Vibration meter calibration verifies accelerometer sensitivity and frequency response by comparison against a reference sensor on a calibrated vibration exciter per ISO 16063-21. A typical acceptance criterion is ±3% sensitivity deviation at the reference frequency (159.2 Hz). This procedure covers mounting, sensitivity check, frequency sweep, amplitude linearity, and as-found/as-left documentation for predictive maintenance programs.
Inspect the accelerometer sensor for damage, cable integrity, and connector condition. Verify the meter display functions correctly, battery is adequate, and all measurement modes (acceleration, velocity, displacement) are selectable.
Mount the accelerometer on the reference vibration exciter alongside the reference accelerometer using appropriate mounting method (stud, wax, or magnetic). Ensure both sensors are securely attached with consistent coupling.
Apply a known vibration level at the reference frequency (typically 159.2 Hz for 1000 rad/s) and compare the meter reading to the reference accelerometer output. This verifies the basic sensitivity (mV/g or mV/ms²).
Test at a minimum of five frequencies spanning the meter's specified bandwidth (e.g., 10 Hz, 100 Hz, 1 kHz, 5 kHz, 10 kHz). Record the meter reading and reference value at each frequency to verify flat frequency response.
At the reference frequency, verify linearity by testing at three vibration levels (low, mid, high) within the measurement range. Calculate the sensitivity error at each level.
Record all data including sensitivity values, frequency response deviations, and amplitude linearity. Issue the calibration certificate with measurement uncertainty and apply the calibration label.
Sensitivity error at the reference frequency must not exceed ±5% of the stated value. Frequency response must be within ±1 dB across the specified bandwidth. Amplitude linearity must be within ±3% across the measurement range.
12 months
1) Improper accelerometer mounting using inadequate torque or incorrect stud material, causing resonance shifts and measurement errors exceeding ±10%. Always use calibrated torque wrench and steel studs per ISO 5348. 2) Failing to verify reference exciter's amplitude stability before calibration, leading to systematic errors in sensitivity measurements. Monitor exciter output continuously during calibration sequence. 3) Using incorrect frequency sweep rates during frequency response testing, causing dynamic effects that mask true instrument response. Follow manufacturer's recommended sweep rates, typically 1 octave per minute maximum. 4) Neglecting to account for cable capacitance effects on accelerometer sensitivity, particularly critical for PE accelerometers where cable length affects charge amplifier gain. Document cable specifications and include in uncertainty budget. 5) Performing calibrations at single temperature without considering thermal sensitivity coefficients, missing temperature-dependent errors that can exceed ±5% over operating range. Verify performance at minimum three temperature points across specified range.
| Issue | Cause | Remedy |
|---|---|---|
| Sensitivity readings drift during calibration sequence | Thermal effects on reference exciter or test accelerometer | Allow 30-minute thermal stabilization period and monitor ambient temperature within ±2°C |
| High noise floor obscuring low-level vibration measurements | Ground loops or electromagnetic interference affecting charge amplifier | Implement proper grounding scheme and use shielded cables with isolated BNC connectors |
| Frequency response shows unexpected resonances | Improper accelerometer mounting creating spurious mechanical resonances | Verify mounting torque to 2-5 N⋅m and ensure flat, clean mounting surface per ISO 5348 |
| Amplitude linearity fails at high vibration levels | Reference exciter reaching displacement limits or accelerometer overload | Reduce test amplitude and verify exciter specifications; check accelerometer overload indicators |
| Inconsistent results between measurement axes | Transverse sensitivity effects or misalignment with exciter axis | Use precision mounting fixture and verify alignment within ±1° using laser alignment tools |
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The frequency 159.2 Hz corresponds to 1000 rad/s angular frequency, which simplifies the mathematical relationship between acceleration, velocity, and displacement. At this frequency, 1 g of acceleration equals approximately 9.81 m/s² and the velocity and displacement values are convenient round numbers.
The mounting method determines the usable frequency range. Stud mounting provides the widest bandwidth (up to the sensor's resonance). Adhesive and magnetic mounts reduce the effective bandwidth due to compliance in the mounting interface. Calibration should use the same mounting method as field measurements.
Portable vibration calibrators (hand-held exciters providing a known vibration level at a reference frequency) enable field verification of vibration meters. This checks basic sensitivity but not full frequency response. A full laboratory calibration is still required at the specified interval.
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