FIELDNote 9401

Environmental Data Recorders: Transducers & Calibration

One of the most important specifications for an environmental shock & vibration recorder is the choice of accelerometer transducer. The accelerometer is truly the heart of the recorder. Any field data measured with the recorder is only as good as the transducer used to measure it. Any limitations of the transducer will be limitations of the environmental recording system itself.

Internal Accelerometer Type

IST chose to use what are called piezoresistive (or PR) accelerometers as internal transducers. Piezoresistive accelerometers were chosen for the benefits offered to the customer, particularly in measuring transportation-type environments. PR accelerometers are more difficult to design into a recording system, electronically, and tend to be slightly more costly to implement. However, PR accelerometers offer tremendous advantages to the user:


They provide exceptionally accurate low frequency response ( i.e. they have true DC response.) This is very important, for example, in accurately measuring over-the-road truck vibration levels.

Analogy:

if you can "see" the event occurring with your eyes, your environmental data recorder's accelerometers need DC response in order to measure it accurately:

Examples:

Truck going over a curb
Truck thrusting side to side while crossing railroad tracks
Package falling through the air, impacting ground or another package
Turning a package upside down
Truck braking heavily, coming to stop
Railcar coupling with another railcar
Vehicle crashing

1. With DC response, piezroesistive accelerometers can be used to measure constant acceleration during a package free fall (or drop). Most other types of accelerometers cannot accurately measure this event. Free fall data measured using piezroesistive accelerometers can be used to reproduce drop trajectories with IST's Package Animator. This is technically feasible only with transducers offering true DC response characteristics. As an example, the following dynamic events cannot be measured accurately unless the environmental recording system utilizes piezoresistive accelerometers:
Free-fall Drop Heights
Package Drop Trajectories
Drop Classifications vertical toss, horizontal toss, drop, impact (no free-fall)
Package Orientation
Linear Truck Acceleration or Deceleration (e.g. heavy braking)
Radial Truck Acceleration (e.g. cornering or turning)

2. Calibration Stability:

Once PR accels are calibrated, they tend to maintain calibration very well, over long periods of time. Again , your environmental data is only as good as the calibration of the system used to measure it!

3. Easy Calibration Verification:

The calibration of an environmental recording system using PR accelerometers can easily be checked without sophisticated laboratory equipment. If the recorder is simply flipped (rotated 180 degrees relative to Earth gravity,) it will measure a 2.00g change. This can easily be recorded and checked by the user at any time, anywhere.

4. Temperature Range

for calibrated operation: - 40 degrees Celsius To + 70 degrees Celsius.

External Accelerometer Type

Although PE accelerometers do not have DC response, they do perform well in measuring transient shock data, as well as vibration data in situations where low frequency measurement is non critical.

Voltage-mode PE accelerometers with integrated electronics, utilizing shear mode construction are the best choice for environmental recording, because of their high reliability, noise immunity, stable calibration, and immunity to temperature changes.

Charge-mode type PE accelerometers have been used in the past, before the newer voltage mode4 technology was developed. However, charge-mode accelerometers, although often less costly initially, can lead to serious problems in field measurements:

Charge mode accelerometers' calibration is a function of the cable length used with the accelerometers, which is very inconvenient in field applications where mounting and configuration are constantly changing.
Charge mode accelerometers are extremely sensitive to picking up signal noise from cabling, particularly when used in dynamic environments. Shaking, vibration or even movement of the accelerometer cables alone can often result in false acceleration readings and calibration changes. This is obviously a serious problem in field applications!
In addition, most PE accelerometers are sensitive to temperature changes, especially charge mode type. By merely holding a PE accelerometer, body heat can generate large (false) acceleration signal levels. IST eliminates this problem by utilizing "shear mode" construction in their accelerometers. Shear-mode construction also yields better sensitivity.
Charge mode accelerometers are best suited to laboratory measurements under controlled environments, or for certain very high temperature applications where other piezoelectric devices cannot operate.

Calibration & Accuracy

IST calibrates all data recording systems traceable to the National Institute of Standards & Technology. Calibration documents certifying this traceability are provided with all IST recorders. Many of IST's aerospace and defense customers require this traceability, as well as compliance with MIL-STD 45662A. Data measured with IST recorders has a high degree of integrity, and is accurate to this national standard to within approximately +5 %.