How to make your own porosity meter.
Interpretation of the measured time of a DIY porosity meter is controversial for the following reasons:
1) The time measured with this model DIY Porosity Meter is very close, but not exactly equal, to the measurement results of an "official" porosity meter.
2) One manual of an "official" porosity meter says: Remark: the repeatability with cloths is not as good as specified (repeatability of ± 2% with a 0.3 mm diameter reference hole ) because it depends a lot on its setting (position, traction, folds, etc.).
3) Different glider manufacturers come to different conclusions based on porosity measurement results.
Following the definition, the height of the water column should be 10 cm during the entire measurement. That is very difficult to attain, and would in practice be a significant source of error.
Therefore we calculated the start height and end height of the water column that result in the same measured time as a water column of 10 cm would give. This depends on the diameter of the bottle, hence the table 02:40 . It gives for a number of bottle diameters the correct start and end height (the volume between the green and red ring is for all bottle diameters 250 cm^3). As you can see in the table, the distance between the blue and green ring (and red as well) is NOT 10 cm ! The water column starts at >10 cm (leaking air faster) and ends at < 10 cm (leaking air slower). With the given start and end height both effects average out.
The time also depends slightly on the diameter of the bucket, but if the bucket diameter > 25 cm, the error caused by this can safely be ignored.
Different glider manufacturers use different places on their gliders for porosity measurement.