As I stated in my post on mop design decisions, the maximum water capacity (and by extension the area that can be mopped by the robot) is heavily dependent on the how much of water the mop can absorb. Ideally, the water tank should run out just when the mop head is saturated with water (and thus not leave dirty water that it can't suck up behind on the floor). To figure what kind of a range I'm working with, I decided to measure the absorbency of the microfiber cloths I purchased to use as my mop head. The product page says the cloth can carry up to 8x its weight in water, but I've been burned before and wanted to put that to the test.

Methodology

The way I tested the absorbency was by measuring the weight of the microfiber cloth at various levels of wetness; dry, damp, saturated, and oversaturated. I performed the dry measurement by weighing the cloth fresh out of the packaging; the damp measurement was performed by soaking the cloth in water and wringing it out; for the saturated measurement I simply held the soaked cloth over the sink until it no longer dripped; and lastly the oversaturated measurement was taken by weighing the cloth immediately after soaking it in water.

The most likely wetness of the cloth in a real-world scenario is probably somewhere between damp and saturated, since a dripping mop head would still leave dirty water on the floor.

Results

Dry

The dry cloth weighed in around 33g, in line with the data found on Amazon. If I trust the advertising, this cloth should hold around 250mL of water. Let's see about that...

Damp

The wet cloth was 106g, so roughly 75mL of water was absorbed (since 1 mL of water weighs a gram).

Saturated

In the saturated case, the cloth weighed 207g and held about 175mL of water.

Oversaturated

Even the unrealistically oversaturated cloth held only 200mL of water, which is still less than product's claim of 8x its weight.

Conclusion

While this microfiber cloth clearly is not as absorbent as was promised, the 175mL of water held by the saturated cloth still bodes well for the overall capacity of the mop module. It would only take a single cloth to match the capacity of the Braava Jet, and four or five to match the capacity of the Scooba, assuming that the entire cloth gets saturated during cleaning. I still have to design the mop module and determine how I'll arrange the cloths and attach them to the module, thus dictating how many cloths I can fit in the design, but these initial results seem promising.