There are no published standards are requirements in regards to weight calibration frequencies for your “precision” calibration weights. “Precision” would be defined as any calibration weights having the following, or equivalent, Accuracy Classes; ASTM Class 0, ASTM Class 1, ASTM Class 2, ASTM Class 3, and ASTM Class 4.

Calibration weight manufacturers will recommend one year to six months, depending on usage of the calibration weights and criticality of the weighing process. In servicing the pharmaceutical industry for over 30 years, most of the industry seems to have followed the calibration weight manufacturer recommendations of one year and six months for their weight calibration frequencies. About seventy five percent of the weight calibrations we perform for the pharmaceutical industry are at a one year frequency, with the rest being at six months.

With that being said, unless your weighing process is part of a cGMP critical operation, or getting very heavy usage, or the calibration weights are being used for calibrating laboratory balances, especially analytical and micro balances, then a one year calibration frequency for the calibration weights should be sufficient. If the calibration weights are being used in a cGMP Pharma critical operation, or getting very heavy usage, or are being used for calibrating laboratory balances, then a six-month calibration frequency is highly recommened for the calibration weights.

The other scenario in which the weight calibration frequency would best be at six months is if an Out of Tolerance “as found” condition would be an issue for any previous measurements or daily checks that particular calibration weight may been used for. This is a common issue in the pharma industry that usually triggers investigations and a look at available historical data.

A weight calibration frequency of once every two years would be acceptable if the calibration weights are not getting used on a regular basis or the weighing application is not critical in the least. But no matter how non-critical the weighing process or how little the calibration weights are being used, a once every two year weight calibration frequency would not be recommended for calibration weights that are Class 0, Class 1, or Class 2.

For a variety of reasons, (as found out of tolerance conditions, weight handling, traceability and confidence in your weighing process) anything beyond a 24 month weight calibration frequency for precision calibration weights would not be recommended.

So in summary, a quick and dirty breakdown of calibration weight frequencies would be as follows:

6 Months
cGMP critical weighing operations; the calibration weights are being used for calibration of laboratory balances; avoidance for Out of Tolerance conditions and investigations; the calibration weights are getting very heavy usage.

One Year
The calibration weights are used on a moderate basis; there is a normal level of critacality to the weighing process; the pharma industry has adopted a one year weight calibration frequency as their industry standard.

Two Year
The calibration weights are not being used that often; the calibration weights are not part of critical weighing process.

Our Weight Traceable Certificate is a NIST Traceable report documenting weight calibration results.

The Weight Traceable Certificate documents the testing environmental data (Temperature, Pressure, Relative Humidity and the Air Density) taken at the time of the weight calibration. The air density is used to apply and adjust for the required air buyoancy corrections for the weight calibration results. In addition to the environmental conditions the weight certificate also contains the descriptive and identification info (S/N, ID No., Range, Accuracy Class) for the respective calibration weights under test.

Both the as found and as left values, for all the calibration weights that were calibrated, are documented on the weight certificate. The weight calibration values documented on the certificate are conventional mass values. The conventional mass values could also be called “mass in air”. These weight calibration values are relavent when using the values in measurements with balances. The conventional mass values should not be referenced when using the calibration weights in a vacuum chamber or when “true mass” values are required.

Also documented on the weight certificates are uncertainty values. These values are the uncertainy of measurement values for both the as found and as left weight calibration values. For example, if a 100g Class 1 weight has an as found value of 0.050mg and an uncertainty value of 0.034mg, then the weight certificate is stating that the value of that weight is 0.050mg +/- 0.034mg. The uncertainy values are calculated at a 95% confidence interval using a coverage factor of k=2.

Lastly, the stated tolerance and the as found disposition columns are contained in the weight traceable certificate. The stated allowable tolerance for the calibration weights is based on the fixed accuracy class values, which are referenced from national or international standards. The most recognized domestic calibration weight standard would be ASTM E 617-97 which uses a numerical accuracy system. The OIML R 111-1 calibration weight classification standard would be most recognized internationally. The OIML standard has an alpha-numeric accuracy breakdown.

The as found disposition columun is stricly a “pass/fail” disposition for that calibration weight for the as found value only. If the resulting as found value of the calibration weight is outside of the allowable tolerance for that respective weight class then the as found disposition would state “Out of Tolerance”. If the resulting calibration weight value is within or at the allowable tolerance for the respective weight class then the as found disposition would state “In Tolerance”. Again, this would be for as found calibration weight values only.

It is also of note that a calibration weight will not be certified to a respective accuracy class if the resulting weight calibration value when added to the uncertainty of measurement value brings the calibration weight out of the allowable tolerance range. For example, if the resulting weight calibration value for a 500mg ASTM Class 1 calibration weight is -0.0089mg and the uncertainty of measurement of that weight calibration is 0.0016mg, then the weight calibration value added to the uncertainty of measurement would bring the potential calibration weight value to -0.0105mg which is outside of the allowable tolerance range of 0.010mg. The calibration weight would then have to be removed from service or downgraded to ASTM Class 2.

In addition to having your balances calibrated at the appropriate intervals (externally or internally), to really get the most value out of your calibration weight measurements in your daily checks it is good practice to apply the following steps to maximize the precision and accuracy of your measurement results.

Before taking the calibration weight measurements, it is alway good to “warm up” the balance, especially if the balances are analytical or micro balances. Warming up the balance consists of placing the heaviest available calibration weight, either at balance capacity or the calibration weight used to test the high end of the designated user range, and place the calibration weight on and off the balance five to six times in a repetitive manner. This action will help excercise the balance pan and also heat up the coils of the balance. It will help greatly in both the accuracy and precision (repeatability) of your calibration weight measurements.

You then want to do a quick check of the level bubble on the balance and make sure it is centered or fairly center in the circle before taking the calibration weight measurement. This is incredibly important in providing the most accurate calibration weight measurements. If the bubble is not centered, every other precautions taken in your daily checks are out the window.

In regards to enviroment at the time of peforming your daily checks with the calibration weights, you want to make sure that there is no vibration, air currents, temperature swings, opening and closing of doors, or people traffic in your lab near the balance. These environmental issues are sometimes out of our control, but whenever possible should be addressed at the time of setting up your calibration weight daily check program. Very accurate and precise balances (micro, analytical, top-loading, and even high precision scales) are all extremely sensitive to “not ideal measurement” environments. So whenever possible try to be aware of when you are taking your calibration weight measurements. You can’t always control the enviroment or all that is going on around you, but you can somewhat control when exactly you are taking your calibration weight measurements.

Finally, in the placing of the calibration weight on the pan itself, there are a few things we want to remember. Make sure you have a good and steady zero captured. You don’t want the balance to be “noisy” (having the display values running up down five to eight counts very rapidly) at the time of placement of the calibration weight. You also want to take a degree of care and technique in your placement. Try not to be too heavy or too rough in placing the calibration weight on the pan. Lastly, place the calibration weight in the most center of the balance pan. Having the calibration weight, or any measurement sample for that matter, properly centered will help minimize the effects of any corner-load error.

It seems like a lot, but if we do keep the above in mind and apply these when taking our calibration weight measurements and also our sample measurements we will surely be getting the most accurate measurement values available.