19 Tools & Measurement Practice Questions & Answers

1. 1. Which statement correctly describes the difference between a thermocouple and a thermistor used as temperature sensors?

   • A.Both devices generate a voltage; the only difference is the length of the lead wire supplied with them
   • B.A thermocouple changes its electrical resistance with temperature, while a thermistor generates a small voltage at a junction of two dissimilar metals
   • C.A thermocouple generates a small voltage at the junction of two dissimilar metals, while a thermistor changes its electrical resistance as its temperature changes✓ Answer
   • D.Both devices change resistance; the thermocouple simply responds faster because it is larger

   A thermocouple works on the principle that a junction of two dissimilar metals produces a millivolt signal proportional to the temperature difference at that junction, which is why it covers a very wide range and is read on a millivolt-capable meter. A thermistor is a semiconductor whose resistance changes sharply with temperature, giving high sensitivity over a narrower range. Knowing which principle a probe uses tells you how to read it and what its practical range is.

   Source: NATE Core knowledge areas, taking temperature and humidity measurements — thermometers (electronic temperature sensors)Report a problem with this question

2. 2. A technician points an infrared thermometer through the face of a supply register to obtain the supply-air temperature. What is the instrument actually measuring?

   • A.The average temperature of the air moving through the duct behind the register
   • B.The wet-bulb temperature of the supply air
   • C.The surface temperature of whatever solid object falls inside its field of view, not the temperature of the air✓ Answer
   • D.The dew point of the air leaving the register

   An infrared thermometer reads the infrared energy radiated by surfaces; air is essentially transparent to that energy, so the reading comes from the register vanes or duct wall the instrument happens to see. To get supply-air temperature you must insert a contact or air probe into the moving airstream, which is why infrared is best used for comparisons and hot-spot hunting rather than for air-side measurements.

   Source: NATE Core knowledge areas, taking temperature and humidity measurements — thermometers (infrared/non-contact)Report a problem with this question

3. 3. A technician measuring return-air dry-bulb temperature slips the probe just inside the duct so the tip rests against the sheet-metal wall a short distance from the coil, and records the number immediately. What is wrong with this measurement?

   • A.Dry-bulb temperature must always be taken with a wetted wick over the sensor
   • B.The duct wall is the correct location, but the probe simply needs to be taped in place
   • C.The tip is picking up duct-wall temperature and radiant influence instead of the moving air, and it was never allowed to stabilize✓ Answer
   • D.Nothing is wrong; readings taken against the duct wall are the accepted practice

   A sensor touching metal conducts heat to and from that metal and also sees radiant energy from the nearby coil, so it reports the duct rather than the airstream. Good technique is to insert the probe far enough that the tip is in the moving air, keep it off the walls and out of the coil's line of sight, and wait until the display stops changing before recording.

   Source: NATE Core knowledge areas, taking temperature and humidity measurements — thermometers (probe placement and stabilization)Report a problem with this question

4. 4. Which field check best verifies that a thermometer reads correctly at a known reference point?

   • A.Hold the probe in a pot of boiling water, because water boils at the same temperature at every elevation
   • B.Compare it to the temperature shown on the customer's thermostat
   • C.Lay it on a metal surface in direct sunlight and compare it to the reported outdoor temperature
   • D.Immerse the probe in a stirred bath of crushed ice and water, because that mixture holds at the freezing point of water regardless of elevation✓ Answer

   A stirred slurry of crushed ice and water is a reproducible fixed point: as long as ice and water coexist the mixture sits at water's freezing temperature. Boiling water is a valid reference only when the local atmospheric pressure is accounted for, because the boiling point drops as elevation increases; a thermostat display or an ambient comparison is not a reference at all. Dial thermometers found to be off can be corrected at the adjustment nut under the dial.

   Source: NATE Core knowledge areas, taking temperature and humidity measurements — gauge and meter calibrationReport a problem with this question

5. 5. Two thermometers placed side by side in the same return-air stream disagree by several degrees. What should the technician do?

