Rain gauge to measure the volume increase of the fermentation

What does ferment to double size actually mean? How do you know when it's double? Difficult! Here is your solution. In our book, we describe exactly how you put a piece of dough in the tube and watch it grow and can measure the increase in volume precisely all the time. We have been using this aid since day one and will never stop.
A bit funny is that these rain gauges are manufactured in the plastic kingdom of Småland by Nyby and not in China. You just have to promote that.