Calibration
Calibration establishes the relationship between known quantity values and an instrument’s indications, including the measurement uncertainty. It is the basis for accuracy, consistency, reliability, and metrological traceability across noise and vibration monitoring systems.
Calibration is the process of establishing a relationship between quantity values and corresponding indications, ensuring measurement accuracy, consistency, and reliability.
What Is a Calibration?
- Per ISO/IEC GUIDE 99:2007, calibration is an operation that establishes a relation between quantity values and corresponding indications.
- A calibration may be expressed as a:
- Statement
- Calibration function
- Calibration diagram
- Calibration curve
- Calibration table
- Per the International Vocabulary of Metrology (VIM4), calibration also requires determining the uncertainties associated with the performed measurements.
- Calibration can be executed on:
- A measuring instrument
- A measuring system
- A measurement standard
What Is the Meaning of Instrument Calibration?
Instrument calibration determines how far off an instrument’s measurement is from the true value of what you are measuring. This is done by comparing instrument readings to a known reference value, and determining how uncertain the result is.
Calibration is not the same as verification or adjustment:
- Verification: Confirms whether the instrument meets specified requirements (e.g., whether a system performs as intended).
Adjustment: Changes the instrument’s readings so the indication corresponds to the true value. Adjustment typically occurs after calibration.
What Is the Adjustment of a Measuring System?
Calibration is often confused with adjustment. Adjustment is a set of operations performed on a measuring system so it provides correct measurements for the quantity being measured. After adjustment, the measuring system usually must be recalibrated.
What Are Calibration Verification and Validation?
- Calibration verification: Confirmation that a system’s performance properties or legal requirements fulfill specified requirements.
- Calibration validation: A verification process that confirms the specified requirements are adequate for the intended use.
- Not every verification is a validation.
Why Do Calibrations Need to Be Done?
Common reasons calibrations are necessary:
- To ensure accuracy and consistency by establishing and demonstrating metrological traceability.
- To ensure instrument readings are consistent with other measurements by calibrating before taking readings.
- To determine instrument reading accuracy by accounting for potential sources of error.
- To confirm instrument reliability so results can be trusted.
Who Calibrates Equipment?
What Is an Accredited Calibration Laboratory?
What Is ISO/IEC 17025?
What Is ILAC?
ILAC is the international organization for accreditation bodies operating under ISO/IEC 17011. ILAC supports accreditation of conformity assessment bodies, including:
What Calibration Services Does the Lab Offer?
An ISO/IEC 17025 laboratory performs calibration services such as testing, calibration, and sampling associated with subsequent testing or calibration.
The laboratory uses equipment required for correct performance of lab activities, including:
- Measuring instruments
- Software
- Measurement standards
What Is a Calibration Hierarchy?
A calibration hierarchy is a sequence of calibrations from a reference to the final measuring system. Each calibration outcome depends on the preceding calibration.
Calibrations are performed using:
- Measurement standards
- Measuring systems operated according to measurement procedures
Example hierarchy:
- National metrology institute
- Accredited laboratory
- User calibration
What Is a Calibration Report?
Calibration results are documented in a calibration certificate or calibration report. Measurement uncertainty must be stated.
Calibration information may be expressed as:
- A statement
- A function
- Additive or multiplicative correction of the indication
- A calibration diagram
- A calibration curve
What Is a Calibration Diagram?
- A calibration diagram is a graphical presentation of the relationship between an indication and the corresponding measurement result.
- Alternative expressions include:
- Calibration curve with associated uncertainty
- Calibration table
- Set of functions
What Is a Calibration Curve?
What Is Metrological Traceability?
What Is a Metrological Traceability Chain?
- A traceability chain is a sequence of measurement standards and calibrations used to relate a measurement result to a reference.
- A metrological traceability chain is defined through a calibration hierarchy and is used to establish traceability of a measurement result.
What Is an Indication?
An indication is a number or value given by a measuring instrument or measuring system.
What Is the Sensitivity of a Measuring System?
Sensitivity is the amount an instrument’s indication changes when the corresponding measured quantity value changes.
What Is the Sensitivity of a Measuring System?
