The chief disadvantage of a potentiometric transducer is
Answer: C
A sliding contact wears, generates noise and eventually becomes unreliable, and the friction it adds loads the mechanism being measured. Its large output and simplicity are real merits, which is why it survives where a limited life is acceptable.
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The gauge factor of a strain gauge is defined as the ratio of
Answer: D
Written out it is \( (\Delta R/R)/(\Delta L/L) \), a dimensionless number of about two for a metallic gauge. It combines the change of dimensions with the change of resistivity under strain, and it is the figure that turns a resistance reading into a strain.
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The gauge factor of a metallic strain gauge is typically about
Answer: A
Around two is the value for the copper nickel and nickel chromium alloys used for bonded foil gauges. A semiconductor gauge reaches a hundred or more through the piezoresistive effect, at the cost of poor linearity and a large temperature coefficient.
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A semiconductor strain gauge has a much larger gauge factor than a metallic one because of
Answer: B
Straining a semiconductor alters the mobility of its carriers, so the resistivity changes by far more than the geometry does. That gives gauge factors above a hundred, but the effect is nonlinear and strongly temperature dependent, so a metallic gauge is preferred for accurate work.
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A bonded strain gauge is one that is
Answer: C
The cement transmits the strain of the surface to the gauge, so the gauge undergoes the same strain as the material beneath it. An unbonded gauge instead has wires stretched between insulating supports and is used in transducers rather than on structures.
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A dummy strain gauge is included in a bridge in order to
Answer: D
The dummy is identical to the active gauge and mounted on the same material at the same temperature but unstrained. Placed in the adjacent bridge arm, its resistance change with temperature matches that of the active gauge and cancels in the bridge output.
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A half bridge arrangement of strain gauges on a beam in bending uses
Answer: A
One gauge on the top surface goes into tension and one on the bottom into compression, and placing them in adjacent arms doubles the output while cancelling any change common to both, temperature included. A full bridge with four gauges doubles it again.
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A thermistor differs from a resistance thermometer in that its resistance
Answer: B
The common thermistor has a large negative temperature coefficient and a markedly nonlinear characteristic. That gives a large signal over a narrow range, whereas a platinum resistance thermometer is nearly linear over hundreds of degrees but changes only slowly.
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The metal almost always used for a precision resistance thermometer is
Answer: C
Platinum is chemically inert, may be obtained very pure and has a resistance-temperature relation so reproducible that it defines the international temperature scale over a wide range. A hundred ohm element at zero degrees is the usual form.
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The self-heating error of a resistance thermometer or thermistor arises because
Answer: D
The element is a resistance carrying the measuring current, so it dissipates a little power and sits slightly above the temperature it is meant to report. The remedy is to keep the current small, which conflicts with wanting a usable signal.
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