Measuring viscosity of vapours

If you have a constant mix, recalibrate your flowmeter empirically (by using a metering pump, and running at known temperatures).

If you don't know the mix proportion/viscosity of your gas at any given time, you're a bit stuck using a meter that is viscosity-depenent.

What units are you looking for in your result (ie l/min etc.)?

The concentrations of the different components are known, I forgot to tell that there is an analyser running on the gasstream ( not for correcting the flowmeter) but for monitoring a connected patient.

That is why I want to know the vapour viscosities.

The flowmeter is selected for low flow resistance and dynamic range and robustness ....limits the selection ;-)

and is already fixed. :-(

best

Biber

Best I can suggest is getting onto the flowmeter supplier, or, unless you're doing something completely unique, your colleagues (medical physics, anaesthesia, ??) who must have come up against the same problem.

Thanks It more in direction unik, very little work seems to be done and we shoot for good model, the agent concentration varies and also type of agent used

Looks simple but is a little tricky breath out in and breath have different conditon ....

You asked about units ... depends on country and user most people look for l/min ATPS ( Ambient temp and pressure wet)

Some prefer STPD ( standard temp pressure dry) and standard is different in US and Europe ...and some BTPS body temp pressure sat) the relations are known and and it is intended that the customer can select in software.

Thanks for your interest

Can you access the output from the analyser?

It looks like you're going to have to try to calibrate the system yourself, with known concentrations,temperatures and flowrates (from a metering pump etc.), then use your results to build a correction algorithm for the flow sensor. You could feed the output from the analyser (and a temperature sensor) into your system to do the correction automatically, or have an operator enter the concentration data.

Sorry I can't be more help. Good luck,

John

Thanks John. As a practical man this is what I have suggested and it can be done but some of the modern guys more clever with computer than gases (I´m an old analog man also good in gases ) strongly believe in having a solid model for the mixed viscosities and the other parameters and then just validate it in som points .... different people have different methods and different rates of success;-) The flowmeter supplier shows up this week but our group a miles in front in understanding his flowmeter ... bur they have done the flowmeter .....

thanks for all support

... they want a solid model for the gasses? Ausgetzeichnet! (just kidding!)

Hope you get some good results from the supplier.

My first reaction is to suggest metering the gas with a constant volume pump like a peristaltic pump that does not rely on the viscosity of the gas to provide flow accuracy.

viscousity affects the reynolds number and once you are in turbulent flow the adjustment for reynolds number is in the 4th decimal place.

Thanks Guest !

Unfortunately with this breath flowmeters you pass from laminar to turbulent twice per breath so its hard to neglect viscosity influence .

For some young patients you may even stay in laminar region. But a good point I was not completly aware about that the influence was so small .

Thanks

B

I think a way could be to generate a laminar flow in a tube (capillar) and measure the relationship between pressure drop and flow. The flow will be directly related, for a given pressure drop, to cinematic viscosity (parabolic velocity distribution). If the tube is long the inlet and outlet effects are small. Measurement of pressure drop is less a problem than the measurement of the flow. If both pressures are constant the flow is constant and it can be determined as the ration volume/time.

The flow must be laminar i.e. the function flow(pressure drop)= linear. Since you do not know the viscosity you cannot pre-compute the limits so that you should estimate and control if you within the laminar flow domain or not.

To determine the temperature influence the tube can be put in a heated/cooled water bath. A heat exchanger through which the mixture will flow before going in the capillar is in same bath to make sure that the gas does not flow under heat transfer conditions in the measuring tube.

Thanks! Yes you are right, a tube or a bundle of tubes called the Fleisch Pneumotachograf the gold standard in respiratory measuerments... and take bunch of measurements with the different anaestetic agents in different background gases and vary the temperature and make a set of equations that the best way .

My faint hope in starting this thread was that someond in the chemical field familiar with softwares like Chem Cad ( I have no access) and maybe a big database for viscosities could advise about a method --- maybe

I´ll try to convince the people with the problem to start the hard work measuring with temperated gases at known flows ( they have made a calibrating device from this 3l syringes driven by a motor at know speeds .

Thanks for you support