   • A.Check both against a known reference such as a stirred ice bath, then use the instrument proven accurate and adjust or remove the other from service✓ Answer
   • B.Assume the digital instrument is correct because it displays decimal places
   • C.Assume the older instrument is correct because it has been in service the longest
   • D.Average the two readings and use the result for the diagnosis

   When two instruments disagree, neither reading is trustworthy until one is proven against an independent reference; averaging simply blends a good number with a bad one, and age or decimal resolution says nothing about accuracy. Verifying against a fixed reference point identifies which instrument is in error so it can be adjusted, repaired, or taken out of service.

   Source: NATE Core knowledge areas, taking temperature and humidity measurements — gauge and meter calibrationReport a problem with this question

6. 6. In one space a psychrometer shows the dry-bulb reading well above the wet-bulb reading; in a second space the two readings are nearly identical. What does this tell the technician about the air in each space?

   • A.The air in the first space is drier — a large wet-bulb depression means low relative humidity — while the second space is close to saturation✓ Answer
   • B.The two readings say nothing about humidity until a separate dew-point meter is used
   • C.The air in the first space is saturated, because the two thermometers differ
   • D.The air in the second space is drier, because readings that are close together mean little moisture

   Evaporation from the wetted wick cools the wet-bulb thermometer, and the drier the air the faster that evaporation proceeds, so the wet-bulb depression (dry bulb minus wet bulb) grows as relative humidity falls. Wet bulb can never read higher than dry bulb, and when the two are equal the air is saturated at 100 percent relative humidity.

   Source: NATE Core knowledge areas, taking temperature and humidity measurements — humidity measurement and calculationsReport a problem with this question

7. 7. Which procedure produces a reliable wet-bulb reading with a sling psychrometer?

   • A.Hold the instrument close to your body while whirling it so the wick stays warm
   • B.Wet the wick with distilled water, whirl it until the wet-bulb reading stops falling, and read the wet bulb first and quickly✓ Answer
   • C.Read the dry bulb first and then wait several minutes before reading the wet bulb
   • D.Soak the wick with tap water and let the instrument hang motionless in the return-air stream

   Minerals in tap water build up in the wick and slow evaporation, body heat and direct sun bias both thermometers, and a still instrument does not move enough air across the wick to reach the true wet-bulb temperature. Because the wet bulb begins climbing back toward the dry bulb as soon as the air movement stops, it must be read first and without delay.

   Source: NATE Core knowledge areas, taking temperature and humidity measurements — sling psychrometer, wet and dry bulb thermometersReport a problem with this question

8. 8. A furnace heats a stream of air without adding any moisture to it. What happens to the relative humidity and to the dew point of that air?

   • A.The relative humidity stays the same while the dew point rises
   • B.Both the relative humidity and the dew point rise
   • C.The relative humidity falls while the dew point stays the same✓ Answer
   • D.Both the relative humidity and the dew point fall

   Dew point and humidity ratio describe the actual amount of moisture in the air, and heating adds no moisture, so they do not change. Relative humidity is a ratio of the moisture present to the maximum the air could hold at its current dry-bulb temperature, and that maximum rises as the air is heated, so the ratio falls. This is a purely sensible process, which plots as a horizontal line on a psychrometric chart.

   Source: NATE Core knowledge areas, taking temperature and humidity measurements — fundamentals of humidity and the psychrometric chartReport a problem with this question

9. 9. A pressure gauge connected to a system that is open to the atmosphere reads zero. What does that tell you about the pressure inside the system?

   • A.The system is at a perfect vacuum, with essentially no molecules left inside it
   • B.The gauge is defective, because a system open to the atmosphere always reads above zero
   • C.The system is at atmospheric pressure; the gauge is referenced to atmosphere, and absolute pressure equals the gauge reading plus atmospheric pressure✓ Answer
   • D.Gauge pressure and absolute pressure are the same value in every situation

   A service gauge is calibrated so that surrounding atmospheric pressure reads zero; it measures the difference between system pressure and atmosphere. Absolute pressure counts up from a true vacuum, so it is the gauge reading plus the local atmospheric pressure. That distinction matters because gas-law calculations and deep-vacuum work must be done in absolute terms, not gauge terms.

   Source: NATE Core knowledge areas, using basic science — pressure measurement (gauge versus absolute pressure)Report a problem with this question

10. 10. During evacuation, why can the compound gauge on a manifold set not verify that a deep vacuum has been achieved?

