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.

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.

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.

USP Section 41 states that “when substances are to be ‘accurately weighed’ for Assay the weighing is to be performed with a weighing device whose measurement uncertainty (random plus systematic error) does not exceed 0.1% of the reading.” The formula for arriving at this measurement uncertainty value is three times the standard deviation, of at least ten replicate measurements, divided by the amount weighed. The resulting value shall not exceed 0.001.

We always want to remeber that as we increase in weight, even though there may be a greater variance between the replicates with each increase, the measurement uncertainty value decreases. The heavier the weight applied, the greater the chances of that particular weight applied passing the test.

MINIMUM SAMPLE WEIGHT: The minimum sample weight, for Assay weighing, is calculated using the above mentioned test method. The minimum sample weight is determined by identifying the smallest mass value that passes the USP Uncertainty Test.

If a 10mg test load was desired on a 5-place analytical balance and the results of the test came to 0.16% of the applied weight. This particular balance, at that particular location, could not be used because its measurement uncertainty exceeds 0.1%. The unit would then need to be tested at 20mg or the balance would be needed to be upgraded to a 6-place micro balance. If the resulting value had been 0.14% instead of 0.16%, then the value could be rounded down to the same significant figures as the stated criteria (0.1%) and therefore would pass becaue it would be equal to the tolerance.

One of the more controversial aspects of this section is what in fact constitutes the “sample weight”. Is the sample to be measured alone considered the “sample weight”? Or is it the “sample weight” with a tare vessel? The “sample weight” in this case would be just the sample of product alone without the tare vessel. So a tare vessel can not be used to increase the weight and bring the sample into a desired range that will pass the USP Uncertainty Test.

As a general rule of thumb, the recommended minimum sample weight will is approximately 3000 to 5000 divisions (i.e. 30.00 mg or 50.00 mg on a 5-place analytical balance). In laboratory environments that are close to ideal (i.e. little to no vibration, air currents, people traffic etc.), then 2000 divisions (20mg) may also pass the Uncertainty Minimum Sample Weight Test. As stated earlier, when applying the USP Uncertainty Test, the heavier the weight applied, the greater the chance of the value coming within the 0.1% tolerance. In referencing the above example again, at a test load of 1000 divisions (10.00 mg), this balance would be required to produce 10 almost perfect replicates, leaving very little room for any deviations. At this mass, only 10 replicates with 1 deviation of 1 count would allow this 10mg to pass, which in real-world conditions, is not probable.

DEFINITIONS

Assay: A quantitative or qualitative analysis of the composition of a material.

Random Error: Errors due to the play of chance. Variability between successive measurements due fluctuating environmental conditions, different observers, etc. An error, which is in general, different each time a measurement, is made.

Systematic Error: Factors that consistently affect the variable being measured. An error that affects all measurements similarly, (mechanical problem with balance) and or can be eliminated by upgrading the system.

Balance calibration weights, also known as precision weights, mass standards, mass sets, calibration masses, and certified weights, give the process a safety net in ensuring the balances and weighing process are accurate and traceable. We all want to be weighing to the same standard, whether it’s at 1kg, 100g, 1g, or 1mg. With that in mind it is important that our weight calibration provider meets the needed criteria for ensuring accuracy and NIST traceability.

To start, balance weight calibration providers should be ISO/IEC 17025 and or ANSI/NCSL Z540-1 laboratory accredited, preferably through A2LA or NVLAP as the accreditation body. This calibration accreditation for precision weight calibration will ensure that a recognized third party regulatory agency is continually assessing and holding that respective precision weight calibration provider accountable for ensuring NIST traceability and accuracy and overall technical competence.

In addition to having the calibration accreditation, the weight calibration provider’s quality program should also be implementing the following: having their Primary Mass Sets calibrated directly by NIST with a rigorous measurement assurance program implemented and maintained for the integrity of the NIST traceable values on their Primary Mass Sets; technical personnel are qualified and experienced (NIST trained and certified preferably); precision weight calibration procedures and weighing designs are that of the metrological industry standard and or strongly based on NIST calibration procedures and recommendations; and finally the weight calibration provider is an active participant in a proficiency testing program (periodic round robins) to measure the weight calibration provider’s ability to maintain the NIST traceability and accuracy on their mass sets. All of the above will help ensure that the resulting weight calibration value provided on the issued NIST Traceable Calibration Certificate will be accurate, traceable, and valid throughout the world.