Stability is the property by which an instrument’s metrological properties remain constant over time. A drift occurs when an indication changes over time, usually due to changes in the instrument’s metrological properties.
What Is Instrumental Measurement Uncertainty?
Instrumental measurement uncertainty is determined through calibration of a measuring instrument or measuring system. It arises from using that instrument or system.
What Is an Accuracy Class?
An accuracy class is a set of measuring instruments that meet stated metrological requirements, keeping measurement errors or instrumental uncertainties within specified limits under specified operating conditions.
What Is a Calibrator?
A calibrator is a measurement standard used in calibration. It serves as a reference to measure other quantities of the same kind, helping establish values and measurement uncertainties.
Acoustic Calibration
What Are the Standards for the Acoustic Calibration of Instruments?
The IEC 61672 standards define sound level meter specifications and calibration requirements:
- Part 1: Specifications for sound level meters
- Part 2: Pattern evaluation tests to confirm conformance with Part 1
- Part 3: Periodic tests (limited set) to confirm the meter remains within required performance limits
What Is the Calibration Process for Sound Level Meters?
To demonstrate conformance with IEC 61672-3, a sound level meter undergoes acoustic and electrical tests. Electrical tests are performed by replacing the microphone with an electrical signal input device whose impedance loading (capacitance) matches that of the microphone.
The sound level meter is also tested across specified temperature and humidity ranges. The standard also specifies how the meter responds to external radio-frequency radiation.
All configurations stated in the instruction manual as conforming to IEC 61672-1 must be tested, including:
- Windscreen
- Installed optional facilities
Self-generated noise of each microphone model specified for use with the meter must also be measured.
How Often Should a Sound Level Meter Be Calibrated at the Laboratory?
Per ISO 1996-2, compliance of the sound pressure level measuring instrument, filters, and sound calibrator must be verified using a valid certificate of compliance with measurement parameters specified in:
- IEC 61672-3
- IEC 61260
- IEC 60942
- or equivalent
Compliance testing must be conducted by a lab meeting ISO/IEC 17025 requirements and ensuring metrological traceability. The recommended interval is once per year. Maximum allowable interval is 2 years.
When Should the Sound Level Meter Be Adjusted?
The sound level meter should be adjusted at the calibration check frequency of the sound calibrator to indicate the required sound level for the environmental conditions under which tests are performed.
Per ISO 9612:
- Field calibration must be performed before measurements
- Field calibration without adjustment must be performed at the end of measurements
- If the sound level indication differs by more than 0.5 dB, measurement results must be discarded
Do Calibration Requirements Differ for Class 1 and Class 2?
How Is a Noise Dosimeter Calibrated?
Noise dosimeters should be calibrated periodically at accredited laboratories following IEC 61252. Per IEC 61252, dosimeter performance aligns with Class 2 sound level meters under IEC 61672-1 for:
- A-weighted SPL range: 70 dB to 137 dB
- Nominal frequency range: 20 Hz to 8 kHz
Field calibration follows ISO 9612. A dosimeter must allow the user to check and adjust sensitivity at the calibration check frequency using a sound calibrator to optimize electroacoustical performance over the complete frequency range.
Per IEC standards:
- The sound calibrator must conform to IEC 60942 Class 1 or Class 2
For ISO 9612 measurements, a Class 1 sound calibrator is required
What Is the Use of a Sound Calibrator?
Per IEC 61672-1, a sound calibrator is used to check and adjust sound level meter sensitivity to optimize electroacoustic performance across the frequency range.
Requirements:
- For Class 1 sound level meters, calibrator must meet IEC 60942 Class 1
- For Class 2 meters, calibrator may meet IEC 60942 Class 1 or Class 2
How Often Should a Sound Calibrator Be Calibrated?
Per ISO 9612, sound calibrator calibration must be verified at an ISO/IEC 17025 accredited laboratory at intervals not exceeding 2 years.
Difference Between Pattern Evaluation and Periodic Verification
Pattern evaluation tests in IEC 61672-2 are more thorough than periodic tests in IEC 61672-3 and include directional response measurements.
What Is the Calibration Check Frequency?
