Approval of calibration certificates and Permissible Error Limits – a process approach

It is not uncommon to find in the Quality or the Instrumentation environment those who are concerned with the calibration certificate or with a calibration certificate that is expiring on a certain date, always focusing on to be approved on some evaluation or audit. But does the calibration certificate alone guarantee the reliability of the process?

We do not intend here to summarize the answer to this question in a simple “yes” or “no”, but to lead to an approach that can induct the reader to a safer path in the search for the answer.

In some cases we only think about the instrument calibration when an audit – either by a client or by a certifying agency – approaches. Only the calibration certificate – without a more systemic analysis – does not ensure suitability for an evaluation process, nor does it ensure a high level of quality for your product. Just certify that your equipment has been compared to a reference standard.

The correct use of statistical techniques and a standards-based analysis can help make metrology a revenue-generating activity for the company. In this, metrology contributes a lot, giving reliability, economy and agility to the process, reducing rework, avoiding losses of raw material and raising the level of quality perceived by the customer.

Looking at the company’s assets, calibration ensures profitability and productivity and increases useful life, with more hours of production per machine and less volume of stock of spare parts.

Step by Step

To be successful in this endeavor, it is necessary to follow some steps to establish a consistent metrology plan that will fit the calibration plan.

Firstly we need to verify the size of the instrument cluster that we have by area and define which are impacting in the process, either by criticality in the final quality of the product, by some normative requirement or by reduction of costs or waste. All equipment included in the metrology plan must be subjected to periodic calibration. It is necessary to make a carefully analysis of each process equipment.

Once the survey is done, describe the unit of measurement involved and the instrument resolution (reading division). Then, it is necessary to study – among the process engineering or the production management – how much tolerance each productive stage has, which in fact translates into how much the production process can vary without causing problems in the product, quality loss or loss of raw material / rework.

This is the most complex and time-consuming stage because it requires a lot of alignment between the company’s departments, a good level of knowledge about the objective that is to be achieved and a careful approach to each case.

There are equipment that serves various processes within a company, such as temperature measurement, for example. Based on a food industry, a thermometer can serve to monitor a refrigeration circuit in ammonia compressors and another thermometer with the same capacity can be used to monitor the temperature of the finished product. Or, in a drug production process, you can monitor the pressure of a line of liquids and use a similar manometer to monitor the pressure of an autoclave in the process of sanitizing inputs.

We see that are similar instruments, but they are being used in very different processes. How to adopt a single tolerance field in this case?

We can choose between two paths: creating tolerances by processes, or adopting a more judicious process tolerance.

In the first case, there is a closer result to the reality of each equipment, but the volume of documents and information handled is much larger. This can lead to mismatches between what is described in the plan and what actually happens in practice of the process. In the second case, adopting tolerances by a more critical process, the volume of information described is considerably smaller, but there is a risk of narrowing both the criterion that will undermine the analysis of a somewhat coarser process.

As it turns out, it is not a task for just one person or for just one department of the company. A more holistic approach to the process is needed.

After this stage, the collected data is worked to reach the tolerance field – which is nothing more than the total amplitude of the process tolerance – and will serve as the basis for the next task, which is the determination of the permissible error limit.

For the purpose of example, we take as reference the fact that this thermometer operates in a process of measurement with value of 120 ° C and that the tolerance of the process is of ± 5 ° C.

We then proceed to adopt 1/3 of the process tolerance (amplitude) to determine the permissible error limit.

When the permissible error limit value is found, it will serve as a parameter for comparison with the measurement result expressed in the equipment calibration certificate, which is the sum of the error value found in the measurement process plus the measurement uncertainty assigned to this same process.

If the value of the measurement result is less than or equal to the permissible error limit, the calibration certificate is approved and as a consequence the equipment can be released for use in the process.

In case the measurement result is bigger than the permissible error limit, the equipment must be adjusted to correct the reading error and be able to correctly answer the need of the process in which it operates. The adjustment operation is not always carried out by a metrology laboratory, and it is often recommended that the manufacturer himself carry out the adjustment, ensuring the accuracy levels required according to the factory specification or in more extreme cases a repair intervention is indicated by the same manufacturer to place the meter again.

In general terms, by performing some of the steps described in this article, it is possible to demonstrate in the client and certifying agency evaluations a consistency in the metrology plan that goes well beyond showing only the calibration certificate, since it takes into account the application of the measurement equipment and the process to which it is inserted.

Complementing this system, in Brazil, for example, it is highly recommended to implement the ABNT NBR ISO 10012: 2004, which deals with generic requirements and provides guidance for the management of measurement processes and metrological verification of measuring equipment used to support and demonstrate compliance with requirements measures. It specifies quality management requirements for a measurement management system that can be used by an organization that performs measurements as part of a comprehensive management system and ensures that metrological requirements are met.

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