    • A.It is accurate, but its readings must first be converted to absolute values using a chart
    • B.It reads only in pounds per square inch and cannot indicate any vacuum at all
    • C.Its vacuum scale is far too coarse to resolve the extremely low pressures involved, so an electronic micron gauge is required✓ Answer
    • D.It reads vacuum accurately, but only while the vacuum pump is still running

    The vacuum portion of a compound gauge is compressed into a small arc and cannot distinguish a rough vacuum from a deep one; its needle sits against the stop long before the system is dry. A micron gauge measures absolute pressure in the very low range where the remaining moisture and non-condensables actually matter, and it should be connected away from the pump, at the far end of the system, so it sees the system rather than the pump.

    Source: NATE Core knowledge areas, using basic science — measurement of vacuum with a micron gaugeReport a problem with this question

11. 11. What actually demonstrates that an evacuated system has reached and is holding an acceptable vacuum?

    • A.Feeling the service hoses become cold to the touch
    • B.Valving the system off from the pump and confirming that the micron reading holds instead of climbing✓ Answer
    • C.Listening for the vacuum pump to change sound and shutting down at that point
    • D.Watching the micron gauge touch a low reading once while the pump is still pulling on the system

    A running pump can hold a low reading at the gauge while moisture is still boiling off or air is still leaking in, so a momentary number proves nothing. Isolating the system and watching the reading is the decay test: a rise that levels off points to moisture still vaporizing inside, while a reading that keeps climbing indicates a leak that must be found and repaired before charging.

    Source: NATE Core knowledge areas, using basic science — vacuum measurement and verification by pressure decayReport a problem with this question

12. 12. How must a meter be connected to measure the voltage supplied to a load that is running?

    • A.Across the load with the disconnect open, so that no live parts are exposed
    • B.Clamped around a single conductor feeding the load
    • C.In series with the load, with the circuit de-energized so the reading is safe to take
    • D.In parallel, with the leads placed across the load's terminals while the circuit is energized and the meter set to the correct alternating- or direct-voltage function✓ Answer

    Voltage is a difference in electrical potential between two points, so the meter must bridge those two points in parallel and the circuit must be live for a potential to exist. Selecting the wrong function — direct voltage on a low-voltage alternating control circuit, for example — produces a meaningless reading, and opening the disconnect first simply gives zero volts.

    Source: NATE Core knowledge areas, basic electricity — digital meter use and setupReport a problem with this question

13. 13. An ohmmeter placed across a relay coil that is still wired into the control circuit shows a resistance lower than the coil's expected value. What is the most likely cause?

    • A.Ohmmeters always read low when they are placed across an inductive coil
    • B.The meter leads are too short to make a proper measurement
    • C.The circuit was not energized, and resistance can only be read on a live circuit
    • D.Other components wired in parallel with the coil provide additional current paths, so at least one coil lead must be disconnected — with the power off — before the reading is valid✓ Answer

    An ohmmeter supplies its own small test current, so any parallel branch still connected to the component gives that current another route and the meter reports the combined, lower resistance. Isolating one lead removes those paths, and the power must be off because outside voltage applied to a meter on the resistance function gives a false reading and can damage the instrument.

    Source: NATE Core knowledge areas, basic electricity — resistance measurement with a digital meterReport a problem with this question

14. 14. Why is a true-RMS meter specified for taking voltage and current readings on electronically commutated blower motors and inverter-driven equipment?

    • A.Because those motors run on direct current only, which ordinary meters cannot read
    • B.Because a true-RMS meter may be used on energized circuits while other meters may not
    • C.Because true-RMS meters are the only meters that never need calibration
    • D.Because the waveforms in that equipment are not pure sine waves, and an averaging meter assumes a sine wave and therefore reports an incorrect value✓ Answer

    An averaging meter measures the average of the rectified waveform and multiplies by a factor that is only valid for a clean sine wave. Electronic commutation and variable-frequency drives chop the waveform, so that scaling factor no longer applies and the displayed value can be well off. A true-RMS meter computes the actual heating-equivalent value of whatever waveform is present.

    Source: NATE Core knowledge areas, basic electricity — meter selection for non-sinusoidal waveformsReport a problem with this question

15. 15. A clamp-on ammeter placed around the entire two-conductor cord of an operating appliance reads approximately zero. What is the explanation?

