From the point of view of the owner, weighing instruments, usually called scales or balances, should provide the correct weighing results. How the weighing instrument is used and how reliable the weighing results are can be very different. Using weighing instruments for legal purposes must have legal verification.

If a weighing instrument is used in a quality system, the user must define the measurement capability of it. In any case, it is the owner or the user of the instrument that carries the final responsibility of measurement capability and who is also responsible for the processes involved. (S)He must select the weighing instrument and maintenance procedure to be used to reach the required measurement capability.

From a regulatory point of view, the quality of a weighing instrument is already defined in OIML regulations, at least in Europe. Calibration is a means for the user to obtain evidence of the quality of weighing results, and the user must have the knowledge to apply the information achieved through calibration.

Calibration and legal verification
Weighing instruments may also possess special features. One of these features includes making measurements for which legal verification is required, for example when invoicing is based on the weight of a solid material. The features may vary slightly from country to country, but in the EU they are the same, at least at the stage when the weighing instrument is being introduced into use.

Verification and calibration abide by a different philosophy. Calibration depicts the deviation between indication and reference (standard) including tolerance, whereas verification depicts the maximum permissible amount of errors of the indication. This is a feasible practice for all weighing. The practical work for both methods is very similar and both methods can be used to confirm measurement capability, as long as legal verification is not needed. The terminology and practices used previously for verifying measurement capability, and for weighing technology in general, are based on these practices of calibrating and verifying, even if it was a question of general weighing (non-legal).

Confirmation is the collecting of information
Confirming the capability of weighing instruments should happen by estimating the quality of the measuring device in the place where it will be used. In practice, this means investigating the efficiency of the weighing instrument; this operation is known as calibration (or verification). One calibration provides information on a temporary basis and a series of calibrations provides time-dependent information. The method of calibration should be selected such that it provides sufficient information for evaluating the required measuring tolerance. The method should be precise for achieving comparable results during all calibrations.

Comparing the indication of weighing instruments with a set standard gives the deviation or error. However, to be able to define the measuring tolerance, we need more information about the weighing instrument, such as repeatability, eccentric load, hysteresis, etc. We must remember that the quality of the evaluation of measuring tolerance depends on the collected information through calibration.

Using a calibration program, which goes through the same steps for every calibration – calculates deviation and measuring tolerance, and, if necessary, produces a calibration certificate – is the best way to achieve reliable information to use in comparisons. This type of program is able to store all the history of calibrated weighing instruments, including information for other measuring devices. It is also handy for monitoring measuring systems. The most important aspect of a calibration program is that it allows the user to select the calibration method that corresponds to the required level of measuring tolerance, and it displays the history of calibrations and in this way provides the user with comprehensive information concerning measuring capability.

The purpose of calibration and complete confirmation
Calibration is a process where the user is able to confirm the correct function of the weighing instrument based on selected information. The user must define the limits for permitted deviation from a true value and required measuring tolerance. If these values are exceeded, an adjustment or maintenance is necessary.

Calibration itself, however, is a short-term process; the idea is that the weighing instrument remains in good working condition until the next calibration. For this reason, the user must determine all of the external factors which may influence the proper functioning of the weighing instrument. The factors in question may include the effect of the environment where the weighing instrument is used and how often the instrument needs to be cleaned, regular monitoring of the zero point and the indication number with a constant mass.

Today, the function of weighing instruments, as well as many other instruments, is based on microprocessors. They possess several possibilities for adjusting parameters in measuring procedures. Calibration should be carried out using settings based on the parameters for normal use. It is very important that the users of the weighing instruments, as well as calibration personnel, are familiar with these parameters and use them as protocol. Since there are several parameters in use, it is important to always have the manual for using the weighing instrument easily available to the user.

The content of the calibration certificate
Very often the calibration certificate is put on file as evidence of a performed calibration to await the auditing of the quality system. However, a quality system is usually concerned with the traceability of measurements and the known measuring tolerance of the measurements made. The calibration certificate of a single measuring device is used as a tool for evaluating the process of measuring tolerance and for displaying the traceability of the device in question.

Performing calibrations based on the measuring tolerance is better than doing routine measuring. Therefore, the user must evaluate the process of measuring tolerance and compare this value with the required measuring tolerance of the process.

Summary
Calibration (or verification) is a fundamental tool for maintaining a measuring system. It also assists the user in obtaining the required quality of measurements in a process.

The following must be taken into consideration:
•the type of procedure to be applied in confirming measuring tolerance
•the interpretation of information while abiding by the calibration certificate
•changing procedures based on received information

Quality calibration methods and data handling systems offer state-of-the-art possibilities to any company.

Aimo Pusa
Lahti Precision
Finland