Why bother with calibration?
Since 2004, the LAB-EL Humidity, Temperature and Pressure Laboratory has held the status of an accredited laboratory. Accreditation is granted by the Polish Centre for Accreditation, after a positive organisational and competency evaluation. Re-evaluations are carried out in four-year cycles, and “in-supervision” evaluations — once a year. The LAB-EL Laboratory holds the. The Calibration Department carries out calibrations of measurement instruments in accredited fields of expertise: temperature, humidity, pressure and vacuum. The Testing Department carries out environmental measurements in rooms. In this paper we will try to answer same of the questions about calibration.
What is calibration?
The calibration is a set of operations to establish the relation between the values of a measured quantity, indicated by a calibrated measuring instrument, and suitable values of physical quantities of standard measurement units, together with uncertainties of measurement. Basically, this means determining the difference between a standard responds and response of the calibrated instrument, taking into account the measurement uncertainty. The proof of the metrological properties of the calibrated instrument is the document issued by the Laboratory, know as the calibration certificate, designated with accreditation symbols. The proof of metrological properties of the calibrated instrument is the document issued by the Laboratory, know as the calibration certificate, designated with accreditation symbols. During calibration, a measurement consistency — or traceability — must be maintained that constitutes an uninterrupted series of references of the calibrated instrument to the national or international standard.
Why bother with calibration?
The question that is also the title of this article is quite commonly asked by the clients of LAB-EL who purchase control and measurement equipment. Calibration is a labour-intensive operation and, with a large number of instruments used, it is quite expensive, so — why bother with calibration? Is it an obligation? Calibration is a requirement of quality management systems, e.g. GMP, to provide authorised confirmation of the reliability of measurements taken with measuring instruments. Calibration is also one component in the verification of instruments. It is not only worth knowing the accuracy of the measurement, but also its discrepancy with the actual value.
Re-calibration — how often should it be carried out?
The calibration of a given measuring instrument should be repeated as long as the instrument is used to fulfil its intended purposes. As yet there are no legal regulations defining calibration intervals, nor the period of validity of calibration certificates. Re-calibration intervals should be set by the user of the measuring instrument and should be documented in the client’s management system or on the basis of the client’s experience. The frequency of re-calibrations depends on the following factors:
- importance of the accuracy of the measuring equipment (e.g. the measurements in a warehouse containing expensive medicine will be of greater importance than in an office space);
- the operating conditions of the instrument (i.e. it should be the more frequent, the more it is exposed to extreme temperatures and humidity levels);
- mechanical and chemical contaminations, especially in the presence of corrosive factors (e.g. fumes of caustic substances or molecules of mud with salt that are ubiquitous in winter in the vicinity of roads with heavy traffic).
The higher the exposure, the faster the ageing process may be (and following this, measurement error may occur faster). And thus:
- if the operating conditions are less extreme (e.g. in a residential or office space), thermo hygrometers should maintain their accuracy over 24 months (and this is the interval in which they should be subject to maintenance, testing and regulation in a laboratory);
- if the operating conditions are close to the measurement limits (e.g. high humidity), and the instrument is used in such conditions only occasionally, with the instrument being used most of the time in less extreme conditions, thermo hygrometers should maintain their accuracy over 18 months (and this is the interval in which they should be subject to maintenance, testing and regulation in a laboratory); despite such provisions — for the sake of simplicity — it is, at times, assumed that a given instrument should be re-calibrated at least once a year. Portable instruments, exposed to shocks, with mechanical or electric regulators and operated extensively should be calibrated more often, but, on the other hand, those that do not feature regulations, are stored in laboratory conditions and are rarely used, are usually calibrated less often. When the re-calibration interval is to be extended (especially pertaining to calibrations with laboratory standards), it is recommended to maintain the so-called standard monitoring, that is an elaboration (e.g. in the form of a chart) of vital parameters coming from subsequent calibration certificates, proving that the parameters do not change considerably over a period of time that is to constitute the interval extension between subsequent calibrations. The user of the measuring instrument (e.g. of a thermo hygrometer, thermometer) should note the necessity for periodic testing of the measuring instrument responses;
- if the operating conditions are close to the measurement limits (e.g. high humidity), and the instrument is used in such conditions continuously, the first testing and maintenance should be carried out after approx. 6 months, and then after every 12 months;
- if an instrument is used in extreme conditions in the presence of high levels of contamination, testing and maintenance should be carried out every 6 months;
- if control measurements taken with another calibrated instrument feature an unacceptable response error, testing and maintenance should be carried out immediately.
Is the maintenance, re-calibration (adjusting) and response testing carried out as a warranty service?
The maintenance, re-calibration (adjusting) and response testing (calibration) of instruments are standard operational tasks, and as such are not subject to the manufacturer’s warranty, and are subject to a fee when ordered by the client.
According to the Order to the Minister of Economy and Labour of 30 March 2005, on types of measuring instruments subject to legal metrological control and the scope of the control — the text is available in the website of the Central Office of Measures, thermometers, hygrometers and barometers for air control (i.e. environmental conditions monitoring) are (among others) not subject to verification, for as such they are not indicated in the Order. It is not possible for the client to obtain a verified thermometer, hygrometer or barometer, as the Central Office of Measures, nor any other authorised body, does not issue verification certificate for such instruments. Currently, the verification is mainly mandatory for instruments, whose responses are used in financial settlements between contractors (e.g. scales, electricity and heat meters, etc.).
Thermometers, hygrometers and barometers are, however, subject to calibration procedures. On the ordering party’s request, the LAB-EL Humidity, Temperature and Pressure Laboratory performs calibration, possibly preceded by adjustment (or ‘regulation’).