General FAQ
A balance normally has two types of calibration modes, internal and external. The internal calibration is performed with internal weights or an electrical compensation to mimic the calibration weight. Performing the internal calibration procedures are an important part of the preventative maintenance program for your scale or balance. It is also important to note that internal calibrations should be verified prior to use. The weight used for verification should be calibrated yearly to provide traceability to the International System of Units (SI).
The external calibration requires a calibrated weight to be placed on the balance when prompted. Certified external weights provide traceability to the International System of Units (SI). The instructions for calibration and the size of the weight required is in the user manual for the balance.
Yes, if you need traceability to International System of Units (SI). A balance’s internal calibration process makes a comparison to the internal calibration and the calibration weight that is internal is not traceable to the SI unit. Since no calibration value or uncertainty is associated with the internal calibration, an external weight is used to provide traceability of the internal calibration.
A calibration certificate provides required information on the weight to make the weight traceable to International System of Units (kg). The calibration laboratory also keeps a record of the calibration and all data that was used to make the calibration report. What is the difference in calibration certificates?
• There are two types of calibration reports, accredited and non-accredited. While both provide traceability to the International System of Measurement (SI), the differences are significant. The accredited calibration process meets the quality standards required of ISO/IEC 17025. In addition, the measurement process is inspected by an independent assessor group like NVLAP or A2LA to insure compliance. Accredited calibration certificates provide more detailed information about the measurement to meet the calibration requirements of regulatory agencies like the FDA, ISO and cGMP compliant manufacturing processes. Non-accredited certificates are controlled solely by the calibration laboratory.
The National Institutes of Standards and Technology (NIST) is a national measurements lab. It's part of the United States Department of Commerce, created in 1901, by an act of the United States Congress to maintain measurement standards and measurement infrastructure. For mass and weight measurements, the unit of mass is the kilogram (kg) and must be traceable to the primary reference standard that is kept by the Bureau of International des Poid et Mesures (BIPM). The prototype kg is provided to each member country, such as United States national laboratory NIST, to propagate the unit throughout the world. "Traceable to NIST" means the standard used by the calibration laboratory was calibrated by NIST directly or by a laboratory that has a reference standard calibrated by NIST. Traceability is a property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
Accuracy is the closeness of the measured value to the actual value. The balance will indicate the measured value of a weight based on the accuracy of the weight used to calibrate the balance. The balance can only be as accurate as the calibration weight. Accuracy is normally associated with the tolerance of the weight used to calibrate the balance.
The precision is the closeness of a set of independent measurements. Repeatability, linearity and corner loading are some factors associated with the precision of the balance. Precision is associated with the uncertainty of the indication or accuracy of the balance.
The cost of calibration is related to the accuracy of the weight and the amount of uncertainty to ensure confidence in the calibration. To illustrate: for small tolerances as with ASTM class 1, the process to achieve uncertainties that are 4:1 (or the requirements of ASTM E617) require a more detailed measurement process than a Class 6 weight with a larger tolerance and uncertainty.
Accreditation is governed by the International Laboratory Accreditation Cooperative (ILAC) that has created an international arrangement between other member accreditation bodies based on peer evaluation and mutual acceptance for global accreditation conformity. A2LA and NVLAP are both members of ILAC and are signatories to the Mutual Recognition Arrangement and therefore recognize each other accredited laboratories to meet their standards.
Weights must be calibrated regularly, but the calibration interval is determined by the amount of drift between calibrations. Weights that are handled properly can have larger calibration intervals that will save money with less frequent calibrations. Remember, extended calibration intervals that are not supported with good statistical data could cost more if the weights are found out of tolerance – impacting product quality and initiating costly impact investigations.
The calibration cycle can be extended but remember, it is not only how often the weight is used but also how it is handled and stored between calibrations.
To receive the proper weight for the balance or scale there is required information required to determine the type of weight and class based on readability, meeting USP 41 or laboratory limits. The questions are meant for the user to acquire the proper weight for the balance.
Weights must equilibrate to the laboratory environment. The minimum amount of time is 24 hours for Class 1 and 72 hours for Class 0. Weights that require adjustment or cleaning to meet the tolerance requirement require time before the “as left” values can determined.
In accordance with ASTM E617:2013, the pass/fail criteria is the conventional mass correction ± uncertainty reported ≤ tolerance to pass.
The tolerances for nominal values that are not listed in the tolerance chart can be calculated using a simple formula. Determine the difference in the tolerance of the nominals listed above and below. Divide the nominal weight tolerance needed by the next nominal listed above, multiply the difference in tolerance by the proportion of the nominal values. Intermediate values only exist for ASTM E617 and NIST HB 105 class weight. OIML R111 does not allow intermediate weight values.
True mass is the mass of the weight regardless of location. Conventional mass is defined as a mass that has a density of 8.0 g/cm3 and weighed in air of a density of 0.0012 g/cm3 at a temperature of 20°C. Balances indicate conventional mass and any product compared on a balance will have a conventional mass value. True mass will require compensation for any forces acting on the weight such as gravity and air density. True mass is normally used with force or torque measurements.
There are two basic changes to USP 41, accuracy limit and repeatability to determine balance range instead of minimum weight. The accuracy limit is 0.10% instead of 0.1% and the accuracy is determined from 100% - 5% of the balance maximum range and is only checked at 1 point. Repeatability defines the range of the balance by determining the repeatability of the balance and using the formula 2* repeatability *1000 = smallest in the range.
Weights come in various sizes but the design is the same. If you have more than one set of the same class or type weight it is harder to distinguish one set from another.
Laboratories are accredited for the calibration discipline that are performed under their lab code. There are different levels of accreditation for weight calibrations, based on the ability of the laboratory to meet the uncertainty requirements for the tolerance class. Laboratories may perform mass calibrations but may not be accredited for mass. Check the scope of accreditation to make sure the laboratory is accredited for the mass level needed.
Depending on the size of the weight, spot cleaning can remove small amounts of contamination and for a Class 1 weight the effect on the calibration value will be minimal. Class 0 weights should be calibrated before and after cleaning.