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Long Term Stability Considerations

 Rev. C

The importance of long term stability increases as the cost of shutdown for calibration increases. As automated systems and installations become more and more complex and costly, the need to reduce downtime becomes more and more important. If calibration drifts beyond the required accuracy, application system shutdown would be needed for recalibration.

TAT technology is designed to be inherently stable in the long term and in most applications, virtually eliminates the requirement for periodic shutdown for recalibration.

The total accuracy specification used by TAT includes an allowance for at least one year of time instability. TAT warranties that accuracy for two years. 

The TAT warranty is valid in laboratory type environments. The only environmental factor would could significantly effect the stability of TAT loadcells beyond that period is for unsealed configurations use in an environment where there are substances in the air which could eventually form significant deposits on the force sensing crystals. Even for non-laboratory environments, most breathable atmospheres will not cause out of calibration drift.

In contrast, long term stability specifications are usually not available with conventional analog strain gage based systems. The reason is that conventional analog strain gage technology has quite a number of sources of potential long term instability the magnitude of which can not be readily determined for a particular unit or production lot. Even if the manufacturer has tested the basic design and shown that it can be stable long term, the nature of an analog strain gage design allows unit to unit variations still give a substantial possibility that any unit may be unstable in the long term.

There are at least four major potential sources of calibration drift in that older analog technology, none of which are major considerations with TAT technology for the following reasons: 

The first source is power supply drift. The voltage to the loadcell will directly effects the magnitude of the voltage signal. A 0.1% change in the voltage to a strain gage bridge will change the signal by 0.1%. Such a voltage change to a TAT loadcell would produce no detectable change in calibration. 

A second source is changes in the glue joint attaching the strain gage as a function of moisture, load cycling and time. TAT technology has not glue joint directly in the signal path.

A third is electrical component value drift. All electrical components including resistors used in all transducer technologies are not very stable over time. Even metal film and wire wound resistors are significantly unstable over time. With older technologies, such component instability is a major source of calibration drift over time. With TAT technology, the nature of the crystal is such that is divides down the effect of such drift by a large factor. Virtually eliminating calibration drift as a result of component value drift.

A fourth factor in digital systems is the stability of the circuitry which digitizes the signal. In the case of analog technology the A/D converter is not perfectly stable over time even with internal auto-zeroing since auto-zeroing effects bias only and has no effect on the scale factor. In contrast, the FM signal coming from the TAT crystals is digitized by digital counters which has essentially no source of time instability in respect to either bias or scaling.

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