The purpose of a process plant is to convert raw material, energy, manpower and capital into end products. In some cases the quality of the product can be verified by doing necessary measurements of the final product, but in many cases the quality cannot be verified only by measurements of the final product. The only way to assure the quality of the end product is to perform measurements during the process. Typical examples are processes that require certain conditions during the process, such as temperature treatments during the process of the product.

In any case, we can safely say that the quality of the end product is based on various measurements. Obviously, if the measurements are not working correctly the quality will be jeopardized.

So how do you assure that the measurements are correct? This can be done only by an appropriate calibration management system.

In this article we’ll discuss what calibration is, traceability of calibration and uncertainty related to calibration, as well as why and how often to calibrate. In addition, we’ll take a quick look at regulatory requirements.

What is calibration?

In short, calibration is a documented process where the measurement instrument to be calibrated is compared against a known value, typically a more accurate reference standard.

It is good to remember the old fact that all measurements have error and calibration tells how much! An everyday example could be that when driving your car you see that the speedometer is showing exactly 75 mph. Then a police officer stops you and you realize the speed limit was 60 mph. The police give you a ticket which shows that you were driving 72 mph. So you can see that your speedometer has an error of 3 mph—you just performed an expensive calibration of your speedometer.

The used reference standard should be traceable and the uncertainty of the calibration should be known. More on these in the next sections.

Traceability of calibration

Traceability is the unbroken chain of measurements from the process measurement all the way to international standards. In practice this is achieved in several steps; the process measurement is calibrated using the company’s working standard or calibrator. The company’s working standard must be regularly calibrated using higher level reference standards, which are typically available in accredited calibration laboratories.

The accredited calibration laboratory ensures that its reference standards are regularly calibrated against national standards; the national standards are compared with international standards.

If that traceability chain is kept unbroken and maintained valid, it ensures that the measurements performed on the factory floor are traceable to the correct value and are good.

If you buy a calibration standard and keep using that for years without any recalibration of the standard, it may be drifted out of its tolerance or damaged, and all your calibrations are erroneous. This kind of calibration will cause only damage.

Uncertainty of calibration

In every calibration it is important to know the total uncertainty of
that calibration.

For example, if you would use a cheap meter with ±5% accuracy to calibrate a measurement with required accuracy of ±0.5%, obviously that measurement would not be any good. If you only document the calibration results (numbers) without stating the uncertainty of that measurement, it is not possible to know the uncertainty and if the calibration was good or not.

A calibration without assessment and statement of the related uncertainty is not really any calibration. In the chain of traceability, the uncertainty of the calibration gets smaller the higher in the chain the calibration is performed.

Why calibrate?

As discussed above, calibration is vital to ensure the quality of the end products. There are also many other reasons to perform calibrations.

To briefly mention a few, these include: all instruments tend to drift over time and lose their accuracy so periodical calibration and adjustment ensures that the accuracy remains at the required level; compliance with standards and quality system; regulatory requirements; to optimize process; safety reasons, both employee and customer/patient; and for economic and environmental reasons.

Risk of not calibrating

Obviously performing calibrations consumes time and money. Performing too many calibrations is a waste of resources, time and money. But the costs of the calibration process must be compared to the risks of not calibrating. If the calibrations are neglected, it may cause quality problems, failure to meet standard/regulatory requirements, production downtime, risk for employee and customer safety, economical losses and other consequences. The consequences of not calibrating adequately can often be far larger than the costs of maintaining a good calibration management process.

How often to calibrate

Sometimes people ask how often the measurement instruments should be calibrated according to quality standards. But the quality standards do not specify the calibration period. You need to specify the calibration intervals by yourself. The main things to consider when specifying the calibration intervals include: the measuring equipment manufacturer’s recommendation, the uncertainty need for the measurements, the stability history of the equipment, earlier experience of similar equipment, importance/criticality of the measurement in question, and the risks and consequences of an out-of-tolerance situation.

Once you start to get experience in the history trend/long term drift of the measuring equipment and compare that to the uncertainty need, you can adjust the calibration period accordingly.

Quality System, standards and regulatory requirements for calibration

All companies have some kind of quality system, it can be built based on the company’s own needs, or based on some quality system standard. One of the most common quality system standards is the ISO 9001:2008. There are also different variations of this standard aimed at certain businesses.

ISO 10012 gives guidelines for measurement process and measuring equipment.

The ISO 17025 is a standard for calibration laboratories, but it can also be used to consider elements in the quality system for calibrations.

ISO 14000 is an environmental standard and is being used more often.

For different businesses there are different standards and requirements. For example for pharmaceuticals and food industry, there are FDA and GAMP regulations that need to be followed.

ISO 9001:2008

The ISO 9001 is a widespread quality system standard. In addition to the generic quality system requirements, let’s have a brief look at what this standard says about calibration:

For example, the section 7.6 is titled “Control of monitoring and measuring equipment” includes the following concerns:

You need to determinate the measurements that need to be taken into account and also the monitoring and measuring equipment that is needed to ensure that the conformity of products is met. Also, you need to establish necessary processes to ensure that necessary measurements can be carried out and are carried out in a manner that is consistent with requirements.

Measuring equipment shall: be calibrated at specified intervals, or prior to use, against measurement standards traceable to national standards; be adjusted as necessary; have identification in order to determinate their calibration status; be safeguarded from any adjustments that could invalidate the results; be protected from damage during handling, maintenance and storage.

If any equipment is found to be out-of-tolerance during a calibration, you should analyze the previous measurements done with the equipment, and take appropriate actions. This may include product recalls.

Records of calibrations need to be maintained. If any computer software is used, the ability of the software must be confirmed before it is used.

There are also other sections in the standard that needs to be taken into account.

Calibration management

In order to assure the quality of your calibration and manage all your calibrations you obviously need to establish a calibration management system as part of your quality system.

The calibration management includes items such as: the calibration process needs to be organized, procedures written how to perform calibrations, a system to plan and schedule required calibrations, system to document calibrations and to manage the documentation.

Most modern calibration management systems are paperless and computerized, including documenting calibrators which communicate with the calibration management software.

This kind of system eliminates the need for error prone manual entry of calibration records, as records are automatically electrically generated. Also, the pass/fail decision is automated and does not require manual calculation which can sometimes be difficult.

The calibration records in a fully electric system are easily stored and can be easily found and analyzed.

 Not only does the electronic paperless calibration management system improve the quality of the results, but it is also far more effective than an old fashioned fully manual system and saves time and money.