Laboratory and metrology are two words that are completely intertwined through their connection to measuring equipment for tests and measurements. Every measuring instrument is required to undergo various supervision processes. This covers equipment in use and those that are currently out of operation. It also included equipment for which orders have been placed.
Order of Equipment
Purchasing an adequate device is the very first step in starting a new measuring or testing workstation.
The following factors need to be considered when preparing a detailed specification for an order:
- Type of measurements to be performed, i.e. what and how the operator is going to measure
- Exploitation conditions, i.e. what will the ambient conditions be for the test room and workstation, and what exploitation conditions are needed for the equipment
- Metrological characteristics, i.e. data obtained from the manufacturer on basic metrological parameters of specific equipment with regards to expected requirements
- Compatibility to legal regulations (if relevant), i.e. whether the equipment has applicable type approval and markings that conform with the requirements of a European Union Directive or applicable national documents
- Manufacturer documentation, i.e. ensure that the documents supplied by the manufacturer are sufficient for laboratory purposes, including user manuals, equipment checking documentation, and calibration certificate
- Other factors (service, references, price, etc.)
Good laboratory practice should be employed by every operator in a laboratory. Different laboratories (for example, pharmaceutical, biotechnological, or analytical) have to adhere to different regulations. Dependent on how the laboratory functions, some of the procedures may be enforced by specific industrial branches.
The ambient conditions in which the equipment is to be operated is a crucial aspect of measuring equipment. Measuring instruments need to be used in accordance with the conditions outlined by the manufacturer. This condition is extremely important, as the manufacturer will only guarantee its full functioning if specific ambient conditions are met. These stipulations are usually with respect to the temperature or humidity of the environment in which it will be operated.
Retraceability of ambient conditions needs to be maintained between requirements relating to measuring instruments and requirements of testing / measuring procedure.
An error often observed at workstations occurs in its designing stage. The ambient conditions referring to measuring equipment and testing procedures at this workstation should be specified. Requirements of instrument exploitation are often different from the testing procedures at a workstation, making it incompatible. For example, there may be a scenario where a working temperature between 18 to 25 ºC has been specified by the manufacturer but the testing procedure specifies a sample storage temperature of 15 ºC. This would mean that the measuring instrument does not correctly match the workstation.
Such measuring instruments can be calibrated in workstation conditions, and appropriate corrections can be considered during calculations. However, it is necessary to thoroughly monitor instrument indications. In this scenario, the laboratory should have contacted the manufacturer of the specific instrument to ask for an opinion or for test results in different temperature ranges.
The most important aspect relating to measuring instruments is supervision over equipment. A laboratory with a quality management system compatible with ISO/IEC 17025 standard, point 5.5. is devoted to describing measuring instruments. In accordance with this norm, a laboratory should be equipped with measuring instruments that allow for tests and calibrations to be properly carried out. Both the equipment and its software should provide the required level of accuracy to meet the requirements for test and calibration.
A specific timetable for the calibration of key values and characteristic features is required for measuring equipment in scenarios where it has the potential to have a big influence on any of the results. The norm also requires that, before installation, all equipment, including sampling instruments, should be calibrated or checked for its proper operation and to ensure that it meets laboratory requirements.
Section 5.5.2 of the norm, outlines another important aspect stating that all measuring equipment should be checked and/or calibrated before use – this includes a reference to section 5.6, which refers to keeping measuring retraceability.
All equipment used for tests and calibrations should be calibrated before delivering it to the operator to guarantee measuring retraceability. The inclusion of auxiliary equipment (e.g. for ambient conditions), is also referred to. This may have a strong influence on the accuracy of measurements, calibration and sampling.
There should be a timetable and procedure in place in a laboratory for calibration of its measuring equipment. The standard suggests that a timetable for calibration of measuring equipment should cover systems referring to choice, use, calibration, checking, supervision and maintenance of reference masses, reference samples used as reference masses and testing/measuring equipment used for test and calibration procedures.
The following documentation should be available for each of the measuring equipment components:
- Identification of equipment and its software
- Manufacturer name, type marking and serial number or another individual marking
- Results from checking which denote whether the equipment is compatible with the specification
- Current location of equipment, if possible
- User manuals supplied by the manufacturer, if accessible, or data on the location of such manuals
- Dates, results and copies of reports and certificates of all calibration processes, adjustments, acceptance criteria and date of next calibration
- Maintenance activities plan, if accessible, and report from already done maintenance activities
- Each defect, malfunction, modifications or repairs of the equipment
A form of documentation called “Life Cards” or “Equipment Book” should be used by a laboratory to store all the above documentation. Often, the person responsible for the technical functioning of laboratory equipment and auditing personnel have very differing views on the procedure for supervision over measuring equipment.
