Analytical Balances - Looking for Variance Source

G

grzesiek

#11
I understand now, what you'd like me to do and it seems good, but as I wrote before the calibration process seems ok, this is the sample weighting process that is troubling me

that's why I wanted to do the ANOVA, but was having problems with the design creation

my take on this is that if I can have 2 balances, 2 operators, 2 equipments for example than my results will show if balances/operators/equipments are the main source of variation, or am I wrong?

of course this disscusion is excellent and helped me understand so much more (at last :) )

ANOVA seems easier to me with the work to be performed, and going to the heart of the problem - identyfing which part of the system is most variable, but I can be wrong of course that's why I'm here
 
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BradM

Staff member
Admin
#12
balances are universally available, anyone can use them. I think so, this doesn't seem good to calibrate 1mg, when I want accuracy of 0.01mg right?
Ok, I should have asked about the universal use of the balance. Then you probably need to keep to full range calibration.

We really haven't talked about the accuracy of the balances. I guess we could write a book on trying to figure out accuracies for balances.:tg: But in short, I typically set tolerances much looser than the resolution. So for a balance with .01mg resolution, the tolerance might end up being much looser; at least .05mg. Of course, setting your tolerance will have a lot to do with your use/application. As far as tolerance on the balance, give yourself as much as you can. But, you will still need to determine how repeatable the balance is in your use range. That repeatability will be a component to your uncertainty.

Also, the reported uncertainty on your standard weights will probably be a factor. Some of those little whisker weights (1mg) can have a fair amount of reported uncertainty, that you would want to consider.

what if I calibrate with a weight, but I weight solid sample that is powder to a little bottle? that's not the same, is it?
No it isn't. Good point.:yes: I suggested using a weight to solely determine the repeatability of the scale. However, I believe when you are performing R&R of the measurement system, you will want to use the product/sample that is normally used. Hopefully the gauge R&R experts (not me:tg:) might offer a suggestion on that.

Also, since you're talking powder, I don't think evaporation/ temperature density will be a factor. But you probably know that better than I.

"If you are wanting to keep your uncertainty analysis simple, you can assume that many of the variances present would be accounted for during your daily checks." - I want to keep it simple, but as I said I suspect that calibration is pretty good, it's the weighting process that I am concered about
You're probably right. :agree1:But, the scales do contribute to the error, and you would want to know by how much.

don't understand, what do you call a swing? movement? they're stable. what is draft - controlled area?
Good questions; I should have elaborated. As far as swing, operators may notice the balances act a bit erratic when, the trash run is made, or the air handler kicks on and off, or at 2pm when it is the hottest, or the balances are most stable in the morning, etc. Operators are always a powerful source of information, and can help in trying to figure out what's going on with these processes.

Sometimes balances (especially your low range balances) can be affected by air flow in the lab; the A/C kicking on and off. That might have already been considered by you. If not, you may want to look at it. Simple hoods made of Plastic/Plexiglass or something can significantly help with air flow/draft considerations.
 

BradM

Staff member
Admin
#13
I understand now, what you'd like me to do and it seems good, but as I wrote before the calibration process seems ok, this is the sample weighting process that is troubling me

that's why I wanted to do the ANOVA, but was having problems with the design creation

my take on this is that if I can have 2 balances, 2 operators, 2 equipments for example than my results will show if balances/operators/equipments are the main source of variation, or am I wrong?

of course this discusion is excellent and helped me understand so much more (at last :) )

ANOVA seems easier to me with the work to be performed, and going to the heart of the problem - identyfing which part of the system is most variable, but I can be wrong of course that's why I'm here
Certainly.:agree1: There are many ways to approach any process. The thing is, ANOVA will detect significance in difference between the two systems. And depending on your level of significance, you will probably find significance.

Then what?:D Comparing the two processes with ANOVA, t test, or whatever test would be most appropriate will let you know there is a statistical difference in means/variance/ etc. Performing uncertainty analysis will let you know how much the difference is, and then you can decide which component of the measurement process needs to be addressed.

As far as design, some considerations:

1. Same operator, different balances
2. Same balance, different operators
3. Different balances, different operators

Take the two set of values, and determine if there is statistical difference.

Also, that is assuming normality with the data (or near normality), if it is a small data set.
:)
 
G

grzesiek

#14
ok I will have to find some more time to think it trough cos there's so much information

I'll just quickly answer some points

"Ok, I should have asked about the universal use of the balance. Then you probably need to keep to full range calibration." - in the sense that many people use it, normally not more than 100mg is weighted, usually and i'm interested in that 1 - 50 roughly

"We really haven't talked about the accuracy of the balances." - meant the resolution fcourse :)

"So for a balance with .01mg resolution, the tolerance might end up being much looser; at least .05mg" - for me bias of .05 is too much, probably need to invest in balances with better resolution?

"Also, since you're talking powder, I don't think evaporation/ temperature density will be a factor. But you probably know that better than I." - true, sometimes powders are so hard to weight because of electrostatics

"operators may notice the balances act a bit erratic when, the trash run is made, or the air handler kicks on and off, or at 2pm when it is the hottest, or the balances are most stable in the morning, etc." - the balances are stable in morning, all other times a drift is visible, magnitude of which is changing randomly (it seems so)

"The thing is, ANOVA will detect significance in difference between the two systems." - I thought I would get % of the errror coming from balances, operators, equipment, and random error

thank you all for the replies so far :) great help to me it is indeed :)
 
G

grzesiek

#15
"The thing is, ANOVA will detect significance in difference between the two systems."

quick question: in ANOVA table I will have F values for different factors (say A, B, C), is it then possible to derive % of total variance from these?
 
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