A hot-wire anemometer is preferred to a vane type when the velocity is
Answer: A
A vane needs enough momentum to overcome its own friction, so it stalls below perhaps half a metre a second, whereas a heated element responds to the least movement. The hot wire is the more fragile and the more affected by dust.
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A U-tube manometer measures
Answer: B
The difference in the two limbs multiplied by the liquid's density and by gravity is the pressure, which makes it a primary standard needing no calibration. It is the instrument behind the draught gauge and the pitot traverse.
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A draught gauge on a boiler is read to establish
Answer: C
Too little draught starves the combustion of air and too much drags cold air in through every leak, so the figure bears directly on the excess air. It is measured in millimetres of water gauge, which is why a manometer suits it.
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A stroboscopic tachometer is preferred to a contact type when the shaft is
Answer: D
Flashing a lamp until the mark appears stationary needs no contact at all, which suits a guarded or fast shaft. The risk is reading a submultiple of the true speed, so the frequency is scanned upward until the pattern first freezes.
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A stroboscopic reading may be a submultiple of the true speed because the mark appears stationary when the flash rate is
Answer: A
Flashing once every two or three revolutions freezes the mark just as convincingly as flashing once per revolution. Approaching from below and taking the first freeze, or checking against the nameplate, avoids the error.
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Measuring motor speed during an audit is worthwhile because the slip indicates
Answer: B
An induction motor's slip is very nearly proportional to its load, so a machine running close to synchronous speed is lightly loaded and a candidate for replacement by a smaller one. It is the cheapest load estimate available where a clamp meter cannot reach.
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A sling psychrometer establishes humidity from
Answer: C
Whirling the wetted bulb cools it by evaporation, and the depression below the dry bulb gives the humidity through a chart or a formula. It is simple, cheap and a primary check on an electronic hygrometer that has drifted.
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An ultrasonic leak detector finds compressed air leaks by responding to
Answer: D
Turbulence at a small orifice radiates strongly above the audible range, where the factory's own noise does not reach, so a leak is located in a running plant. Compressed air leakage of twenty or thirty per cent of generation is not unusual and is among the largest single opportunities in many audits.
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An ultrasonic steam trap tester distinguishes a failed trap by
Answer: A
A working trap discharges condensate in bursts and then closes; a failed one passes live steam continuously, and the sound is quite different. Temperature alone cannot distinguish the two, since both a working and a blowing trap are hot.
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A TDS meter is used in a boiler audit to decide
Answer: B
Dissolved solids concentrate as steam is raised, and blowdown discards hot water to keep them within limits, so measuring them lets the blowdown be reduced to what is actually needed. Every unnecessary litre carries its heat away with it.
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