Ok...now that I see how specific you all want to be on this...
I have made several of these things for test the flow rate in water in a nuclear reactor.
(damn I do nice work
)
anyways, from what I gather they are highly accurate, like +/- .25%. Of couse this is a larger model, but my thinking is is that it can be scaled down. It is called a venturi flow meter. The liquid (or gas) travels through a series of very specifid tapered diameters ( tolerance of +/- .0005 in) ( on the inside ) & pass by a couple of very specific ports that are a certian length. The fluid passing by these ports creates some sort of vaccum & there ya go. However, at this point we have gotten way past the point of being a hobbiest.
Doing a bit of searching on the web I did find this little unit that looks like it might be ideal for testing flow....does not look expensive & the accuracy looks to be acceptable.
Pretty simple...water flows against a piston that is mounted on the spring. The spring compresses & you read where the plunger stopped on a scale on the outside of the clear tube. There is a option to "build it yourself" on the page, so I submitted a quote for one with 1/2in fittings,brass fittings, with a brass ball valve with a max flow rate of 3gallons per minute. Be interesting to see the price.
http://www.flowmeters.com/catalog_li...tion_set_id=73