What all weights are required for daily verification of Analytical balance having capacity 220 g with least count 0.0001 ?
• Calibrate the balance with 5% and 50% of its full capacity.
• Place the standard weight in the centre of the balance platform.
• Keep the weights in ascending order.
Tolerance : Least count of the balance or 0.1% of standard weight used which ever is higher.
Weight selection for daily check of balances should relate to risk and thus risk should be reflected in your chosen tolerances.
For all USP weighings, the tolerance is 0.10% with a confidence level of 95% (k=2).
For sensitivity test, the weight should indeed be bigger than 50% of the capacity of the balance, better to choose a weight close to 100% of the capacity, and indeed NLT 5% of the capacity. (weight 1)
For checking the minimum weight, a repeatability test should be done, with the desired minimum weight (don’t forget the tare!!) or a weight less or equal to 5% of the capacity. (weight 2)
The relative expanded uncertainty of a least ten weighings should the be calculated and must not exceed the chosen tolerance (again, for USP weighings, tolerance is 0.10%).
For checking eccentricity, the first weight can be chosen. (weight 1)
Refer to Mettler Toledo GWP for more, very detailed info.
What is frequency of all these calibration?
Frequency also would depend on risk, the higher the risk, the more frequent these checks should be performed.
A lot of auditors like to see a daily weight check with weight 1, the repeatability test (weight 2) can be performed less frequently (monthly). Repeatability is strongly affected by the balance surrounding parameters, settings for stability and speed (the higher the speed, the higher the standard deviation mostly is). All these factors should be considered when making up a test plan.
Strongly recommended is to have a ‘real’ calibration certificate with measurement uncertainty done by a certified service provider.