The CMT3 tank interface can accommodate up to four tank senders, measures them independently, and makes the values available to the entire P-Bus. Its true value lies in its versatility: it understands five different signal types and also works with senders from third-party manufacturers.
Which sensors can be connected
The CMT3 supports the signal types common in onboard networks:
- Resistance sender 10–180 Ohm. The European standard characteristic.
- Resistance sender 240–33 Ohm. The American variant — opposite to the first.
- Freely adjustable resistance range 0–300 Ohm. The fallback for senders that do not follow either of the two standard characteristics.
- Current signal 4–20 mA. Typical for hydrostatic submersible probes.
- Voltage signal 0.5–2.5 V. Typical for ultrasonic senders.
- Float switch type 0–1. Two states instead of a fill level — a limit value message.
The freely adjustable range is the reason why existing senders can continue to be used. During a refit, this often makes the difference between "retrofit interface" and "open tanks and replace senders."
Connection and installation
The tank senders are connected via pluggable screw terminals, and the P-Bus connection is made via M12; a suitable T-cable is included. The operating voltage is 8–32 V DC, and the current consumption is low enough at 7.5 mA in standby and 10 mA active to not play a role in the balance. The housing measures 107 x 85 x 40 mm.
For the installation location, the same consideration applies as for any sensor bundling: the interface belongs with the sensors, not with the monitor. The paths from the tank penetrations are then short, and a single M12 cable leads to the bus.
Setting parameters on the monitor
Configuration is not done on the interface, but via the system monitors PSM 3 or PSL. Four parameters must be set for each tank:
- Sensor type. The first and most important value — without it, the interface will interpret the signal incorrectly.
- Tank size. The usable volume, not the nominal volume of the tank.
- Tank characteristic. The correction for tanks whose cross-section changes with height.
- Alarm level. Separated for empty and full, with varying usefulness depending on the tank type.
The display can be in liters or percentage. Both have their place: percentage is better for a quick glance, liters for planning the next leg.
The characteristic curve: why half height is not half volume
A sender measures height. What is of interest is volume. For a rectangular tank, this is the same — but not for a tank that follows the hull or furniture construction.
Typical symptom: The display drops quickly in the upper half and seems to stop at the bottom. The sender is working perfectly correctly; only the conversion from height to volume is missing.
In practice, one proceeds as follows: During the first complete filling, it is noted at several points how much was actually filled and what the display shows. These value pairs create the correction. This is an afternoon's work and never an issue again afterward.
NoteCompensation requires real measurement points. It cannot be set meaningfully on the day of installation — initially, the sensor type and volume should be entered, and the fine correction will follow with the first filling.
Water consumption and watermaker
In addition to fill levels, the CMT3 can also record flow. A watermaker is integrated with two flow sensors: one measures what is consumed, the other what is produced.
This is a fundamentally different measurement than the fill level — it counts quantities instead of reading a level — and is therefore interesting where fill level measurement is difficult or where the balance is more important than the instantaneous value.
If the display is incorrect
- Mirrored display — full is reported as empty: the two resistance characteristics are swapped.
- Never reaches completely full — tank height entered too large, or for ultrasound, the level is in the upper dead zone of the sender.
- Value jumps — for ultrasound, sloshing or foam, for resistance senders, a loose ground contact.
- Correct at the top, wrong at the bottom — the characteristic curve is not compensated.
- No value at all — first check the sensor type, then the terminal, and only then suspect the sender.
Typical errors
- Sensor type guessed. A moving display does not prove that the type is correct.
- Interface mounted near the monitor. It belongs with the tanks — the paths there are short.
- Nominal volume instead of usable volume. Reports residual quantities that cannot be accessed.
- Characteristic curve skipped. The most common reason for a display that no one believes anymore.
- Four senders planned, five needed. The CMT3 accommodates four — the fifth tank requires a second interface.
