Viscometer

[Missy K]

For a physics college project i have to build a viscometer which measures viscosity using the torque of a DC motor.
It will have to be a rotational viscometer.
I'm thinking of making a stormer viscometer.
The rotational speed of the viscometer must be variable.
Any ideas about how to make one?
Any ideas about the programming required for this, in order to have variable velocity settings?
We work using Labview and/or Matlab.

 

[Gary B]

I would think a small low voltage DC motor turning paddles would do. Vary the voltage to vary the speed. Since the more viscous the fluid, the slower the motor will turn and the less CEMF so current would be directly related to viscosity of the sample

 

[DerStrom8]

Yes, I agree with Gary's idea. A "loaded" motor draws much more current than a motor with no load. Therefore, by measuring the amount of current the motor is drawing, you can figure out whether the fluid is less viscous (more like an "unloaded" motor) or more viscous (more like a "loaded" motor). I think this is the simplest way you could do this.
Der Strom

 

[jokerswild]

I've used Viscometers when I was in the flexo print trade and they were very complicated things. Good luck!

 

[Reloadron]

I would think a small low voltage DC motor turning paddles would do. Vary the voltage to vary the speed. Since the more viscous the fluid, the slower the motor will turn and the less CEMF so current would be directly related to viscosity of the sample

I would work from there amd as suggested measure motor current at a given voltage or if budget allows find a small rotary torque sensor and measure torque. Additionally a temperature sensor should figure in so temperature(s) can also be recorded.

Ron

 

[MrAl]

Hello,

Do you have to measure torque with that kind of visc meter?

 

[Reloadron]

Hello,

Do you have to measure torque with that kind of visc meter?

Hi Ya MrAl, actually likely not. I would guess if you had some standard viscosity solutions at a known temperature, you could plot the motor current and see what you come up with while keeping the motor voltage a constant. Long as you had some known standards to work from. Maybe Fisher Scientific or another retailer for the standards?

Ron

 

[Pommie]

If you drive the motor with a constant current then it will produce a constant torque and the speed will be proportional to the viscosity. The voltage across the motor will be proportional to the speed and so will also be proportional to the viscosity. A current limited lab supply would act as a constant current source and enable some quick and easy trials.

Mike.

 

[bryan1]

I agree with Pommie on this, I just had a talk with my wife and she was saying the viscometers at her work were one precise unit and the moment they went out of calibration a service tech was called. My wife used to work in the medical industry so yes I can agree a designing a devise of this nature would entail some serious research. The answers given above should give a good idea on your quest.

Cheers Bryan

 

[Pommie]

Actually Brian, some solids are viscous. Glass being one example. However, I doubt the OP had glass in mind when they started this thread.

Mike.

 

[MrAl]

Hi Ron,

Yes that would be interesting. I've never made one of these myself.

I agree that measuring current could lead to measuring torque, whatever function that produced (linear or not), and that measuring speed with a constant torque would probably lead to some usable results, but now that i re read the original post im a little confused about what kind of meter he/she wants to build as one type uses torque and another type doesnt, so there appears to be a conflict in the OP.
I guess we'll have to wait for a comeback

 

[Reloadron]

Actually this link is a pretty informative write up on the subject. Well illustrated as to the math behind the scenes and may prove useful to the OP.

We use them at work for testing the viscosity of the resins before the resin goes into large vats where we impregnate motors. It was a graduation from Zhan Cup Methods that were previously used. Not my department.

Ron

 

[Mr RB]

Actually Brian, some solids are viscous. Glass being one example. However, I doubt the OP had glass in mind when they started this thread.

Mike.

Sorry to go off topic Pommie, but I believe that is a myth. Glass is a solid an evidenced by perfectly preserved examples from ancient Egypt and Greece etc. It's generally believed the myth started from the old small glass panes where the glass was found to be thicker at the bottom, but this was just a construction method because the glass of the time was mis-shapen and a good window smith would install the panes with the thick side at the bottom.

 

[Missy K]

Thank you all for the ideas....and to MrAL I have a choice to work with torque or current,
The viscometer has to be able to work at different speeds, which implies a variable power supply.
I am thinking that i will work with current as the more viscous the solution the more current drawn.
If i use a brushless DC motor then the current-torque have a linear relationship.
I will have to calibrate the current / viscosity(resistance) relationship for my motor using another viscometer!

 

[MrAl]

Hi again,

Sorry i must have missed your reply earlier.

That sounds like a good idea. I wish i could specify a motor for you, but all i can do at this point is suggest possibly a smaller type motor to get better resolution.
Yes calibration should get you there pretty quick.

I've also thought about the effect of flow on the measurement, and this might require a constant container (sample) size too.

 

[Reloadron]

Just remember to note your standard temperatures.

Ron

 

[MrAl]

Hey Ron, very good point! I didnt even think about that until i read your post. Sometimes we get stuck in the 'room temperature' mode

 

[Missy K]

Thanks for all your help with this project.
It is gonna be a bit of work but i think i have a few good ideas
The varying temperature is still a sticky issue but i think i will have a temperature prob which will b interfaced so i can view the temperature and viscosity on the front panel of my computer program!

 

[Reloadron]

Good, take your ideas and run with them. If when all is said and done, and if you think about it, please do share with us how things went. There are lab standard viscosity solutions out there if you really get into it. Hope it all goes well...

Ron

 

[dabeak]

All true of course, and now it gets complicated. I’ve “been there done that” and you have to be very careful about defining the test, the instrument, the substance, and the phase of the moon.
The speed itself is a factor. Look up thixotropic (wiki is a good start). All materials are thixotropic (or dilatant) to some extent. Worse yet the scale of the measurement is also a factor. So the size and shape of the paddle as well as the speed and time are also factors. It turns out that in practice the instrument and the paddle are also specified for the particular application. The most common viscosity instrument is the Brookfield but the Ford and Zahn cups are also common. Many viscosity test are specific like for catsup. Conversions of the results of one method to another are a PHD thesis. So a specific instrument is just that, specific to the application.