A simple and clear process of metrological acceptance is presented by the ISO 10012:2003 norm:
Figure 1. Process of metrological acceptance of a measuring equipment – according to ISO 10012:2003. Image Credit: Radwag Balances & Scales
A calibration procedure is presented in the above process, but it may also refer to other processes (for example, checking). Its purpose has to be defined at the beginning of the process.
If the purpose is a calibration procedure, then this process is a technical comparison of measuring equipment with reference mass. The calibration process is complete when a document, in most cases a calibration certificate, has been issued. An identification for its calibration status should also be available for the measuring instrument.
Metrological verification is the next step according to procedure. If not, then such verification is not possible, and the instrument is handed to the operator. The instrument should be verified in accordance with metrological requirements if they are specified. An instrument will receive a document confirming its status (for example, an entry to “life card” of an instrument) if it is compatible with all requirements. Following this, a decision will be made on whether to appoint a confirmation identification status.
The measuring instrument will then be given to the operator. If the equipment fails to meet metrological requirements, then a decision needs to be made and action taken as to whether it is possible to adjust or repair the equipment. If this is not possible, then a report should be prepared from tests, indicating that it has failed verification and identifying the instrument status before returning the instrument to the operator. If possible, then the instrument should be adjusted or repaired or handed over to personnel who can carry this out. Once adjusted or repaired, the instrument should go through a complete calibration procedure.
This is a simple process that can be easily introduced into an organization. However, the process should be adapted in accordance with the requirements of each specific laboratory.
A metrological confirmation label should be placed on all instruments to specify which instrument it refers to clearly. According to requirements specified in norm PN-ISO 10012-1:1998, a basis for Polish Accreditation Centre, instruments can be marked with an adhesive label or binding label, or the instrument can be marked permanently.
The date of the last and next acceptance according to the management system should be indicated on each label. These labels should also identify the person responsible for the current status of the instrument. Labels should be attached to an instrument in such a way that it is not possible for accidental or intentional damage or improper use of the label.
As previously mentioned, retraceability is one of the main factors relating to measurements and their results.
A feature of measurement of reference mass, measuring retraceability provides a way to bind it with pre-determined references through means of a continuous chain of comparisons, in which each element has its uncertainty values determined.
Repeatedly measuring retraceability is an unequivocal measuring process that allows for its comparison to a reference unit.
Figure 2. Visualization of measuring retraceability as in case of reference mass. Image Credit: Radwag Balances & Scales
Figure 3. Schema of measuring retraceability. Image Credit: Radwag Balances & Scales
Calibration of measuring instruments in accredited calibration laboratories and checking them according to internal calibration is the best way to measure retraceability.
Auditing of Measuring Equipment
Internal and external audits carried out by accredited or calibration units need to include in their content sections as a reference to measuring equipment. In laboratories, information should be provided on how to audit and where to search for documents from an audit in the case of measuring equipment.
The equipment’s documentation is the first source of information. Documentation may consist of:
- Instrument card: this contains all data referring to product identification as well as data on product metrological acceptance (calibration and checking), any repairs, maintenance activities, or additional adjustment on the storage location.
- The user manual supplied by the manufacturer of supplier of the instrument - this typically includes the original user manual, which should be used to prepare workstation manuals, and shortened versions of the original user manual.
- Calibration certificate of an instrument
- EC declaration of conformity in case of instruments covered by legal metrological control (purchased after 1st May 2004); documents need to be properly stored as they are necessary for controls performed by national bodies and are required in case of second verification of an instrument after conformity evaluation (requirements of legal metrology)
- Verification markings or certificate of second verification in case of instruments with legal metrological control (purchased before 1st May 2004) - such documents are required in case of control within legal metrology.
The measuring instrument itself is the second source of information during an audit. It is possible to identify confirmation features for its metrological status and its legibility directly on the measuring instrument. All the above-specified data should be included on the label.
The auditor should check the system requirements for storing and accessing the user manual.
According to the auditor’s knowledge, it is also possible to assess the general technical condition of the instrument and test the authorized personnel’s knowledge of the instrument’s user manual.
Corresponding records are another source of information on measuring equipment, including:
- Maintenance, service and metrological confirmations
- Weighing results during the checking procedure
- Calibration certificate
- Authorization documents for personnel
- Work safety documentation (if necessary)
The fourth source of information is any data referring to ambient conditions at the workstation and, as such, an auditor may show interest in records of specific parameters, for example, temperature and humidity. A more technical auditor may also look for ways to eliminate disturbance factors, for example, application of an anti-vibration table.
All of the above are places where records may be searched for audit. However, a laboratory can determine its sources of audit information.
In relation to measuring equipment of a laboratory, reference is made in terms of control over measuring equipment, including calibration and periodical checking of the instruments, referring calibration and checking results to national and international reference masses and data (see section 5.6 of norm ISO/IEC 17025), along with continuous monitoring of instruments operation. The above activities are fundamental for all operators of measuring instruments in laboratories that have introduced an accredited management system.
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