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Applications Note 1-31. The presence of any physical sensor inserted into any mechanical or thermodynamic systemwill alter, to some extent, the system itself. This phenomenon can be visualized by consideringthat the addition of an accelerometer to a structure, for the purpose of determining the dynamicbehavior of the structure, implies the addition of the accelerometerÕs mass, thereby changing thestructureÕs dynamic characteristics. This fact is very similar to the Heisenberg Uncertainty Principleof nuclear physics, in that, the act of observation changes the system being observed. Asanother example, the flush-diaphragm pressure sensor must experience bending to produce outputas we shall see. The bending of such a diaphragm implies that the volume of the measuredcavity must increase with increasing pressure, thereby altering the measurand.2. All sensors that are capable of providing information at zero hertz or ÒDCÓ, when the measurandis static or very slowly changing with respect to time, must shed energy in order to provideinformation regarding the measurand. Sensors that do not dissipate heat or energy by someother mechanism, are not capable of providing information (organized energy) as an outputwhen the input is static and unchanging (invariant) with respect to time. It is important to notethat the piezoelectric sensor is of the nondissipative variety and cannot provide informationconcerning time invariant parameters. It is equally important to realize that energy must be subtractedfrom the measurand by the piezoelectric sensor, for any information in the form of anoutput to be realized, regardless of the time rate of change of the measurand. This situation isvisualized best by considering the piezoelectric accelerometer mounted to a vibrating structure.Since the accelerometer possesses mass and Newtons Law states that force will equal the productof mass and acceleration (F = ma), the accelerometer will therefore require a given forceinput to be displaced. Since work energy is equal to the product of force and displacement (E=Fd), the energy required to cause the cyclic displacement of the sensor is equal to the product ofmass, acceleration and displacement (E =mad), where this energy is supplied to the accelerometerby the structure to which it is mounted.3. The presence of a physical sensor inserted into any mechanical or thermodynamic system willexchange energy in many different forms with the measured system and the measurement systemto which it communicates. This statement means that the mere presence of the sensor massin a small chemical reaction vessel will imply that calories will flow, either from the sensor intothe vessel or from the measurand to the sensor mass, and thus could influence the behavior ofthe measurand. In the case of the static-capable (zero hertz) sensor, a portion of the energyrequired to be shed, in the provision of information regarding the measurand, will be absorbedby the measurand thereby altering its state. Additionally, the energy state existing at the outputof a sensor is changed when the sensor is connected to any other element of the measurementsystem.Just as the product of force and distance equals mechanical energy, the product of charge ßowper unit time (current) and voltage equal energy per unit time or power. Since all elements of themeasurement system possess some Þnite input impedance, it follows that, when a voltage differenceexists between these elements, some current must ßow between them for each element toperform itÕs intended function. At close to zero hertz, impedance becomes simply resistance andThe Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-9