Sorry I missed your second question. I'm building what's called a "flowmeter", a device that measures flow speed. Depending on the application you need to do that in a specific manner. For instance this application requires high precision for very low flows, for which standard venturis are unfit, so a special method that relies con capillaries is used to get the poiseuille flow, which allows to get more precise and stable measurements for low flow ranges (below the speed at which we breathe).

That same one Sr. The pre-analysis for rectangular sections exists of course, but I had to finish it for my specific case (constant pressure variation). It's simple, but you have to know what you are doing.