Flow rate measurement techniques

[pattarkutty]

hey,
Can anybody suggest a method of measuring very small flow rates of a few ml per hour precisely.This is a diffusion experiment where water is allowed to pass through a membrane at a constant pressure.we have to know the amount of water passing through the membrane.There will not be any marked changes in the level due to flow.

 

[FredGarvin]

Will this be in a closed vessel, i.e. a tube or will it be in an open channel/weir? Very low reynolds number flows are very tough to get good accuracy on. If you can provide a bit more information on your set up perhaps we can come up with something.

 

[pattarkutty]

Sorry for the late reply.I am giving u more information on this.
This will be a closed tube. But the purpose of the experiment is to study convection at high rayleigh no: at the limit of large prandtl no:...but this will primarily be concentration driven convection from a lower tank through a membrane to a tank above, and the lower tank will be fed with water (low viscosity and lighter compared to the liquid in the upper tank). we have to measure the amount of water entering the upper tank through the membrane. The lower tank will be fed with water through a tube from an aspirator bottle kept at a height. We better not disturb the upper tank since the plumes resulting from diffusion have to be studied. The expected flow rate is around a few ml per hour. And the level of liquid in the aspirator bottle has to be kept constant inorder to keep the pressure head constant.

 

[FredGarvin]

That's a toughie. A few ml/hr is nothing. The first things that comes to mind are a hot wire anemometer and a pelton wheel. If all else fails, is there a way to do the test in two parts in which one of the parts would have the water flow diverted to a graduated cylinder in stead of the upper tank? You could get an average flow by doing a timed bucket check.

Will the flow eventually reach a steady state, or will it be of a constant transient nature?

 

[pattarkutty]

Thanks for the suggession. but there was a change of consideration. I realize that the rate we will deal with is too small for any accurate measurements. A loss of a few drops may make huge errors. So i have decided to change the membrane pore size to increase it to something like 1ml/sec. Measuring that should be easier. But i still need ur advice on stratification. I need to know of some method by which one can stop liquid from falling to the lower tank through the memberane from the upper tank until the start of the experiment. What we need here is something like an water tight lid that can be removed without much disturbing the liqiud in the upper tank. And also suggest me possible ways of fastening both the tanks with the membrane in between.
And is there any other way of drilling holes in glass plates other than using a diamond coated drill bit.