Waste Indicator Light Purpose

The waste full indicator is there to warn the cassette is nearing full, but there is some inbuilt leeway to allow further use of the toilet in the event you are unable to empty the cassette straight away. Imagine the fuel light on your car; it will illuminate to tell you are going to run out of fuel shortly. If it were to illuminate at the point the tank is empty it would be too late.

As you rightly say, the actual usable capacity of the tank is less than the ‘brimmed’ capacity to allow for expansion and gas build-up etc. This is not usually a problem in typical caravans or motorhomes, but our toilets are commonly used on boats where owners will have multiple cassettes, storing them full until they get to somewhere to empty them. If there wasn’t any expansion room, the cassette could rupture with, as you can imagine, undesirable consequences.

For these reasons, the warning light comes on earlier than you would expect. It is not possible to recalibrate it.

If you are close to an Elsan point and are not planning on storing a full cassette, there should be no issue with using the toilet past the point the light comes on to get extra capacity, and to make the additives go further.

With regards additives, the dosage is standard across our toilets and can treat up to 20 litres of wastewater. It would be theoretically possible to reduce the dosage proportionally to the actual usable capacity of the toilet but given the end result would be a saving of only a few milliliters of additive, it is not generally worth the effort.

I hope this helps explain sufficiently.