Feature

Calibration of Electronic Measuring Instruments

Posted on 27 January, 2010 | Tags: Precise Assessment

British scientist Lord Kelvin (1824 -1907) had said, "I often say that when you can measure what you are speaking about and express it in numbers, you know something about it, but when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind: it may be the beginning of knowledge but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be."

The adequacy of the quality functions is intimately associated with the quality of measurements. Therefore, the measurements made should not only be adequate but must be valid, accurate, dependable and reliable. In order to fulfill these essential requirements associated with Quality Assurance/Quality Management Systems, a well-structured Measurement System with International compatibility becomes the foremost necessity.
Process converts Input to Output. Relation between Input and Output can be controlled. To control the process, we need to know the status of the process. Measurement gives information regarding this status. Control of a process can never be better than the measurements made on the process. The more accurate the data obtained from the process, with more accuracy can the process be controlled.
By definition, Calibration is a set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument and the corresponding values realized by reference standards.
In practice, Calibration is essentially a comparison with a higher standard. A higher standard means higher accuracy and better resolution. For establishing higher accuracy, the minimum accuracy ratio of 1:3 (Reference standard: Unit Under calibration) is followed. For establishing better resolution, the normal thumb rule of 1:10 is followed.
The grey areas in calibration are lack of national / international standards for calibration method / calibration report, too many steps in the traceability chain and 'measurement uncertainty' and its evaluation.
Calibration has to be viewed as an essential element of reliable measurement. Inspection, measuring and test equipment should be recalibrated periodically. The frequency of calibration is one of the most argued points in calibration control program and considered to be grey area however, we need to assign periodicity of each equipment.
The periodicity should be decided based on, 

• Type of equipment, 
•  Manufacturer's recommendation, 
•  Frequency of use, 
•  Statistical data of the equipment's behavior, 
•  The criticality of the process in relation to quality / productivity / efficiency, 
•  Environmental condition of use,
•  Tendency to wear and drift,
•  Maintenance and Service,
•  Frequency of cross-check,
•  Penalty of an incorrect data getting accepted because of the measuring equipment has become faulty.

Basic Requirements of Calibration     

•  Reference  / Calibration Standards
• Other peripheral equipments.
• Controlled environmental conditions
•  Competence of Calibration Lab personnel
•  Traceability of Reference / Calibration standards
•  Documentation

Errors Related to Instrumentation

•  Misreading the instruments
• Making incorrect adjustment
• Applying instrument improperly 
• Computational error
• Recording improperly
• Loading effect
• Impedance mismatch
• Thermals
• Short and Long Term Drift
• Leakage Current 
• External noise
• Range and Resolution limits
• Observational (Parallax)

Benefits of Calibration
Calibration facilitates to create an infrastructure of distributed metrological competence for tracking back measurement results to national standards. It ensures quality control and assurance in production. Calibration ensures that accreditation criteria are met in the ling term and promote international recognition. It fulfils the requirement of traceability to national or international standards such as ISO9000, ISO14000 etc and to comply with the requirements of global trade. Contractual obligations between vendor and customer can be fulfilled satisfactorily. Calibration proves that the instrument is in proper working condition and this reduces the rate of rejection and failure thus yields higher return. Improved product quality and service results in satisfied customers. Proper calibration can help monitor various parameters accurately such as power etc and significant power, hence cost savings can be achieved. It is critical to calibrate the instruments such as pressure gauges to measure boiler pressure, LPG cylinder or weighing scale to measure chemical contain of medicine.

Care to be Taken During Purchase of Instruments
While purchasing an instrument, it is important to see that the calibration certificate and if required, standard calibrating device must be supplied for the instrument. It is important to have maintenance and calibration instructions along with the operating instructions, which are handy when, need be while the instrument is in use. It is also good to have information about the sources to maintain and calibrate the instruments as and when required or the supplier takes the responsibility under the service contract. Appropriate measures should be taken for routine maintenance and calibration of instruments and operation training of personnel.

Vinodkumar D Patel is Manager, Cal. Lab, CSD, QMS with Masibus Automation and Instrumentation Pvt Ltd. He may be contacted at sales@masibus.com