | approx. 5 cm |
| Gobius Pro | Threshold, non-contact | yes | yes | yes | conditionally | from outside, side | — |
| DFS 24 | Flow | yes | no | no | no | in the line | — |
For shallow tanks, the dead zone is the decisive criterion: With an internal height of 20 centimeters, a bottom dead zone of 5 centimeters costs a quarter of the measuring range.
Question 1: How is the tank constructed?
What matters is not the volume, but the usable measuring distance – the height between the lowest and highest realistic fill level. Tanks deeper than two meters rule out the pressure sensor; very shallow tanks make any sensor with a dead zone problematic.
Irregularly shaped tanks – the norm in fitted installations – have a second problem: the fill level is linear in terms of height, but not volume. Ten percent height in the wide lower part represents significantly more liters than ten percent in the narrow upper part. Displays with characteristic curve correction compensate for this; without this function, the display remains a height indication, not a quantity indication.
Question 2: What medium?
| Medium | To be observed |
|---|---|
| Drinking water | food-grade materials required |
| Diesel | density significantly below water – must be set for pressure sensors |
| Gasoline | additional requirements for ignition protection and resistance |
| Greywater | deposits on immersed parts |
| Blackwater | float sensors are out; pressure sensor or non-contact measurement |
For blackwater tanks, a legal requirement is added: the three-quarter fill level must be monitored and displayed. A threshold sensor is sufficient for this; a full display is not mandatory.
Question 3: How do you access the tank?
This question often dictates the choice during retrofits more than all technical considerations. If the tank lid is accessible and has a standardized bolt circle, a float sensor is straightforward; suitable adapter rings compensate for different bolt patterns. If the tank is built-in, glued, or under a fixed floor panel, practically only external measurement remains – and for radar, this requires a plastic tank.
Before ordering, three measurements should be noted: usable internal tank height, diameter and bolt circle of the existing opening, and the clear height above the tank. The last point is often forgotten – a sensor that requires 15 centimeters of installation height at the top cannot be installed under a floor panel with 8 centimeters of clearance.
Question 4: Which display?
The sensor and instrument must be compatible. Philippi tank sensors can be connected directly to BTM2, VTM 2 monitors and the PCU, without an additional interface. For the P-Bus system monitors PSM 3 and PSL, the CMT3 tank interface is required, which integrates up to four sensors and transmits their values to the P-Bus energy management system.
For third-party sensors, it's worth checking the resistance range: Besides 10 to 180 ohms, other conventions are in circulation, and some show the highest resistance when the tank is full, while others show the lowest. A sensor on the wrong instrument will provide a display that moves but doesn't say anything. If several tanks with different methods are to be displayed on one screen, a universal tank interface solves this more cleanly than trying to install the same type of sensor everywhere.
NoteIf the values are to be combined with battery data on a single display anyway, this decision should be made before choosing the sensor. The display determines which signal types are compatible – not the other way around.
Quick decision guide
- Blackwater tank? Pressure sensor TDS/TDN 200, or Gobius C for plastic tanks. Float sensors are out.
- Retrofit without drilling? Gobius C for plastic tanks, Gobius Pro as a simple threshold detector.
- Highest accuracy? Pressure sensor or ultrasound.
- Level on the chart plotter? Gobius C in the NMEA 2000 version, or via BTM2 or VTM 2.
- Most affordable and proven solution? Float sensor TGW or TGT.
- Know daily water consumption? Flow sensor DFS 24, in addition to the tank sensor.
- Shallow tank? Short float sensor or pressure sensor – consider the dead zone.
Typical errors
- Sensor length chosen by tank volume. The internal height is decisive, not the liters.
- Dead zone not planned for. In shallow tanks, it consumes a significant part of the measuring range.
- Float sensor in a blackwater tank. It is not designed for this.
- Radar on a metal tank without drilling. Radar waves do not penetrate metal.
- Density not set. A pressure sensor in a diesel tank with a water characteristic curve will permanently show too little.
- Resistance range not checked. The display moves but is incorrect at every point.
- Installation height above the tank forgotten. The sensor fits in the tank but not in the installation space.
- Characteristic curve not corrected. In irregularly shaped tanks, half the height is rarely half the quantity.
