Introduction 

In accordance with the requirements for measuring processes and equipment, a periodic inspection of the measuring equipment is imposed. Such periodic metrological control of measuring instruments is the calibration and the issued document is the calibration certificate. However, the issue of a calibration certificate alone does not decide whether the instrument is suitable for measurement or not. The calibration certificate is only a statement of the metrological condition of the measuring instrument under test and does not normally evaluate individual indications. The purpose of calibration itself is to determine the indication errors together with their uncertainties, and the user of the measuring instrument shall assess for himself whether he can perform measurements with a particular meter, determine the measuring range of the instrument. When performing calibration in the Sonel Accredited Testing and Calibration Laboratory, the customer is certain that if his meter exceeds the maximum permissible errors, he will be informed about this fact by the laboratory staff. It should be remembered that the meter - electronic device undergoes degradation related to its use.  The lack of maintaining metrological parameters is related to its frequency and conditions of use as well as its storage. For example, a lux meter, in which the photoelectric cell that is a sensor, ages over time and should therefore be subject to periodic metrological inspection. 

First, let’s clarify what is the calibration certificate and the calibration itself.

Calibration certificate - this is a proof that certifies the metrological properties of the measuring instrument are fulfilled. Otherwise, it is a document that defines the relationship between the standard and the indication of the instrument presenting the measurement uncertainty. Measurement uncertainty is a parameter related to the result of a measurement that characterizes the spread of values that can be attributed to the measured value. It contains many components that can be determined from the statistical distribution of the results of many measurements and can be described by standard deviation. Others are estimated on the basis of assumed probability distributions based on experience or other related corrections or benchmarks. When calibrating measuring instruments, the measurement uncertainty shall be estimated in accordance with document EA-4/02 [1]. It is a guide that unifies the requirements for expressing measurement uncertainty across Europe.
The calibration process consists in comparing the display of the calibrated instrument with the display of the calibration instrument. The calibration instrument should be much more accurate than the calibrated instrument.
We must remember that calibration is a statement of the actual state and in the case of any irregularities, it does not include the adjustment/tuning of a the instrument causing a changes in this state.

How to read a calibration certificate? 

The Polish Centre for Accreditation determines the appearance - a specimen of a calibration certificate for accredited calibration laboratories. Apart from the graphical appearance, it imposes what information should be included in it. It shall contain the 'Calibration' accreditation symbol, under which its accreditation number marked 'AP XXX' shall appear. It is thus a confirmation that the calibration of the measuring instrument was performed in an accredited laboratory (with competences confirmed by the Polish Centre for Accreditation).

Other relevant information proving compliance with the requirements of EN ISO/IEC 17025 is

• calibration method (standardized method or calibration procedure/instruction), 
• measurement consistency (a specific way of referring calibration results to a national measurement standard). 

The most important for the customer are the results of the calibration (carried out under appropriate environmental conditions). Most often presented in the form of a table containing:

• eference value - reference value to which the indication of the meter is compared, 
• indication of the measuring instrument to be calibrated, 
• uncertainty of measurement.

Additionally, at the customer's request, the certificate may include calculated measurement errors (or corrections) of the tested instrument and its acceptable measurement errors, in order to determine compliance with the specification and the evaluation results. A statement of conformity also may be included at a request. There are no requirements to always place these information. It is up to the user to determine whether the tested instrument is suitable for the specified measurements. Of course, the laboratory can perform such assessment, relying on the metrological knowledge of its staff.

Statement of Conformity

The SONEL S. A. Testing and Calibration Laboratory applies as a standard the method of decision rule using a non-binary statement of conformity with a guard band (according to ilac-g8:09/2019, section 4.2.3)<font face="Arial, sans-serif"> [2]. </font>With the statement of conformity there is a risk from the accepted principle.

1.    Definitions (according to ILAC-G8:09/2019):
Tolerance (specification) limit  (TL) 
specified upper or lower bound of permissible values of a property

Acceptance limit (AL)
specified upper or lower bound of permissible measured quantity values

Guard band (w)
interval between a tolerance limit and a corresponding acceptance limit where length 𝑤=|𝑇𝐿−𝐴𝐿|
Specific risk
is the probability that an accepted item is non-conforming, or that a rejected item does conform. 
This risk is based on measurements of a single item

2.    Decision rule.
In the Calibration Certificate, the document on which the requirements for maximum permissible errors (the tolerance limit) were defined is clearly identified.

A guard band with a width equal to the expanded measurement uncertainty (w = U) was adopted.

Points for which the measurement uncertainty is greater than the permissible error (w > TL), as well as points without a defined specification, were excluded from the statement of conformity.

Statements of conformity are presented as:
•    pass  - the measured result is below the acceptance limit,
•    conditional pass  - the measured result is inside the guard band and below the tolerance limit,
•    conditional fail  - the measured result is above the tolerance limit but below the tolerance limit added to the guard band,
•    fail -  the measured result is above the tolerance limit added to the guard band.

The markings for the statement of conformity are placed on the right side of the results table for each measurement point. Their meaning is described in a legend located at the end of the Certificate.

Below is a graphical presentation of a non-binary statement of conformity with a guard band w = U. 

3.    Risk associated with the decision rule.
Every measurement result is subject to uncertainty. The laboratory estimates the level of risk associated with the decision made.
Risk associated with the method:
•    pass  - the specific risk of false acceptance is up to 2,5%,
•    conditional pass - the specific risk of false acceptance is up to 50% if the result is close 
to specification limit,
•    conditional fail - conditionally non-conforming result - the specific risk of false rejection 
is 50% if the result is close to specification limit,
•    fail -  the specific risk of false rejection is up to 2,5%.

In summary, the user can analyze the suitability of the instrument for the tasks to be performed himself, or have the laboratory perform a conformity determination.
In the absence of available instrument documentation, the Laboratory may refrain from performing the compliance determination.
 

How often should the instruments be calibrated? 

The calibration certificate issued by the calibration laboratory does not indicate the date of subsequent calibration. The user of the measuring instrument is responsible for the subsequent calibration. The laboratory does not suggest in any way when the next calibration should take place, as it does not know the conditions of storage and use of the measuring instrument, nor its purpose or scope of application. The methods that can be used to determine the interval(s) between calibrations are defined in ILAC-G24 [3]. Bearing in mind how many factors influence the frequency of instrument calibration, it can be said that it is very important to analyze the results of measurements included in calibration certificates, especially the values of measurement errors and uncertainty. On the basis of these calibration results, the user can always determine whether the instrument meets the specified requirements or not.

Summary 

The decision to use a particular measuring instrument and the interpretation of data from the calibration certificate is always at the discretion of the instrument’s user. He's the one responsible for his measurements. The mere possession of a calibration certificate for a measuring instrument does not confirm its efficiency. A calibration certificate may be issued for a meter with incorrect indications. Only an actual calibration certificate and the correct interpretation of the results contained therein can ensure the correct execution of measurements. In the case of instruments used for tests related to protection against electric shock, the person carrying out the measurements shall have total confidence in the efficiency and correctness of the indications of the instrument used. Measurements taken with a faulty meter can contribute to an erroneous assessment of the effectiveness of shock protection, which can endanger human health and even life.

Literature:
[1] EA-4/02 „Determination of measurement uncertainty in calibration"
[2] ILAC G8:09/2019 Guidelines on Decision Rules and Statements of Conformity
[3] ILAC-G24:2022 „Guidelines for determining the calibration interval of measuring instruments”
[4] M. Borkowski, Automation and Measurements, Analysis of Calibration Certificates. How to read them?