The charge sensitivity test by comparison method is the internationally recognized secondary calibration technique used to determine the charge output sensitivity of piezoelectric accelerometers with fully traceable accuracy. By mounting a device under test back-to-back with a reference standard accelerometer and exciting both with identical vibration, the method delivers reliable calibration results in accordance with ISO 16063-21 and GB/T 20485.21. This article explains the measurement principle, applicable samples, tested parameters, step-by-step procedure, typical applications, and frequently asked questions related to this essential vibration calibration test.
Technical Overview
Charge sensitivity is the defining metrological characteristic of a charge-output piezoelectric accelerometer. It expresses the ratio of the electrical charge generated by the sensing element to the mechanical acceleration applied to the transducer, and it is typically stated in picocoulombs per unit of acceleration, such as pC/(m/s²) or pC/g. Because the charge output of a piezoelectric element depends on material properties, preload, and assembly details, every transducer must be individually calibrated and periodically recalibrated throughout its service life.
The comparison method, also known as back-to-back calibration, is a secondary calibration procedure described in ISO 16063-21 and the corresponding Chinese national standard GB/T 20485.21. In this method, the transducer under test is rigidly mounted back-to-back with a reference standard accelerometer whose sensitivity has been established by primary calibration, such as the reciprocity method or laser interferometry in accordance with ISO 16063-11. Both transducers are then subjected to the same sinusoidal acceleration, and the charge sensitivity of the device under test is obtained directly from the ratio of the two measured outputs and the known sensitivity of the reference transducer.
The comparison method is the workhorse of industrial accelerometer calibration because it combines accuracy, speed, and traceability. Its principal advantages include:
- Typical expanded measurement uncertainty of approximately 1% at the reference frequency and 2% over the full working frequency range
- Wide frequency coverage, typically from a few hertz up to 10 kHz or higher depending on the exciter and reference standard
- High throughput, making it suitable for both factory production calibration and periodic laboratory recalibration
- Direct metrological traceability to national measurement standards through the calibrated reference transducer
- Significantly lower equipment cost and shorter measurement time compared with primary calibration methods
Testing Samples
Charge-Output Piezoelectric Accelerometers
Standard industrial and laboratory accelerometers with charge output are the primary test objects. These include general-purpose shock and vibration sensors, high-sensitivity seismic accelerometers, and miniature high-frequency accelerometers. The test determines their charge sensitivity in pC/(m/s²) or pC/g at a reference frequency and across their rated frequency range.
High-Temperature Charge Accelerometers
Piezoelectric accelerometers designed for elevated-temperature service, such as those used on gas turbines, aircraft engines, and exhaust systems, rely exclusively on charge output because integrated electronics cannot survive the thermal environment. The comparison method establishes their charge sensitivity at ambient conditions, which serves as the baseline for evaluating sensitivity drift at operating temperature.
Back-to-Back Reference Standard Accelerometers
Reference standard accelerometers with a calibrated mounting surface on top are the reference devices used in every comparison measurement. They themselves are periodically verified, either by primary calibration or by comparison against another traceable standard, to maintain the integrity of the calibration chain.
Impedance Heads and Dual-Output Transducers
Impedance heads that combine a force sensing element and an accelerometer in one housing are also calibrated by the comparison method. The acceleration channel is calibrated against the reference accelerometer, while the force channel is verified using a known dynamic force input or by comparison against a calibrated force reference.
Testing Items
The comparison method charge sensitivity test typically includes the following measured and reported quantities:
- Reference charge sensitivity at a specified reference frequency, commonly 160 Hz as recommended in ISO 16063-21, and at a defined reference acceleration such as 100 m/s² or 10 m/s²
- Charge sensitivity as a function of frequency across the rated working range, for example from 20 Hz to 5 kHz or 10 kHz, at a constant controlled acceleration level
- Frequency response deviation, expressed as the percentage difference between the sensitivity at each frequency and the reference sensitivity
- Reported calibration results with measurement uncertainty, reference conditions, and traceability information for the reference transducer
- Optional items performed on request, including amplitude linearity at the reference frequency over the rated acceleration range and phase response evaluation when phase-matched measurement channels are used
Testing Methods
Equipment and Setup
A typical comparison calibration system consists of an air-bearing electrodynamic vibration exciter, a sinusoidal signal generator with closed-loop acceleration control, a back-to-back reference standard accelerometer, low-noise charge amplifiers or precision charge meters, a digitizer or dynamic signal analyzer, and a calibration workstation that automates the measurement and uncertainty calculation. The laboratory environment is maintained at stable temperature and humidity, and all reference instruments hold valid traceable calibration certificates.
