Torque meter Calibrations to BS7882:2008

[spggodd]

Hello all,

I'm currently trying to calibrate a torquemeter using BS7882:2008 and creating a spreadsheet which calculates the various uncertainties and errors in the calibration.
I'm using the worked example (B1) within the standard as a guide and have come up against 2 problems that I'm hoping someone can shed some light on, as I am stumped!

1) How is the lower limit of calibration calculated?

To would seem to me that the lower limit suggests the lowest calibration class attainable. I.e. with classes of 0.05, 0.1 and 1.0 the lowest would be a class 1.0 but this doesn't seem consistent with the example.

2)How is the value for Uncertainty Contribution "Resoultion U[sub]4[/sub]" derived?

The standard previously references resolution as being (for a digital display) "one increment of the last active number on the numerical indicator" which I would have thought would be something like 0.001 etc... However the values presented in the example are 0.1, 0.05, 0.025, 0.017, 0.013 which would suggest the resolution is changing based on the load applied.



Many thanks for taking the time to reply.
Steve

 

[TED7]

Hope you haven't forgotten this thread completely!

I don't have a copy of BS7882 to hand so I can't answer 1 but your answer to 2 is 0.001 (in the example numbers), the reason the example is showing 0.1, 0.05, 0.025, etc is probably just the way trailing zeros are truncated in metric. The sequence of numbers would really be 0.100, 0.050, 0.025, 0.017 and 0.013.
The vernier on my desk for example has 0.00 on the display when closed and the smallest detectable change on the screen is 0.01 in metric so it's metric resolution is 0.01mm, in inches it has 0.0000 and the last digit increments in 5's so the resolution is 0.0005".

I imagine "lower limit of calibration" would refer to the smallest torque value where your calibration is valid - big torque meters are too coarse to measure small torques. If it has classes, I guess this is to limit the number of 'lower limits' people are using. It would be pretty silly to have a catalogue of meters with lower limits of 11.9Nm, 11.2Nm and 11.5Nm. Restricting them to a lower limit class of 12Nm would make sense as it would catch them all.