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Tutorials : Apr 15, 2011 (Vol. 31, No. 8)

Minimizing Weight Measurement Uncertainty

Software Helps Labs Comply with ISO, GxP, and USP Regulatory Requirements
  • Stephen Wilent

Precision can be defined as the closeness in agreement of results during a series of repeat measurements under presumed identical conditions. These results are often expressed as a standard deviation. Precision is often mistaken as a measure of accuracy; it is distinctly different from accuracy, and the confusion can lead to significant regulatory violations associated with weights and measurements.

The fact is, precision between instruments or labs can be very good, but the accuracy can be off by orders of magnitude. Poorly maintained or low-quality balances can provide good repeatability, while at the same time their accuracy can be very poor. Simply put, precision describes the scatter of results achieved when repeatedly measuring the same weight.

Precision is not a factor of the true value of the weighed item and should not be used to describe accuracy. It should be noted that when measuring small quantities of a substance on an analytical balance (i.e., a few percent of the balance’s capacity), repeatability is the dominant contributing uncertainty of measurement, accounting for greater than 90% of the total uncertainty.

Accuracy can be defined as the closeness of agreement between the result of a measurement, or series of measurements, and the true value of a traceable, certified standard. Accuracy is establishing a relationship between values displayed on the balance and values known. In essence, accuracy is obtained by comparing values obtained on the balance against the value of the standard as reported on the weight’s certificate of calibration.

Having a clear understanding of the difference between precision and accuracy will allow you to maintain your weights and measurements in a compliant manner. The quality-management functionality within Mettler Toledo’s LabX 2010 Balance Excellence software contains all necessary components to completely support establishing measurement uncertainty (minimum weight) for determining the precision of your lab balance (Figure 1). LabX balance software also contains all necessary calibration and weight traceability support for determining the accuracy of your lab balance.

Established Regulatory Requirements

A standard that is used by a huge variety of industries around the world, ISO 9001, states “Measuring equipment shall be calibrated or verified at specified intervals…against measurement standards traceable to international or national measurement standards.”

Another important regulation often requiring compliance, 21 CFC pat 211.68 (a), which also affects weighing operations, states, “Automatic, mechanical or electronic equipment…shall be routinely calibrated, inspected or checked according to a written program designed to assure proper performance.” USP section 41 also succinctly outlines measurement uncertainty requirements by stating “measurement uncertainty is satisfactory if three times the standard deviation of not less than 10 replicate weighings divided by the amount weighed, does not exceed 0.001 (0.1%).”

What is measurement uncertainty? VIM defines measurement uncertainty as “a parameter characterizing the dispersion of the quantity values being attributed to a measurement.” When the repeatability test is executed, you can then establish the minimum weight for each balance, regardless of readability. The minimum weight is the lowest amount of sample mass that can be weighed on a balance with a high degree of confidence while complying with the required weighing accuracy.

Many environmental influences affect measurement uncertainty, including: temperature, airflow, electrostatics, vibration, and magnetism. As a result, measurement uncertainty must be established on the balance in situ. Once a minimum weight is established, it must be monitored and controlled. So how do you know you are minimizing your weighing risk?

LabX-Avoiding Risk

The dictionary defines calibration as: “to determine by measurement or comparison with a standard; the correct value for each balance reading on a device.” Risk assessment is an integral aspect of each day for lab managers and technicians. But, have you considered the importance of, and the risks associated with the calibration of your balance and the accuracy that is provided?

Can your calibration provider back you up in times of trouble, and supply documented evidence that the equipment was functioning within specification? Can their calibration procedures withstand independent scrutiny and be shown to be traceable to National Standards for accuracy of measurement, thus reducing the risk of inaccurate weighing events? These are important documents that provide tremendous support that lab and weighing activities are professionally and expertly conducted.

The built-in test and adjustment scheduler for LabX ensures that balance calibrations are carried out as planned according to your quality management standards. Whether it’s using the internal or external weight verifications, or even performing repeatability tests, lab managers have the option to configure the calibration setting according to their requirements.

For example, if the user accidentally forgets to execute the test, LabX can block the weighing task from running until the test is performed. LabX can also block the weighing task if the test is performed and did not pass per your established standards. This ensures that all weighing jobs will and can only be executed on a well-calibrated and tested weighing instrument, avoiding weighing risk and providing strict compliance to quality-management objectives.

In order to support required regulation and accreditation standards, it is fundamental that LabX offers full traceability of all data and information generated within the system. This includes electronic signature acceptance and complete system security. Components of this include support for password policy, user rights management and role definition, and control of access to specific functionality. By utilizing built-in LabX features, the user can easily conform to these tough standards.

Audit Friendly

LabX 2010 Balance Excellence provides users step-by-step instructions on weighing tasks directly on a Mettler Toledo balance touch screen (Figure 2). All individual process steps are incorporated to provide complete workflow SOP compliance, including weight checks before the process begins. All calculations are automatically performed and all data is saved in a central database. Transcription errors are eliminated, and full traceability is guaranteed.

Weighing methods are created and tailor-made for each specific weighing task. The methods require users to always follow the same procedures according to the assigned SOP. The user can not deviate from this SOP, providing strict process compliance. Written instructions can be defined in the method to offer guidance to the user when the task is executed on the balance. This eliminates risk as all instructions are pre-set and users can simply follow the step-by-step instructions.

With LabX, manual transcription error is a thing of the past, as all data are automatically captured. Key metadata, which are crucial to the weighing operation such as: weighing date and time, operator name, procedure used (during each weighing task), balance settings, weighing status and instrument used, and also when the balance was last adjusted are all recorded. Once the job is complete, a customized report can be generated that contains all necessary data for total compliance reporting needs.