    • A.The appliance is not actually drawing any current and must be defective
    • B.The meter has to be set to the voltage function to read through a jacketed cord
    • C.A clamp-on meter cannot measure alternating current at all
    • D.The jaws enclose current flowing in both directions and the opposing magnetic fields cancel, so the clamp must surround only one conductor✓ Answer

    A clamp meter senses the magnetic field produced by current in the conductor it encircles. With both the supply and return conductors inside the jaws, the two fields are equal and opposite and sum to nearly nothing. Splitting out a single conductor — with a line splitter or at an accessible single-conductor point — restores a valid reading.

    Source: NATE Core knowledge areas, basic electricity — clamp-on ammeter useReport a problem with this question

16. 16. A technician needs the total volume of air, in cubic feet per minute, being delivered by a supply register. Which instrument gives that quantity most directly?

    • A.A rotating-vane anemometer held at one point in front of the register
    • B.An infrared thermometer aimed at the face of the register
    • C.A manometer connected across the register
    • D.A flow hood that captures all of the air leaving the register and displays volumetric flow✓ Answer

    A flow hood integrates the entire discharge from the register and reads volumetric flow directly. A vane or hot-wire anemometer reads velocity at one point, so it requires a multi-point traverse plus the free area of the grille before flow can be calculated. A manometer reads pressure difference, not flow, and an infrared thermometer reads surface temperature.

    Source: NATE Core knowledge areas, instruments for airflow measurement (awareness level)Report a problem with this question

17. 17. A system is losing charge slowly, and brushing soap solution on the accessible joints produces no visible bubbles. What is the appropriate next step?

    • A.Sweep the suspect areas with a more sensitive method such as an electronic leak detector, then pinpoint the exact spot with bubble solution✓ Answer
    • B.Release the remaining refrigerant so the escaping gas can be heard more easily
    • C.Conclude that the system does not leak and simply add refrigerant at the start of each season
    • D.Replace the compressor, since a slow loss of charge always means compressor failure

    Bubble solution is a good confirming tool but has limited sensitivity, so a very small leak can escape it entirely. Electronic detectors respond to far smaller leak rates and are used to narrow the search, after which bubbles confirm the precise location for repair. Deliberately releasing refrigerant to the atmosphere is never an acceptable diagnostic method, and the leak must be repaired before the system is recharged.

    Source: NATE Core knowledge areas, service instruments — leak detection methods and relative sensitivityReport a problem with this question

18. 18. Why is a gentle flow of nitrogen kept moving through copper tubing while joints are being brazed?

    • A.Nitrogen displaces the oxygen inside the tube, preventing the copper-oxide scale that would otherwise flake loose and foul filters and metering devices✓ Answer
    • B.Nitrogen lowers the melting point of the filler metal so that a torch is not needed
    • C.Nitrogen is needed only for soft-soldered joints, not for brazed joints
    • D.Nitrogen cools the joint so that the filler metal solidifies faster

    Brazing works at a much higher temperature than soft soldering, and at that temperature the oxygen inside the tube reacts with the hot copper to form a black oxide scale on the inner wall. A low-pressure purge of nitrogen sweeps the oxygen out so the scale never forms, protecting the filter drier and the metering device downstream. The nitrogen is flowed, not pressurized, so the joint can still be made.

    Source: NATE Core knowledge areas, tubing tools and joining — brazing versus soldering and nitrogen purgingReport a problem with this question

19. 19. A system is to be charged by weighing in a measured amount of refrigerant. What does that require of the technician's charging scale?

    • A.Nothing, because the amount can be judged well enough from how the cylinder feels as it empties
    • B.Nothing beyond switching it on, because charging scales do not drift
    • C.That it be set on a level, stable surface, zeroed with the cylinder in place, and checked against a known weight from time to time, because the accuracy of the whole charge rests on it✓ Answer
    • D.That it be returned to the factory for recalibration after every job

    Weighing in a charge is only as accurate as the instrument doing the weighing: an unlevel or drifting scale silently overcharges or undercharges the system, and cylinder feel is not a measurement at all. Setting the scale level, zeroing it before the transfer, and periodically verifying it against a known weight keeps that error out of the job, and the charge should still be confirmed by operating measurements afterward.

    Source: NATE Core knowledge areas, service instruments — charging and recovery equipment, instrument calibrationReport a problem with this question

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