With the above being said, it is also important that the weight calibration and certification provider has the capability to calibrate your precision calibration weights ranging from 1mg up to 30kg. And of course that the weight calibration measurement values are precise and accurate enough to meet the extremely tight tolerances for Ultra Class Weights, ASTM Class 0 Weights and ASTM Class 1 Weights, along with the comparative OIML Class E2 Weights and OIML Class F1 Weights. You do not want to be sending your balance precision calibration weights to separate weight calibration vendors because the weight calibration provider can only calibrate up to 10kg due to capability or accuracy limits.

And finally, the weight calibration provider must have the flexibility and turnaround time for weight calibrations that will meet your schedule and requirements. A standard NIST Traceable Weight Calibration Certificate usually has a next calibration due date of 12 months, if the balance calibration weights are sent in to the weight calibration provider and are at the facility of the weight calibration provider for a month, not only is it one month less that you can be using your precision calibration weights but in many companies, especially in the pharmaceutical industry, certified weight calibrations are date sensitive and must have the calibrations completed within that specified calendar month or it may cause paperwork headaches.

In summary, like anything else that is critical, there is a lot to be considered in assessing who should be calibrating your balance calibration weights. Not only do you need to ensure the technical competence of the weight calibration provider to ensure traceability and accuracy, but you also need to take into account the weight calibration provider’s capabilities for measurement range and finally their timeliness in getting your balance calibration weights back to you and your process.

CARE AND HANDLING
It all starts with the handling of the weights. Precision calibration weights are stainless steel or sometimes aluminum for very small milligram weights (20mg and below). Very good tweezers, preferably stainless steel with fine carbon tips, are best for weights 100g down to 1mg. This will help prevent the calibration weight being dropped, damaged during handling, or even lost. For the larger calibration weights, 200g and up, a clean cotton glove, that is changed as needed, works best.

Never handle the weights with bare hands, abrasive equipment, or a glove that could be dirty or contaminated. The oils from the bare hand or other contaminents will cause a weight to change its value and if it is an Ultra Class, ASTM Class 0, ASTM Class 1, or OIML Class E2 weight might even cause the weight to go out of tolerance.

Abrasive handling equipment will scratch and damage the weight causing the weight to lose mass and have its weight calibration certificate value altered and eventually go out of tolerance.

The calibration weight should be handled and placed on the balance with great care. Most of out of tolerance conditions on calibration weights are caused by wear and tear occurred during regular use. The weight class tolerances on ASTM Class 1, ASTM Class 0, Ultra Class, and OIML E2 weights are extremely fine (and can go down to 0.010 mg, 0.006mg and 0.005mg respectively) and a continuously used weight can easily go out of tolerance. A calibration weight should be handled and placed on the balance platform as carefully as possible. This technique will help reduce wear and tare and out of tolerance conditions. Again, having stainless steel fine carbon tip tweezers will also help in reducing wear and tear.

Cleaning weights is usually not something that is done other than by the weight calibration vendor. But I do recommend, for the very precise and smaller calibration weights (10g down thru 1mg), if not a periodic cleaning then at the least a camel hair brush to dust the weight off each time prior to use on the balance. There is almost always a build up of dust and debris (either from the case itself or the balance and or environment). The build up can become considerable, changing the weights value and causing an out of tolerance condition for the precision calibration weights. Also, if a weight becomes contaminated (testing material, fingerprints, etc.) it could probably be cleaned without having to go back to the weight calibration company. When a cleaning is needed and desired, again just for the 100g calibration weights and smaller, the calibration may be easily cleaned with a very fine cheesecloth and ethyl alcohol. Great care must obviously be taken so as no to damage or alter the weight during cleaning, but a very light cleaning with the ethyl alcohol should do the trick and bring the weight back to its weight calibration certificate value.

If an analytical balance that has a readability of 0.0001g (gram), and has a needed measurement range of 100g (grams) down to 100mg (milligrams), with a critical measurement point at 1g (gram), then the NIST Traceable calibration weights needed for this internal check would be at a minimum 100g, 1g, and 100mg. The accuracy for the calibration weights should be ASTM Class 1. ASTM Class 1 is a very precise calibration weight accuracy and the most common in domestic laboratory applications.

