Efficient Plant April 2022 - 16

feature | data acquisition
Lab calibration report
shows how sensitivity
data from a single point
calibration provided an
incomplete profi le of the
sensor's performance.
Fig. 1: Sensor calibration results
turer's calibration standards and provide assurance
that your valuable assets are in good hands.
When credibility is established, the specifi cation
data can be used to determine whether the sensor
was fully tested aſt er manufacturing and off er clues
to the testing methodology. Physical attributes, such
as size, housing material, hermeticity, and cabling are
good starting points to determine if a sensor is a fi t
for its intended environment. Th ey are included on all
specifi cation sheets. You may fi nd that performance
attributes vary by manufacturer.
A few key specifi cations should always be included
in sensor data, regardless of manufacturer, to guarantee
its operation within an intended application
and performance range. Th ese include measurement
range, frequency range, resonant frequency, output
impedance, output bias voltage, and discharge time
constant. When reviewing the specs, be sure to ask:
Are any of these performance specs omitted?
Was the last calibration single point/limited range?
Is historic trend data available and reviewed?
Reading between the lines for missing data can
provide you with a sense of how a calibration was
performed. Consider the results (Fig. 1) of an accelerometer
that was tested directly from the factory,
with an advertised sensitivity of 50.39 pC/g at 120 Hz.
While the sensor was of very high quality, its performance
was measured within a very specifi c range at
16 | EFFICIENTPLANTMAG.COM
120 Hz, 5 g peak, and 23 C (73 F).
In this example, the sensor failed to stay within
+/-5% range of sensitivity as the frequency range
decreased. Sensitivity data from a single point calibration
provided an incomplete profi le of the sensor's
performance, especially when applied to turbine
testing (this product's intended use), where the sensor
will need to operate in extreme temperatures, be
hermetically sealed against environmental debris, and
live with high electric fi elds.
Consider your own application to determine if any
missing data points might become an issue down the
line. A manufacturer may be able to provide further
performance data upon request. Th e more information
you can get on a sensor's proven performance
over time, the better.
TAKE SENSOR CALIBRATION
INTO YOUR OWN HANDS
Among the must-have attributes, there's one line
on any spec sheet that you'd be better off ignoring:
" Additional calibrations are not necessary. " Don't let
this statement inspire false confi dence in a sensor's
performance over time. A few scenarios illustrate
this point:
shock events (dropping a sensor, exceeding shock
limit), which can result in a broken crystal (sensing
element)
APRIL 2022
http://www.EFFICIENTPLANTMAG.COM
Efficient Plant April 2022

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