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Viscosity Measuring Method
When measuring viscosity of a product, which parameter should be constant and which should be adjusted, is it the speed or the torque?The method in my lab is using Brookfield LVDV-I Prime, adjusting the speed until the torque displays at least 70%. With this method, some very different rheological character fluids having the same viscosity value. This makes a confusion when visualizing a viscosity based on its value.Please enlighten me in this regard. -
3 Replies
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I will start off by saying that adjusting for a torque of minimum 70% is an interesting method. My goal when setting a specification has always been to shoot for a torque range, between 35 and 70 usually. Obviously this would vary from product to product but I digress.
Viscometers measure viscosity, they don’t really give you much insight into different rheological properties of your samples. Unless you have a test for rheology in your lab, it can be difficult to marry these two ideas especially if you find yourself working with many non-Newtonian products.
I find it may be easier to not dig too much into this quandary if you are the type that can do that! It is standard industry practice to say that viscosity is a measurement of the “thickness” of your product but there are many instances where your product is going to have different viscosities being measured vs. being squeezed through a tube vs. pumped during filling/pumped through a component.
Rheology is your measure of flow - when you swirl your product around in a beaker trying to gauge your viscosity your are actually looking at rheology! This is why products that behave differently during these visual “swirl” tests can have similar if not identical viscosity readings.Hope this helped and wasn’t just my old man ramblings!
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When I did viscosity measurements, our company used a Brookfield.
We kept the speed and spindle constant. We took the reading after 1 minute.
Our goal was to have the middle of the specification come out in the middle of the scale.So, for our shampoos the viscosity spec called for a target of 7000, with a range between 6000 - 8000 cps. The dial on our viscometer went from 0 - 100. We chose a spindle and speed where when the reading was 50, that matched a viscosity of 7000.
But it was all relative. If you changed the spindle and speed, you might get a different viscosity value.
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I think the important things to consider are consistency in the testing method first and foremost. You do want your torque to be in a ‘good’ range for the instrument you are using so your readings are reliable. Second why do you want to characterize viscosity- is this an internal QC parameter you want to meet? Is the speed you are choosing correlated to some processing step (pumping through a tube, mixing) that you need to understand your formulations properties during?
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