Accuracy and Calibration
Under normal conditions, piezoelectric sensors are extremely
stable and their calibrated performance characteristics do not change
over time. However, often sensors are exposed to harsh environmental
conditions, like mechanical shock, temperature changes, humidity etc.
Therefore it is recommended to establish a recalibration cycle.
For applications where high accuracy is required, we recommend
to recalibrate the accelerometer every time after use under severe
conditions or at least every 2 years. In some less critical
applications, for example in machine monitoring, recalibration may be
unnecessary.
For recalibration the transducer can be sent back to Metra. Our calibration service is based on a transfer standard which is regularly checked at Physikalisch-Technische Bundesanstalt (PTB).
Many companies choose to purchase own calibration equipment to
perform recalibration themselves. This may save calibration cost,
particularly if a larger number of transducers is in use. It may also
be desirable to calibrate the vibration sensor including all measuring
instruments as a complete chain by means of a constant vibration
signal. This can be performed using a Vibration Calibrator of Metra’s VC2x
series. The VC20 calibrator supplies a constant vibration of
10 m/s² acceleration, 10 mm/s velocity, and 10 µm
displacement at 159.2 Hz controlled by an internal quartz
generator. The frequency of model VC21 can be selected in seven steps from 15.92 to 1280 Hz. The VC110
Vibration Calibrating System has an adjustable vibration frequency
between 70 and 10,000 Hz at 1 m/s² vibration level. It can be
controlled by a PC software. An LCD display shows the sensitivity of
the sensor to be calibrated. The VC110 is also suitable for measuring frequency sweeps.
If no calibrator is at hand, a measuring chain can be calibrated electrically either by
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Adjusting the amplifier gain to the stated accelerometer sensitivity.
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Typing in the stated sensitivity when using a PC based data acquisition system.
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Replacing the accelerometer by a generator signal and measuring the equivalent magnitude.
Understand the limitations of transducer calibration. Do not
expect the uncertainty of calibration to be better than ±
2 %.
For the evaluation of measuring results it is very important
to assess the measuring errors. The following three groups of errors
occur with piezoelectric accelerometers:
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Sensitivity Errors: Calibration errors, linearity errors, frequency and phase response errors, aging errors, temperature coefficients
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Coupling Errors: Influence of transducer weight, quality of the coupling surfaces, transverse sensitivity
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Noise and Environmental Influences: Noise, base
strain, magnetic fields, temperature transients, sound pressure, cable
motion, electromagnetic interference in cables, triboelectric effect in
cables
Systematical errors can be corrected arithmetically if their
process of formation is known. The effect of these errors has been
diminished and well described by the manufacturer.
Most of the systematical errors can be neglected if the
measuring results are compared with another measurement under similar
environmental conditions. This is of particular importance for unknown
and undescribed systematical errors.
Most errors, however, will occur accidentally in an
unpredictable manner. They cannot be compensated by a simple
mathematical model since their amount and their process of formation
are unknown.
For practical measurements, systematical errors and accidental errors are combined in one quantity called measuring uncertainty.
The following example illustrates the contribution of several error components and their typical amounts:
Accelerometer:
Instrument with RMS calculation:
The addition of the squared error components yields for this example an uncertainty of u = 9 %.
An uncertainty better than 10 % can only be reached if
all relevant error sources are considered and if the used measuring
equipment is of good quality.
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