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# MiniTab- Determining Repeatability of an Instrument

## MiniTab- Determining Repeatability of an Instrument

(OP)
Hello,

I am trying to test the repeatability of a new instrument. I am not doing a full on Gage R&R because I'm only testing with 1 operator. This may sound weird to some, but I don't think the operator matters when it comes to this test. This instrument measures surface area of powders. The operator weighs out a certain amount and adds it to a tube. He/she then screws that tube into a port, shuts the door, and presses START in the program. The instrument then does the rest. The only variable that I can think of that the operator could effect would be how tight he/she fastens the tube. I don't think this varies much anyways.

Three samples can be measured at once on this machine. So, I weighed out 3 samples and repeated the tests over and over again; 15 times. So my data consists of 3 samples with 15 measurements each. I plotted them in Excel in SPC form for viewing with upper and lower spec limits. I can tell that each port behaved a bit differently. I calculated variance, st dev, and the coefficient of variance for each. I'd like to know if there is another more concrete method for my test. I'm very familiar with gage r&r's but not so much with this method using only 1 operator.

It really isn't feasible time wise to do multiple operators, it takes 3 hours per test. Due to resources that really isn't an option. If it was I would have done it.

Can anyone help me on this? Thanks in advance.

### RE: MiniTab- Determining Repeatability of an Instrument

What exactly does the tester do? How is it measuring the area? What is the reference standard specified by the manufacturer?

How trained is this one operator? The fact that there are statistically different results from the same operator suggests that there is little operator dependence. Do each of the 15 measurements include a new insertion into the port? Are the 15 measurements internally consistent?

TTFN
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### RE: MiniTab- Determining Repeatability of an Instrument

(OP)
The operator weighs the sample, puts it in the tube, weighs the tube + sample on a calibrated scale, places the tube into a degassing & heating machine, presses START, the material is then conditioned using inert gases and heated to evaporate any moisture, this process takes 2 hours, when complete the operator removes said tube, weighs it again on the same calibrated scale, the tube is then inserted into the surface area machine, then presses START.

The machine uses liquid nitrogen to cool the sample to a cryogenic temperature of around 77K. Simply put, gaseous nitrogen is then pumped into the sample tube and then the individual Nitrogen molecules form a thin film on the surface of powder particles. The machine measures the amount of nitrogen that formed this film and produces a surface area. Of course it is more complex but this thread isn't about surface area calculation of powder.

### RE: MiniTab- Determining Repeatability of an Instrument

You didn't answer my other questions

TTFN
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### RE: MiniTab- Determining Repeatability of an Instrument

How does the machine control the amount of nitrogen on the sample? If the sample is 77K, then what keeps more nitrogen from sticking to the surfaces?

I disagree with your last statement. You have what you claim is a non-repeatability of this test, so how the test is performed is extremely relevant.

TTFN
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