So, if we have those three weights incorporated into our internal check program, we have bracketed our measurement range for that particular analytical balance. In addition, with the very accurate ASTM Class 1 weights, we can feel very confident that when the weight is placed on the analytical balance and we see a very large error or discrepancy (Example: The 1g weight has a balance reading of 1.0030g) we know that there is an inaccuracy with the balance and it is time to contact the balance calibration vendor and more importantly that particular analytical balance needs to be put out of service until the inaccuracy is corrected.

The only time that ASTM Class 1 weights would not be sufficient would be in micro balance applications. Micro balances are extremely fine balances with readabilities of 0.000001g (gram) or even 0.0000001g (gram). For micro balance applications ASTM Class 0 or Ultra Class (Manufacturer Accuracy Class) weights would be needed. These are the most accurate weight classes and are better suited for the extremely fine readability of the micro balances.

The internal calibration weight balance check is not intended to duplicate the balance calibration performed by the balance calibration technicians during regularly scheduled preventative maintenances. It is a part of your quality assurance program to ensure that your weighing instruments are measuring accurately. The checks should be performed as per a standardized operating procedure (SOP) along with results being documented in the respective log book.

Also keep in mind, that the internal calibration weight checks alone are not sufficient. A regularly scheduled preventive maintenance and calibrations with a qualified vendor is still needed. And lastly, make sure your your internal calibration weights are calibrated on an appropriate interval (six months, annually, etc.) to ensure accuracy and NIST traceability.

For more info please see our “Internal Calibration Weight Verifications (Checks)” under Technical Articles in this site.

When choosing balance calibration weights for your weighing and measurement application, the first thing that needs to be addressed is what accuracy class will be needed. Accuracy classifications are predetermined accuracy designations at the time of manufacturer. Currently there are three major categories for classifying precision laboratory weights; ASTM, OIML, and Manufacturing (Ultra Class and Ulti Class).

WEIGHT CLASS BACKGROUND
The ASTM Classification is as per the document ASTM E 617. Most of the domestic (United States) weight classifications adhere to this specification. The Weight Classes are broken out numerically from ASTM Class 0 to 7, starting with the most accurate classes first. ASTM Class 0 would be the most accurate and tightest allowable tolerance of the weight classifications followed by ASTM Class 1 as the next most accurate (tightest allowable tolerance). In most precision laboratory and calibration applications ASTM Class 4 weights would be as far down as you would probably want to go.

The OIML Classification is used internationally (Europe, Africa, Asia, South America, etc.). OIML R 111-1 is the document for the OIML weight classifications. The classifications are broken out as per alpha-numeric designations, Class E1, Class E2, Class F1, Class F2, Class M1, Class M2, and Class M3. OIML Class E1 would be the most accurate and tightest allowable tolerance followed by OIML Class E2 and OIML Class F1 respectively. For precision laboratory and calibration applications OIML Class F2 should be used as a minimum accuracy for the weights.

Manufacturing weight classifications, Ultra Class and Ulti Class, are for very precise laboratory applications. These weight classifications are only recognized by the end users and the manufacturers themselves. The weighing regulating bodies of ASTM, NIST, and OIML do not formally recognize Ultra Class or Ulti Class. The classifications and tolerance values are very similar to the ASTM Class 0 weight classification.

ACCURACY CLASS APPLICATIONS AND USAGE
There aren’t too many standards or references in terms of what weights to use and when.

Speaking in metrological terms, a practical guide would be the following:

ASTM Class 0, Ultra Class, and OIML Class E1 should be used as at the highest level of precision i.e. mass standards (calibrating other weights), micro-balances testing and calibration, and critical weighing applications.

ASTM Class 1, 2 and OIML Class E2, F1 should be used in precision applications i.e analytical balance testing and calibration.

ASTM Class 3, 4 and OIML Class F1, F2 are best suited for Top Loading Balance calibrations and testing, and moderate precision applications (laboratory non-critical).

For more information and background please see our Calibration Weight Accuracies under the Technical Articles in this site.