उपकरण अंशांकन समझाया
The increasing need for known accuracy and uncertainty and the need to have consistent and comparable standards internationally have led to the establishment of national laboratories to maintain primary standards of measurement used in providing traceability to customer's instruments by calibration. Calibration helps ensure accurate measurements in processes, which is key to maintaining quality, safety and innovation of products and services used in day-to-day activities.
Calibration simply refers to comparing the measurement values delivered by measuring equipment under test with those of a standard of known accuracy. The standard is usually another measurement device of known accuracy, a device generating the quantity to be measured such as a voltage, a sound tone, or devices, such as a meter ruler, vernier callipers, micrometer screw guage, weighing balances etc.
The International Bureau of Weights and Measures (BIPM) defines calibration as the follows as an operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties (of the calibrated instrument or secondary standard) and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.
It is the act of comparison which doesn’t include any subsequent adjustment. The reference standard used in calibration is usually traceable to a national or international standard held by a metrology body.
Outcome of a Calibration Exercise
Calibration helps check and verify the accuracy of the measuring device and creates traceability of the measurements to ensure that they are consistent with other measurements. The following are some of the result of a calibration exercise:
- No Observed Errors: This is when there is no significant error being noticed on the equipment under test.
- Significant Errors: This results when there is a significant error being noted but without adjustment made.
- Effecting an Adjustment: Here, an adjustment is made to correct the error to an acceptable level.
When to Conduct a Calibration Exercise
The accuracy of all measuring equipment reduces with time sometimes caused by regular use of equipment. In addition, changes in accuracy can also be caused by exposure to hazardous manufacturing environments such as a chemical laboratory. Depending on the type of measurement equipment or the environment in which it is being used, their accuracy may degrade very quickly or over a long period of time. Calibration enhances the accuracy of measuring equipment which results to improved product quality. Calibration exercise can and may be required due to the following additional reasons:
- When a new instrument or measuring equipment is purchased.
- After an measuring equipment has been repaired or modified.
- After moving from a particular location to another location.
- When a specified time period (operating hours) of the equipment has elapsed.
- Before and/or after an important measurement is conducted.
- After an unforeseen circumstance, such as sudden change in weather condition, shock, vibration or physical damage, this might potentially impact the integrity of its calibration.
- Whenever observations appear questionable or the equipment does not match the output of similar equipment.
- As specified by a customer or recommendation by the instrument manufacturer.
Costs of Not Calibrating
The costs of not calibrating measuring equipment can be damaging which may include the following:
- Manufacturing Costs: It may lead to an increase in production costs due to manufacturing errors.
- It may lead to wrongly accepting non-compliant product.
- It may lead to wrongly rejecting compliant product.
- It may lead to increased plant shutdowns.
- It may result in premature breakage of manufacturing equipment.
- It may result in faulty research and development.
All of the above effects result in damage to the reputation of the concerned business. The possible cost to business reputation, when compared to the cost of annual calibration, means it’s often not worth the risk of ignoring calibration.
Types of Calibration
Equipment calibration can be conducted on different types of instruments across different sectors. The type of calibration services conducted depends on the type of equipment requiring calibration, including the following:
- Pressure Calibration
In this type of calibration service, gas and hydraulic pressure are typically measured using pressure balances and calibrators along with a number of pressure gages. Examples of pressure instruments that are frequently calibrated include the following:
- Analogue Pressure Gauges
- Barometers
- Digital Indicators
- Digital Pressure Gauges
- Test Gauges
- Transmitters
- Temperature Calibration
Temperature calibration needs to be conducted in processes where temperature readings play very critical role and are to be carried out in a controlled environment. Temperature measuring equipment requiring calibration on a periodic basis includes the following:
- Chambers/Furnaces
- Data Acquisition Systems
- Dial Thermometers
- Infrared Meters
- PRTs and Thermistors
- Thermal Cameras
- Thermometers/Thermocouples
- Weather Stations
- Flow Calibration
This is conducted on flow meters that check product or feedstock quality and quantity, fuel/energy quantity or function in a critical process. The types of flow meters that frequently require calibration include the following:
- Laminar Flowmeters
- Rotometers – Gas and Air
- Thermal Mass Flowmeters
- Turbine Meters
- Electrical calibration
Electrical calibration is necessary to check the veracity of electrical instruments across a diverse range of industries. In this type of calibration, the current frequency, resistance and voltage are often checked. Instruments that are frequently sent for electrical calibration include the following:
- Clamp Meters
- Counter timers
- Data Loggers
- Electrical meters
- Insulation Testers
- Loop Testers
- Multi-meters
- Oscilloscopes
- RCD
- Mechanical calibration
Mechanical calibration services are conducted on mechanical equipment taking into consideration a number of elements such as mass, force, dimension, angle, volume, flatness, torque and vibration in a temperature controlled environments. Some of the most frequently tested instruments for mechanical calibration include the following:
- Accelerometers
- Load Cells & Force Gauges
- Micrometers, Verniers, Height Gauges
- Scales/Balances
- Torque Wrenches & Screwdrivers
- Weight & Mass Sets
Applicable Standards in Equipment Calibration
It is important to ensure that calibration process be carried out under recognized accredited standards as it is considered the most credible way. Some examples of these include the following:
- ISO 17025
This standard states that "A calibration certificate (or calibration label) shall not contain any recommendation on the calibration interval except where this has been agreed with the customer. This requirement may be superseded by legal regulations.”
