Helipath Viscometry Data Interpretation

eaxsharpe

When using a helipath spindle, the viscosity values vary greatly from initial impact with the mixture to the end viscosity near the bottom of the sample container, in some cases there is nearly 1 million cP difference. 

How do I know which value to record if I'm being asked to use viscosity as a means of determining sample stability? 

DAS

Those are the inverted t right?. Those are used for high viscosity and thixotropic stuff. You need the right spindle for what you are measuring. You don't always get the error message. Check the manual, there should be a table or a range. Also it could indicate you product is very plastic and the readings are accurate. 

The values you should take are the parameters production will give you as reference. X value measured in X sample with X spindle on X meter during X time at X temperature. If you are dealing with non Newtonian fluids they should give you a table. Measurement without reference means squat. You can consider it "stable" if the sample fits the parameters, nothing else. 

Also a common mistake is not knowing how long has passed since batch production. This brings lots of fights between production and QC. You could be rejecting a perfect batch just because they gave you a fresh sample, or getting funky readings in a short period like you are having. Thixotropy, that bitch...

Window

Are you being asked to assess the stability of old product (previously tested fresh by someone else), or are you designing a process for testing stability from start to finish?

If the latter, you are of course far more limited in what you can do, other than try and figure out what whomever did for the initial tests you're comparing your results to. That is: there should be a standardized test method that specifies how to take the viscosity reading. Maybe the reading is taken after 1 minute, or averaged from a 30 second period starting 1 minute in (as some newer viscometers can do). As DAS notes, there needs to be set parameters.

If you're assessing the product from fresh to aged, I might try to assess if spindle/speed combination is appropriate for the product, or if the helipath is the right choice, even.

I personally would not plan to use any number from initial impact as I would expect a high level of variability. I might also ensure that the guides are set correctly and nothing more than the spindle is in the material during the test (if the weight (or chuck) was hitting the top layer of the product or if the spindle was hitting the bottom of the container either could account for a significantly higher reading).

eaxsharpe

DAS said:

The values you should take are the parameters production will give you as reference. X value measured in X sample with X spindle on X meter during X time at X temperature. If you are dealing with non Newtonian fluids they should give you a table. Measurement without reference means squat. You can consider it "stable" if the sample fits the parameters, nothing else. 

Window said:

Are you being asked to assess the stability of old product (previously tested fresh by someone else), or are you designing a process for testing stability from start to finish?

We are working on bringing a stability program in house as higher-ups discuss the possibility of bringing formulation in house so I don't really have any benchmarks unless I am able to get the data from the CMOs that are currently producing our products.

I spent a bit of time making sure the starting height of the T-bar is the recommended 1/4" above the top of the material and it stops well before the end of the beaker. I've also used the spindle tables from Brookfield to start with what seems like the spindle for high viscosity (T-F, 96). Is it possible the hair mask is too thick to read a consistent value? Or as it spins down into the material its getting increasingly harder to move? 

DAS

It's all possible, it could be the spindle, the product, the method... sometimes is hard to interpret the data, that's why you will have to test it as much as possible and get as much data as you can. For example, do you move the spindle up and down on speed 0 or with movement?. Have you tried both?. At what speed?. What's the difference?. Does it happen in all samples?. And if you mix it before?. 

And of course you should ask the manufacturer what are the parameters they use and what are their results.