Test Procedure
The measurement is performed in the following sequence:
- Inspect and clean the mounting surfaces of both transducers, then mount the device under test directly onto the top surface of the back-to-back reference accelerometer using the specified stud torque, adhesive, or mounting wax in accordance with the manufacturer instructions
- Connect the transducers to their charge amplifiers using low-noise coaxial cables, verify insulation resistance and freedom from cable-generated triboelectric noise, and allow the system to thermally stabilize
- Excite the assembly with a sinusoidal vibration at the reference frequency and reference acceleration, and simultaneously measure the charge outputs of the reference transducer and the device under test
- Calculate the charge sensitivity of the device under test as the reference sensitivity multiplied by the ratio of the two charge outputs, and express the result in pC/(m/s²) or pC/g
- Sweep the frequency across the rated range at constant acceleration to obtain the frequency response curve, and report the sensitivity deviation relative to the reference value
- Verify that the total harmonic distortion of the excitation, transverse motion, and electrical noise remain within the limits specified in ISO 16063-21 so that they do not contribute significantly to the measurement uncertainty
Uncertainty and Quality Assurance
The uncertainty budget accounts for the calibration uncertainty of the reference transducer, mounting effects, excitation distortion, transverse sensitivity, charge amplifier gain error, repeatability, and environmental conditions. Accredited third-party laboratories performing this test operate under quality management systems conforming to ISO/IEC 17025, ensuring that reported sensitivity values are traceable to international system of units standards with stated expanded uncertainty, typically around 1% at the reference frequency for a coverage factor of 2.
Applications
The comparison method charge sensitivity test is applied throughout the life cycle of vibration transducers in nearly every industry that measures dynamic motion:
- Factory calibration and final quality control in accelerometer manufacturing, where every production unit receives an individual sensitivity value
- Periodic recalibration of working accelerometers in test laboratories as required by ISO/IEC 17025 quality systems and customer audit programs
- Machine condition monitoring in power generation, petrochemical, and steel industries, where accurate sensitivity ensures correct interpretation of vibration severity values
- Modal analysis and structural dynamics testing, where normalized frequency response data are essential for building accurate analytical models
- Aerospace, automotive, and rail NVH and durability testing, where sensitivity accuracy directly affects the validity of qualification and certification results
- Secondary standards maintenance in metrology laboratories, where comparison calibration chains preserve traceability from primary national standards to working field sensors
FAQ
What is charge sensitivity and how is it expressed?
Charge sensitivity is the ratio of the electrical charge produced by a piezoelectric transducer to the mechanical acceleration applied to it. It is expressed in picocoulombs per meter per second squared, pC/(m/s²), or in picocoulombs per g, pC/g. For example, an accelerometer with a sensitivity of 10 pC/g produces 10 picocoulombs of charge when subjected to one standard gravitational acceleration.
Why is the comparison method preferred for routine calibration?
Primary calibration methods such as laser interferometry offer the lowest uncertainty but require expensive specialized equipment and long measurement times. The comparison method achieves practical uncertainty of about 1% to 2% while being fast, cost-effective, and fully traceable, which makes it the standard choice for production calibration and periodic recalibration of working transducers.
What accuracy can the comparison method achieve?
With a well-characterized reference standard, an air-bearing exciter, and properly controlled measurement conditions, a typical expanded uncertainty is approximately 1% at the reference frequency and approximately 2% across the extended frequency range, with a coverage factor of 2 corresponding to a confidence level of about 95%.
What is the difference between charge sensitivity and voltage sensitivity?
Charge sensitivity is an intrinsic property of the piezoelectric sensing element and applies to charge-output transducers. Voltage sensitivity is the ratio of output voltage to acceleration and depends on the internal capacitance of the element and any external capacitance in the measurement chain. Charge-mode transducers are specified in pC/(m/s²) or pC/g, while transducers with built-in electronics are specified in mV/(m/s²) or mV/g.
How often should accelerometers be recalibrated?
Most quality systems and manufacturers recommend recalibration every twelve months. Transducers exposed to mechanical shock, high temperature, corrosive environments, or those used in critical applications may require more frequent verification. A significant change in reference sensitivity compared with the previous calibration often indicates internal damage and triggers further investigation.
Which standards govern this test?
The comparison method for vibration transducer calibration is specified in ISO 16063-21 and the corresponding Chinese national standard GB/T 20485.21, with terminology and basic concepts defined in ISO 16063-1. Primary calibration of the reference standard follows ISO 16063-11, and laboratories performing the test operate under the general requirements of ISO/IEC 17025.