Understanding Calibration in Vibration Measurement
Definition: What is Calibration?
Calibration is the process of comparing a measurement instrument or sensor against a known reference standard of higher accuracy and documenting the relationship between the instrument’s output and the true value. For vibration measurements, calibration verifies that accelerometers, velocity sensors, and analyzers measure accurately, and if necessary, provides correction factors to compensate for deviations from ideal performance. Calibration ensures measurement traceability to national or international standards and is required for quality systems (ISO 9001), legal compliance, and reliable condition monitoring data.
Regular calibration is essential because sensor sensitivity can drift over time from aging, temperature cycling, mechanical shock, or environmental exposure. Without calibration, trending data becomes unreliable, fault severity assessments inaccurate, and maintenance decisions poorly informed.
Why Calibration is Necessary
Measurement Accuracy
- Sensors drift from nominal sensitivity over time
- Typical drift: 1-5% per year depending on usage
- Shock, temperature, aging affect calibration
- Verification ensures measurements remain accurate
Traceability
- Chain of comparisons linking measurement to national standard (NIST, NPL, etc.)
- Calibration certificate documents traceability
- Required for ISO 17025 accreditation
- Legal and contractual requirements
Quality Assurance
- ISO 9001 requires calibrated measurement equipment
- Demonstrates measurement process control
- Provides confidence in data
- Supports decision-making
Consistency
- Compare measurements from different instruments
- Ensure all sensors in program measure consistently
- Enables meaningful trending across multiple instruments
Calibration Methods
Primary Calibration (Laser Interferometry)
The absolute reference method:
- Compares sensor to laser interferometer (measures displacement with nm resolution)
- Sensor mounted on precision shaker
- Acceleration/velocity calculated from laser displacement measurement
- Most accurate method (uncertainty < 0.5%)
- Performed by national labs and specialized calibration facilities
Secondary Calibration (Comparison)
- Compare sensor to calibrated reference sensor
- Both sensors on same shaker
- Reference sensor recently calibrated by primary method
- Most common method for routine calibrations
- Uncertainty typically 1-3%
Back-to-Back Calibration
- Mount test sensor directly on reference sensor
- Both experience same motion
- Compare outputs directly
- Simple and quick
- Good for field verification
Handheld Calibrator
- Portable device providing known vibration (typically 1g at 159.2 Hz)
- Quick field check of sensor/system
- Not full calibration but verifies function
- Useful for pre-measurement checks
Calibration Certificate
Essential Information
- Sensor Identification: Model, serial number
- Calibration Date: When performed
- Sensitivity: Measured value (mV/g, pC/g, etc.)
- Frequency Response: Deviation from ideal over frequency range
- Uncertainty: Measurement uncertainty statement
- Traceability: Reference to standards used
- Calibration Lab: Accreditation information
- Next Due Date: When recalibration required
Calibration Intervals
Recommended Frequencies
- Critical Applications: 6-12 months
- General Industrial: 1-2 years
- Infrequent Use: 2-3 years
- After Shock/Damage: Immediate recalibration
- New Sensor: Verify factory calibration
Factors Affecting Interval
- Criticality of measurements
- Usage frequency and severity
- Historical drift rates
- Environmental conditions
- Regulatory or contractual requirements
Field Verification
Between Calibrations
- Handheld calibrator checks (monthly or before critical work)
- Back-to-back comparison with reference sensor
- Zero check (output with no vibration)
- Consistency checks (compare readings between sensors)
Acceptance Criteria
- Within ±5% of certificate value: acceptable for most industrial work
- Within ±2%: good performance
- Beyond ±10%: recalibration or replacement needed
- Sudden changes: investigate cause (damage, connection issue)
Calibration Records
Documentation Requirements
- Maintain calibration certificates
- Track due dates
- Record any out-of-tolerance findings
- Document corrective actions
- Trending of sensor drift over multiple calibrations
Calibration Database
- Centralized tracking system
- Automatic reminders for due calibrations
- Historical calibration data
- Instrument status tracking
Standards and Requirements
Calibration Standards
- ISO 16063: Methods for calibration of vibration sensors
- ISO 5347: Accelerometer calibration methods
- ISO/IEC 17025: Calibration laboratory competence
Accreditation
- ISO 17025 accredited labs preferred
- NIST traceable in USA
- UKAS, DKD, COFRAC in other countries
- Accreditation ensures calibration quality
Best Practices
Calibration Program
- Establish calibration intervals for all sensors
- Use accredited calibration laboratories
- Maintain calibration records
- Track due dates and ensure timely calibration
- Perform field checks between calibrations
Handling and Care
- Protect sensors from shock and abuse
- Store properly when not in use
- Handle cables carefully
- Document any drops or damage
- Recalibrate after suspected damage
Calibration is fundamental to measurement quality in vibration analysis. Regular calibration against traceable standards, proper documentation, and systematic verification ensure that vibration measurements remain accurate and reliable over time, providing the measurement confidence necessary for effective condition monitoring, diagnostics, and maintenance decision-making in industrial facilities.
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