- ANSI/NCSL Z540
This standard states that measuring equipment "...shall be calibrated or verified at periodic intervals established and maintained to assure acceptable reliability..."
- ISO-9001
This standard states that "where necessary to ensure valid results, measuring equipment shall...be calibrated or verified at specified intervals, or prior to use...”
- MIL-STD-45662A
This standard states that measuring equipment "... shall be calibrated at periodic intervals established and maintained to assure acceptable accuracy and reliability...Intervals shall be shortened or may be lengthened, by the contractor, when the results of previous calibrations indicate that such action is appropriate to maintain acceptable reliability."
Basic Calibration Process
The exact process of equipment calibration varies according to the type of instrument, how critical its role is in the operation and standards that are followed for the calibration purpose. However, the following are some generic steps for equipment calibration.
- Determine the purpose and scope calibration
The first stage of the calibration process is the design of the measuring instrument that needs to be calibrated. The equipment design needs to be capable of measurements that are "within tolerance" when used within the stated environmental conditions over some specified period of time. Fundamentally the purpose of calibration is to maintain the quality of measurement as well as ensure the proper working of particular instrument.
- Identify Inspection and Measurement Processes
After identifying the scope of the calibration, then decide whether the calibration will need to be done as part of an existing process or if there is a separate procedure in place for it. Each measurement instrument should be traceable by means of its own calibration record, which should contain:
- Identification number
- Manufacturer and model
- Frequency of calibration
- Reference (traceability) standards used
- Validation certificates and calibration findings
- Details of actions taken in case of unsatisfactory results.
- Determine the Frequency of Calibration
Determine the frequency of calibration depending on the likely level of usage the measuring equipment. Assigning the calibration intervals can be a formal process based on the results of previous calibrations. The equipment manufacturer usually assigns the measurement tolerance, suggesting the calibration interval (CI) and specifies the environmental range of use and storage.
- Assign Responsibilities
The selection and training of competent calibration personnel is an important and the personnel involved with calibration should possess the right education and experience in the area of job assignment, knowledge of metrology and calibration and others.
- Identify Applicable Standards
Determine which standards and procedures applicable to your process to ensure correct calibration. For example, ISO 9001 states that the Quality Manager should administer the calibration procedure.
About the Author
Olanrewaju, Adebayo Bamidele is a Lead Auditor of ISO 9001, FSSC 22000 / ISO 22000, 14001 & 45001 Management Systems, Certified Six Sigma Master Black Belt (CSSMBB), process engineer, and quality management professional with strong working experience and proven skills in manufacturing excellence, ISO management systems implementation, lean / digital manufacturing, and project management.
Adebayo is a Corporate Member of American Institute of Chemical Engineers (AIChE), Associate Member, the Institution of Chemical Engineers (IChemE), Corporate Member, Nigerian Society of Engineers (NSE), Corporate Member, Nigerian Society of Chemical Engineers (NSChE), Associate Member, Nigerian Institute of Management (NIM), and Associate Member, Institute of Strategic Management of Nigeria (ISMN).
He is an author of over 15 books and has published over 45 online courses on various e-learning platforms including Udemy, Alison, Learnformula & Study Plex.
You can reach him, here.
