A level measurement tells you how much is left. A flow measurement tells you how much has been consumed. This sounds like the same information, but it's a different measurement – and often the more accurate one for fresh water tanks.
Counting instead of estimating
The DFS 24 sits in the fresh water line and counts what flows through it. It delivers 1000 pulses per liter with a measuring range of 1 to 25 liters per minute. The monitor then calculates the consumption and the remaining tank content.
The advantage over a level measurement lies in its independence from the tank geometry. A fresh water tank that follows furniture construction and is narrow at the bottom poses difficulties for any height measurement – the flow meter is not concerned with this. It counts liters, and liters are liters.
Added to this is the resolution. A thousand pulses per liter means that even small withdrawals are registered – a height measurement doesn't see this, because the level doesn't move measurably.
What the method cannot do
A flow measurement counts from a starting point. Someone has to set this starting point – typically when filling the tank.
This leads to the only real weakness: if the tank is refilled without resetting the counter, the calculation will diverge. This is the same principle as with ampere-hour counting on the battery – it needs a reference point that the measurement itself cannot provide.
Similarly, the meter does not register what is drawn off past the sensor. A second outlet before the sensor, a drain cock, a line to the deck wash system – all of this falls outside the balance.
NoteTherefore, the sensor should be placed as close as possible to the tank outlet, before any branching. What branches off behind it is counted; what branches off before it is not.
Installation in the line
The sensor has a connection flange for hoses with a diameter of 10–12 mm. A separately available connection flange is available for half-inch hoses.
The housing measures 110 x 23 x 57 mm, the connecting cable is 20 cm long. These 20 cm are the point that is overlooked during planning: the next connection point for the extension must be within this range, or a connection point will be needed in the wet area – which is something to avoid.
Three more points:
- Observe the flow direction. The sensor is directional; installed incorrectly, it will not count correctly.
- Mount accessibly. A water line will eventually be loosened – you'll want to be able to reach that spot then.
- Not at the lowest point. This is where whatever accumulates in a water line collects.
Connection to the display
The sensor can be connected directly to common tank monitors. In the P-Bus system, the connection is made via the CMT3 tank interface – and this is a limitation that should be known: even with a system monitor with an integrated tank interface, the separate CMT3 is still required for the flow sensor.
So, anyone who could manage with four tank senders and therefore wanted to forgo a separate interface must recalculate once a flow sensor is added.
Two sensors for a watermaker
With two flow sensors, a watermaker can be fully balanced: one measures what is produced, the other what is consumed. The difference is the actual change in the tank.
This is the constellation in which flow measurement plays out its greatest advantage – because both directions are recorded and the balance is complete, without the level ever needing to be measured.
Combining both
The two methods are not mutually exclusive, and together they are better than either one alone. The level measurement provides the absolute reference point that the counter needs; the counter provides the resolution that the level measurement lacks.
In practical terms: the level value resets the counter after each filling, and between fillings, the counter provides the more accurate value. This exact interaction also exists with the battery – there, a full charge resets the ampere-hour counting.
Typical errors
- Sensor mounted behind a branch. What branches off before it falls out of the balance.
- Flow direction reversed. The sensor is directional.
- Counter not reset after filling. The calculation will then permanently diverge.
- CMT3 not planned for the P-Bus. It remains necessary for the flow sensor, even with an integrated tank interface.
- 20 cm connection cable overlooked. The next connection point must be within reach.
