I use specifications for a measuring instruments as a Type-B standard uncertainties in uncertainty calculations per GUM.
Some external standards (such as ISO 17025 via A2LA) seem to require specific values for repeatability as Type-A standard uncertainties.
I feel that the repeatability of the measuring instruments is already captured in their specifications. If I determined their repeatability and included it in my calculation I feel I would be double-counting their repeatability. I'd rather not have to increase my uncertainties to explicitely accomodate repeatability, but that seems to be the mandate.
Am I missing something?
Thanks,
Brian
Hi Brian
It's well known that the dwarfs made Thor's hammer Mjollnir, which was a little short in the handle. What is less well known is the dwarfs also made a micrometer for Thor (played by Arnold Schwarzenegger). This micrometer was of equal quality to the hammer and always returned to zero after use (although it was not much use for killing giants, which is why you don't hear much about it).
But the micrometer also had a systematic error. The screw thread pitch was slightly out, so that it always measured 10 microns too big. (The dwarfs were very far sighted by the way, which is why they used metric units over 1000 years before they were invented). The dwarfs were also scared of Thor, so whilst they didn't know exactly what the systematic error was, they included an uncertainty statement which said that the uncertainty in the measurement was +/- 10 microns.
When Thor received the micrometer from Loki (played by Danny DeVito), Thor was suspicious of its accuracy. But since Thor was not too good at statistics, he got Loki to carry out a repeatability study. Loki measured five gauge blocks, twenty times each and estimated the type A uncertainty as +/- 1 micron. When Loki told Thor of the error, Thor was very pleased. "I am making precision pistons for my father's toy train set. They must be correct to the nearest micron or they will not fit. This is perfect!"
However, Loki was deceitful. He had deliberately ignored the 10 micron uncertainty statement given by the dwarfs. "It doesn't matter." he said, "Every measurement will have the same instrument error, so I don't need to include the dwarf's uncertainty statement in my calculations".
After Thor had made the pistons, he gave them to his father Odin (played by Samuel L Jackson) who was initially delighted. "At last I will be able to complete my model of 'The Mallard' ".
But when Odin tried to fit the pistons into the piston rings, they were too loose! Steam would escape from the seals and the train would not work. He demanded to know what had happened, so he ordered his first wife, Frigg, (played by Jessica Alba) to investigate what had happened.
Frigg compiled a report which concluded with the following statement. "Loki failed to take into account the manufacturer's uncertainty in his calculation. He correctly calculated the error due to repeatability, but failed to understand that systematic error is not corrected by repeated measurements"
This, by the way, is the real reason why Loki was bound and left in a cave until the day of Ragnarok. The death of Balder was just an excuse, Odin was actually angry about his toy train.
NC