# Establishing Ranges for Weights for Calibration Verification of a Micro-Balance

M

#### MetroPetro

I'm looking for help in determining the ranges I need to verify calibration of a new micro-balance. I am new to metrology and am wading hip deep in statistics right now, but I feel I have all of the information I need to establish pass/fail ranges for daily verification of calibration. The verification will be for Sensitivity, and the balance has a maximum capacity of 20g. I have calculated the minimum weight to be 0.0015g using the standard deviation from the Repeatability calibration (0.0000005) and the USP formula (3 x St. Dev) x 1000...so now I need tolerances. What am i missing here? Thanks in advance!

Staff member
Not sure how old this micro-balance is. If it is an older balance, it may have trouble meeting the repeatability requirements at a low end. What I would do is begin to test the balance and see what the lowest minimum weight is.

As far as the test ranges, much of that will depend on the specifications of the balance. Namely, you won't be able to achieve a higher accuracy that that which is published for the balance.

From that, a lot of it will depend upon the requirements of the balance. What are they measuring, and how accurate do they need it?

Be sure and match the balance with the requirements. Say, if they are trying to measure to .0001 gram and that is the resolution of the balance, the balance will be out of specification all the time (most probably).

I would find out what the process requirements are for the balance. Then, determine what balance you need for the application; and determine if that balance will suffice.

Now... if you have a facility with multiple balances, what you may find is customer A is using a balance that would better suit customer B; and vice versa.

So start with the process requirements, and then tighten the tolerance from there as much as you can, as long as the balance is capable of producing that tolerance.

Also... be sure you have controlled for vibration, humidity, draft, etc. Since you mentioned "micro-balance, I would assume this thing goes out six/seven places.