Viscosity - a question with pur'Poise' ?

[picclock]

I'm trying to estimate flow rates of two different liquids through a system. This is just flow under gravity. Would I be correct in thinking that the ratio of flows through an identical system will be the same as the viscosity ratio of the two liquids ?

I'm afraid that all I know about viscosities is that it was measured in Poises and I seem to recall that it was a value based on time it took for a funnel to empty - which, if memory serves, would be a pretty good analogue.

Many thanks for your time

Best Regards

picclock

 

[andyf]

It's probably more complicated than the flow rate varying simply in proportion to the viscosity.
If you're like me, you'll be none the wiser for doing so, but have a look at this for flow through a pipe:
http://en.wikipedia.org/wiki/Hagen%E2%80%93Poiseuille_equation
and this seems to deal with flow through a narrow orifice:
http://www.mcnallyinstitute.com/13-html/13-12.htm 

Andy

[picclock]

Hi Andyf

I've been over those equations and they are not nice  .

After I posted the question, I thought I could easily prove it myself by using the flow rate of water and then kerosene through a small pipe. If the viscosity ratios are close match to the ratio of flows then that's likely good enough.

Many thanks for your time

picclock

[John Rudd]

Hmm..just seen this....

Fluid flow through a pipe is influenced by the Reynolds number of the fluid or the gas passing through from my college days when we did Bernoulli's theorem for flow calcs...

Yuck..I hated it..made my brain hurt...

[andyf]

Your college studies would be a great deal more help with problems like this than mine, John, which were law. Still, at least I avoided any troublesome sums 

Picclock, I'm sure you are right - measuring jugs and a stopwatch look like being the easiest way. There's a bit table here
http://www.engineeringtoolbox.com/absolute-viscosity-liquids-d_1259.html which to me seems to say that kerosene is about half as viscous as water. That's  if I understand the units correctly (which I probably don't!).

Andyu

[John Rudd]

This is more appropriate than magic numbers and school theorems......

My dad was a vehicle body refinisher....To get the paint to the right consistency ( viscosity...)

He would thin it with the appropriate thinners and run a sample through a special funnel ( fixed volume)..and use a stop watch to time the liquid fall...

Hope this helps..

[awemawson]

Funny that John - years ago I made a 'dipper' for the same purpose. It comprised a 3" length of 1" copper pipe with a copper disk soldered on the bottom through which a short length of 1/4" pipe protruded axially. I soldered a length of the same 1/4" pipe to the 1" tube as a handle allowing me to dip it into a paint mixing can to test dilution. I actually squashed the end of the 1/4" pipe a bit to slow the flow and extend the time. A quick glance at your watch as it flows out tells you if you have added enough thinners. Worked a treat and avoided juggling funnels and having multiple containers to pour out of and into.

[dsquire]

Funny that John - years ago I made a 'dipper' for the same purpose. It comprised a 3" length of 1" copper pipe with a copper disk soldered on the bottom through which a short length of 1/4" pipe protruded axially. I soldered a length of the same 1/4" pipe to the 1" tube as a handle allowing me to dip it into a paint mixing can to test dilution. I actually squashed the end of the 1/4" pipe a bit to slow the flow and extend the time. A quick glance at your watch as it flows out tells you if you have added enough thinners. Worked a treat and avoided juggling funnels and having multiple containers to pour out of and into.

I wish that I had heard about this 35 years ago when I was into painting cars. It would have been much better than watching the paint drip off the stir stick until you figured that you had the right consistency. 

Cheers 

Don

[awemawson]

It was probably 35 years ago I did it 

[David Jupp]

As well as the inverse of viscosity ratio, you may also have to allow for differences in fluid density.  The density will alter the driving force available due to gravity.  If fluid densities are close then don't worry too much.

It gets much trickier if the viscosities are such that flow with one fluid is laminar, and turbulent with the other.

Chemical Engineering is not the most precise of disciplines, so don't expect any perfect answers...

[picclock]

Using a clear airline water drain 'cup' i marked upper and lower levels with a felt tip, and clamped it in a retort stand above a plastic 1 pint beer 'glass' to catch the fluid.  I filled it with water and waited until the top mark was reached to start my stopwatch. The time to the second mark was 32 secs. Water has a viscosity around 0.89 (or 1) depending on the internet site. Timed again for consistency and spot on. Same test with paraffin (Kerosene) , viscosity 1.64, time 32 secs. Clearly things are not what they seem. Last desperate try with Isopropyl Alcohol, time 32 secs. Obviously the universe gods are tricksters and viscosity is just a myth !!!

OK, I think I'm stumped on this, clearly there are things going on that I have no hope of understanding.

If anyone does understands whats going on here please enlighten me - but for now I'm going to partake of a cold one to ease my addled mind.

Best Regards

picclock

[waggle]

Liquid printing ink is affected quite noticeably by small changes in viscocity, so we had to measure the viscocity frequently, accurately and repeatable. The way we did this was as has been described in previous replies but with a calibrated cup.

http://cdn.c.photoshelter.com/img-get/I0000gOr2CB9d3pE/s/860/860/Fphoto-67711210B-6RM.jpg

It is just a cup with a hole in the bottom, but very accurately made. the cup is dipped into the liquid and at the point it is lifted out a stopwatch is started and at the point that the stream breaks up the watch is stopped. We used to aim for around 21 - 22 seconds which was around the consistency of single cream. The cup needs to be held vertical so that the fluid drains out cleanly, hence the shape of the handle, so if can be just held up on one finger.

If you are just looking to compare the relative viscocities of different fluids then you could just make something similar. Start off with a small hole in the bottom and check the fluid that appears to be thinnest. Enlarge the hole if necessary to achieve an emptying time of between 20 and 30 seconds. Any shorter and the results may be unreliable but if the hole is too small your arm will start to ache before the cup is empty!

Waggle