Advances in monitoring, measuring and analysing equipment, have resulted in industry making increasing use of preventative maintenance, which should prevent unplanned production stoppages and breakdowns, resulting in production cost savings for industry. While true in theory, it is not so in South African industry. This article explains why this is the case.
Every instrument/transducer will have a functional lifetime and a lot of certified operational periods where the unit will comply with the manufacturer's specifications (regular calibration will be able to verify it). In the vibration monitoring industry most end users and distributors believe that vibration-measuring equipment/transducers will comply with their manufacturer's specifications for ten years to life. They will only repair or replace transducers when they are totally useless/broken - often they will be sent for calibration with the hope that calibration will solve the problem with the faulty units. The faulty units will be referred by the accredited calibration laboratory to the distributor for repairs.
More than fifty thousand vibration transducers are in daily use by industry to perform measurements/monitoring on machines, motors, pumps, etc. Less than 5% are annually/regularly calibrated by an accredited laboratory to ensure traceability to national and international standards. Most of industry is required, nationally and internationally, to conform to the ISO 9000 Quality Assurance System, therefore industry should be obligated to perform traceable and meaningful measurements. The question will now be: Why are so few vibration-measuring equipment/transducers calibrated? This question is answered within the rest of the article.
Most of industry in the vibration-monitoring field will also raise the question: Why should it be necessary to perform accurate, traceable and repeatable measurements? Before this question is answered, let us define accuracy, repeatability and traceability:
Accuracy
The closeness of the agreement between the result of a measurement and the (conventional) true value of the measurand.
Repeatability
The closeness of the agreement between the results of successive measurements of the same measurand carried out subject to the following conditions:
* The same method of measurement.
* The same observer.
* The same measuring instrument.
* The same location.
* The same conditions of use.
* Repetition over a short period of time.
Traceability
The property of a result of a measurement or the value of a standard where by it can be related to stated references, generally international via national standards, through an unbroken chain of comparisons all having stated uncertainties.
The above terminology is quoted from VIM 'International Vocabulary of Basic and General Terms in Metrology' issued by BIPM, IEC, IFCC, ISO, IUPAC, IUPAP and OIML.
We should now look at the difference between traceable calibrations performed by an in-house facility of a company; an ISO 9000(only) accredited company, distributors and accredited laboratories. The best that a normal in-house facility of a company, an ISO 9000(only) accredited company and distributors can provide you when performing a traceable calibration will be the following:
The equipment, which they are using to perform the calibration with, will be traceable to national and international standards, may be via a SANAS (South African National Accreditation System) accredited laboratory. The only exception will be, when their so-called calibration facility is SANAS accredited for the specified fields and/or parameters to perform vibration calibrations.
On the other hand, a SANAS accredited laboratory, accredited for the necessary parameters to perform calibrations on vibration transducers and vibration measuring equipment, etc. will provide you with the following service:
* A SANAS certificate, which is acknowledged by the laws of this country, the ISO 9000 assessors and the overseas countries with their industries.
* A calibration, which is performed by a competent metrologist, normally experienced or overseen by an experienced metrologist.
* A laboratory using well-documented and validated calibration methods and procedures, which consists of validated internal laboratory-developed methods/procedures and internationally-developed methods/procedures.
* Documented results, which are kept on hard copy or disk for reference purposes.
* A calibration, which is performed in a controlled environment (temperature, humidity, etc), to ensure that the same measurements can be repeated year after year, which will indicate any slow long-term drifts on client's instrumentation and transducers.
* Traceability to international standards via national standards.
* Reported uncertainties of the measurements, which are calculated and expressed in accordance with the SANAS EAL-R2 document 'International Standard'.
In consultation between the accredited laboratory and the client, re-calibration periods of the calibrated instrument can be determined from the following information that is available to the laboratory:
* The amount of personnel using the instrument/transducer.
* Usage per week/month of the instrument/transducer.
* The environment in which the instrument/transducer is being used.
* The type of measurements performed.
* The end user abuse of the instrument/transducer. (Physical condition)
* The long-term drift rate of the calibrated instrument/transducer, etc.
The accredited laboratory shall comply with the ISO/IEC 17025 specification: The general requirements for the competence of testing and calibration laboratories. This means that the accredited laboratory is audited once a year by SANAS and should comply to the ISO/IEC 17025 specification, which is a quality system for accredited testing and calibration laboratories with the emphasis on calibrations performed to fulfil the client needs.
The calibration of the measuring equipment is now covered, but not the personnel who perform the measurements in industry. More than 50% of the people in industry performing vibration measurements are not adequately trained or trained at all. Training should be done in the following areas:
The measuring methods/procedures to obtain accurate and repeatable vibration data
The correct use of the measuring equipment, which includes the understanding of the technical specifications of the measuring equipment, ie the frequency and level responses of the transducer and measuring equipment should be known to the person performing the vibration measurements, etc.
The correct procedure for recording and storing of the measured data
The person performing the vibration measurements should know the theory and practical working of the unit to be measured (motor, pump, etc). For example, the type of electrical motor and the motor strength (kW), and the normal produced total vibration over a specified frequency band according to the relevant ISO specification.
The processing of obtained vibration data for meaningful conclusions
Example: A person should not change alarm settings without very good reasons. They should also be able to give a good explanation of why their changes were necessary.
The bottom line
In industry, every person taking vibration measurements, obtaining vibration data, analysing obtained vibration data, etc must be adequately trained to perform the required function correctly.
Now the question "Why should it be necessary to perform accurate, traceable and repeatable measurements?" can be answered:
* To cut production costs.
* To reduce the manufacturing of scrap products.
* To prevent reworking of products.
* To prevent breakdowns and unplanned production stoppages.
* To save time (money) and workforce (hours) performing fault finding on systems.
* To use the minimum workforce to produce the maximum output.
Only the most important aspects were addressed, a long and difficult road is still ahead in training the industry users and distributors of vibration measuring equipment/transducers in quality assurance and to perform meaningful measurements using calibrated equipment and not just collecting piles of vibration data without the proper application for it.
Many years ago advice was given to me by a visiting overseas delegate and I would like to share it with the vibration industry: "If it moves - train it, if it does not - calibrate it!"
De Beer Calibration Services
(012) 998 2172
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