A power analyser shows 40 kW on a feeder whose nameplate loads total 90 kW. The likeliest explanation is
Answer: A
Connected load and actual demand routinely differ by a factor of two, since machines idle, run part-loaded or stand still. That gap is often where the audit's opportunity lies, and it is why the nameplate total is never used as a consumption figure.
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An auditor's infrared readings of a steam header vary widely along its length while a contact probe finds it uniform. The explanation is
Answer: B
Correct answer: the surface finish and therefore the emissivity varies
An infrared instrument infers temperature from emitted radiation, and how much a surface emits at a given temperature depends on its finish - bright metal, oxidised metal, paint and lagging all differ greatly. A header that is uniformly hot but variably painted, cleaned or corroded will therefore read as if its temperature varied. The contact probe measures the temperature directly and is telling the truth.
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Assertion: a true rms instrument should be used for every current measurement in an audit. Reason: modern loads draw distorted current.
Answer: C
Drives, rectifiers and electronic lighting are now the majority of many installations, and an averaging instrument understates their current substantially. Since the auditor cannot know in advance which loads are distorted, the true rms instrument is used throughout.
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Assertion: an infrared thermometer gives the most accurate temperature measurement available. Reason: it requires no contact with the surface.
Answer: D
Freedom from contact is exactly its advantage and is why it is used, but its accuracy depends wholly on the emissivity assumed and on what the surface reflects. A calibrated contact probe is the more accurate instrument by a wide margin.
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Put the steps of measuring the flow in a duct with an anemometer in order.
1. Compute the volume flow from the mean velocity and the area 2. Measure the free cross-sectional area 3. Take readings at several points across the section 4. Average the readings
Answer: A
The area is established first because it is fixed, then the velocity is sampled across the section because it is not uniform, and the mean of those samples is what multiplies the area. A single centre reading overstates the mean appreciably.
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A single velocity reading at the centre of a duct overstates the mean because the profile is
Answer: B
Friction at the wall makes the velocity fall to zero there, so the centre-line value exceeds the mean by a tenth or more depending on the flow regime. A traverse of several points, or a factor from a standard, is what corrects it.
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Comparing a permanently installed meter with a portable instrument for an audit, the portable instrument
Answer: C
One good clamp-on meter serves fifty measuring points over a week, which no permanent installation could justify. A permanent meter gives the continuous record and is generally the more accurate, being properly installed.
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None of the following is a reason to log rather than spot-measure except
Answer: D
A varying load is precisely what a single reading cannot characterise, and the night-time base load is often the finding that pays for the audit. Accuracy and calibration are unaffected, and a flat tariff is an argument against logging rather than for it.
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