Calibration check frequency is the nominal frequency of the sinusoidal sound pressure produced by a sound calibrator. Example: SV 36 sound calibrator produces 1000 Hz (1 kHz).
What Is a Field Calibration?
Per ISO 9612, field calibration is an acoustic check of a sound level meter including the microphone using a sound calibrator meeting IEC 60942:2003 Class 1 performance. Field calibration must be done in a quiet location before the daily series of measurements.
What Is a Remote Calibration?
Remote calibration is an automatic check indicating whether a system is functioning correctly or potentially defective. Results are not used to adjust measurement results. For long-term monitoring of several days or more, ISO 20906:2009/Amd 1:2013 requires checking operation of any sound monitor at least once per day.
Vibration Calibration
What Is a Calibration Curve?
Per ISO 16063-21, vibration sensors are calibrated by comparison to reference transducers over 0.4 Hz to 10 kHz.
These calibration methods apply to:
- Human vibration
- Machine vibration
- Building vibration
ISO 16063 replaced the former ISO 5347 calibration series.
What Is a Vibration Calibrator?
What Is a Vibration Plate?
What Is a Reference Transducer?
A reference transducer is a vibration sensor calibrated under reference conditions by primary means with documented uncertainty.
It can be mounted:
- On top of the sensor under test
- Underneath the sensor under test
In vibration calibrators, the reference transducer may be part of a moving element (calibration plate).
What Is a Reference Vibration Signal?
A reference vibration signal is a sinusoidal vibration signal with specified magnitude and frequency used for testing electromechanical performance of a vibration meter.
Per ISO 16063-21:
- Preferred calibration frequency for accelerometers: 160 Hz
Reference acceleration amplitude: 100 m/s²
What Calibration Types Apply to Vibration Meters?
ISO describes three calibration levels:
- Pattern evaluation at a national metrology institute
- Product type testing / pattern approval for a production series
- Product type testing / pattern approval for a production series
- Periodic verification at an accredited laboratory
- Performed periodically (e.g., every 2 years)
- Performed periodically (e.g., every 2 years)
- In-situ check performed by users
Minimum testing to confirm the instrument likely functions within required specs
How Often Should a Vibration Meter Be Calibrated?
How Often Should a Vibration Meter Be Calibrated?
- Mechanical frequency response tests: Performed on a shaker (vibration calibrator) by comparison to a calibrated reference transducer
- Electrical tests: Use sinusoidal electrical signals applied to the instrument’s electrical input
What Are the Calibration Results?
Calibration results include:
- Calibration frequencies and vibration amplitudes
- Sensitivity magnitude values
- Expanded calibration uncertainty
What Information Is in the Calibration Report?
A calibration report includes:
- Calibration method and instrumentation with calibration due dates
- Ambient conditions during testing
- Mounting technique
- Amplifier settings
- Calibration results
Key Takeaways
- Calibration per ISO/IEC GUIDE 99:2007 establishes a relationship between quantity values and indications and includes uncertainty determination.
- Instrument calibration quantifies accuracy and uncertainty against known references and differs from verification and adjustment.
- Verification confirms specified requirements are met; validation confirms requirements are adequate for intended use.
- Calibrations establish traceability, improve consistency, account for errors, and confirm reliability.
- Calibration is typically performed in controlled lab conditions.
- ISO/IEC 17025 labs ensure competence and traceability; ILAC supports accreditation bodies.
- Calibration services include testing, calibration, and sampling using instruments, software, and standards.
- Calibration diagrams, curves, and tables express indication vs measurement relationships; curves alone do not include uncertainty.
- Traceability chains relate results to references through documented calibrations contributing to uncertainty.
- Indication, sensitivity, stability, accuracy class, and calibrators are core metrology terms.
- Acoustic calibration includes acoustic/electrical tests plus environmental and RF checks. Sound level meters and calibrators require periodic verification; typical maximum interval is 2 years.
- Vibration sensors are calibrated against reference transducers; vibration calibrators generate known amplitude/frequency vibration for in-situ checks.
- Vibration meters should be calibrated at least every 2 years at accredited labs.
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