density

http://www.mobrey.com/products/density/7826.php

There are devices that comprise two static pressure sensors at a known distance/depth.

Might be difficult if you have turbulent flow.

You could use water columns for a cheap setup.

I have seen where distillaries use a simple hydrometer floating in a pot to measure the density of the distallate coming off the top of the still. Basically you have a pot with a pipe pouring into it which in turn overflows into another pipe which carries the alcohol off to a receiver. The hydrometer floats in the little pot. These are usually inside an enclosed glass box or a box with a window so you don't have fumes all over the place, but allows you to look directly at the hydrometer to check the density. The hydrometers are relatively cheap, so you can buy several in case you drop one and break it, the rest is basically just made from pipe stock. Its about the cheapest way you can go.

In the distilleries that I have seen, the enclosed glass box is used to allow visual observation/measurement of the density of the alcohol stream from the top product which relates to the alcohol proof. These glass boxes are often sealed by the government to prevent tampering with the hydrometer (which I believe is calibrated by the governmental agency responsible for weights/measures relative to product quality). Taxes are assigned to the distillery based on the readings observed.

...the density of WHAT?!