A tank gauge that is not trusted is worthless. And in practice, the problem is almost never the sensor, but rather three settings: the sensor type, the tank height, and whether the tank actually has the shape the gauge assumes. This article goes through the setup in the order in which it works.
Step one: correctly specify the sensor type
The PCU works with common sensor systems. For it to interpret a signal, it needs to know what kind of signal is coming in. Roughly, there are four families:
- Resistance senders. A float moves a wiper. Common ranges are 10–180 ohms (European) and 240–33 ohms (American). The two run in opposite directions — if they are confused, the gauge will read empty when the tank is full.
- Current signal 4–20 mA. Typical for hydrostatic submersible probes. Robust against long cables because the current remains constant along the entire length.
- Voltage signal, usually 0.5–2.5 V. Typical for ultrasonic sensors that measure from above through the tank lid.
- Switch contacts. Float switches that only know two states. They do not provide a fill level, but a limit value alarm.
The most common setup error is a sensor type that generates a plausible value, but not the correct one. A moving display is not yet proof that the type is correct.
Step two: Volume and tank height
The gauge calculates a fill level from the sensor value and a volume from the fill level. For this, it needs two pieces of information: how high the tank is and how much it holds. Both numbers should be measured, not estimated.
The nominal volume from the tank's data sheet is only a starting point. Rarely is everything usable: the outlet is a bit above the bottom, and filling stops below the top. The range that the gauge should display is between these two points — not the geometric tank capacity.
NoteAnyone who fills the tank once with counted canisters and notes the values will have the most reliable calibration there is — and it takes an afternoon.
Step three: compensate for irregular tanks
A rectangular tank is the simple case: half height equals half volume. However, such tanks are rarely found on board. The usual shapes follow the hull or furniture construction and are narrow at the bottom and wide at the top — or vice versa.
The result is a gauge that drops quickly in the upper range and seemingly stays put at the bottom, even though the sensor is measuring perfectly correctly. The sensor measures height; what is of interest is volume. For this, the setup offers a correction with which the characteristic curve can be adapted to the real tank shape.
The practical approach: Instead of calculating the geometry, during the first filling, note the displayed values at several points — empty, a quarter, half, three-quarters, full — and correct where the display and the actual filled amount diverge.
Step four: Set alarm thresholds
Alarm thresholds are the part that is most quickly set incorrectly — not technically, but in terms of effect. A threshold that triggers too early will be ignored after the second week. One that triggers too late comes too late.
The question is not when it becomes critical, but how much lead time is needed. Fresh water: enough to reach the next filling station. Black water: enough to allow further use before emptying is necessary. Fuel: the usual reserve plus the distance to the next opportunity.
These considerations result in thresholds that should be adjusted again after the first trip. When first setting up, they are inevitably guessed.
If the display is incorrect
Four symptoms, four causes, which should be checked in this order:
- Display inverted — full is shown as empty: wrong resistance characteristic, usually 10–180 instead of 240–33 ohms or vice versa.
- Display never reaches full — tank height entered too large, or with ultrasonic measurement, the fill level is in the sensor's upper dead zone.
- Display jumps or jitters — with ultrasound, splashing or foam on the surface, with resistance senders, a loose ground contact.
- Display is correct at the top, not at the bottom — classic case of an uncompensated tank shape.
Typical errors
- Sensor type guessed instead of checked. A moving display does not prove that the type is correct.
- Nominal volume entered instead of usable volume. The display then reports residual quantities that cannot be accessed.
- Calibration on the day of installation. Without known full and empty states, any characteristic curve is guessed.
- Alarm thresholds by feel. The calculation based on the required lead time is useful, not a percentage value.
- Tank shape ignored. The most common reason for a display that no one believes anymore.
