A transducer whose output is a train of pulses whose frequency is proportional to flow is
Answer: C
A turbine meter with a magnetic pickup gives one pulse per blade, so counting pulses gives volume and their rate gives flow. Because the information is in a count rather than a magnitude, it can be fed straight into a digital system.
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The span of a transducer is
Answer: D
A pressure transducer rated from zero to ten bar has a span of ten bar, and a temperature sensor rated from minus fifty to two hundred degrees has a span of two hundred and fifty degrees. Accuracy and linearity are commonly quoted as percentages of it.
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The resolution of a wire-wound potentiometric transducer is limited by
Answer: A
The wiper moves from one turn to the next, so the output changes in steps of one turn's worth of resistance rather than continuously. A conductive plastic track has no such limit, which is why it is preferred where fine resolution matters.
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A load cell for weighing consists essentially of
Answer: B
The steel column or beam deflects elastically in proportion to the load, and gauges bonded to it report that deflection as a strain. A full bridge of four gauges gives both a large output and good temperature compensation.
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The reason a full bridge of four active gauges is preferred in a load cell is that it gives
Answer: C
Four gauges, two in tension and two in compression placed in opposite arms, give four times the output of a single gauge, and every change common to all four cancels. The output remains proportional to the excitation, so that must still be held stable.
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A pressure transducer specified as 0.1 per cent of full scale over a span of 10 bar has a possible error at a reading of 1 bar of
Answer: D
A tenth of one per cent of ten bar is 0.01 bar wherever the reading lies, so at one bar that absolute error is one per cent of the reading. Specifications given as a percentage of full scale always behave this way at the bottom of the range.
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A transducer is described as having a time constant of 5 s. Its reading reaches 95 per cent of a step change in about
Answer: A
An exponential response reaches 63 per cent in one time constant, 86 in two and 95 in three, so about fifteen seconds. This matters when a slow sensor is used in a control loop, where the lag can make the loop unstable.
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A thermowell fitted round a thermocouple in a pipe
Answer: B
The metal sheath and any air gap within it must be heated before the junction responds, so the time constant may rise from a fraction of a second to tens of seconds. The trade against mechanical and chemical protection is a routine engineering decision.
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An accelerometer of sensitivity 10 pC per g is used with a charge amplifier of 1 mV per pC. An acceleration of 5 g gives an output of
Answer: C
The charge is \( 10 \times 5 = 50 \) pC and the amplifier turns each picocoulomb into a millivolt, so the output is 50 mV. The cable capacitance does not enter, which is the whole reason for using a charge amplifier rather than a voltage one.
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A rotary encoder giving a Gray coded output is preferred to one giving a plain binary output because
Answer: D
In plain binary several bits change at once at some boundaries, and any slight misalignment of the tracks then gives a wildly wrong reading during the transition. With one bit changing at a time the worst error possible is one count.
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