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 change resistance with temperature; the thermocouple reacts faster
   • B.The thermocouple changes resistance; the thermistor outputs millivolts
   • C.The thermocouple outputs millivolts; the thermistor changes resistance✓ Answer
   • D.Both output millivolts at a junction; only their lead length differs

   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 air temperature in the duct behind the register
   • B.The wet-bulb temperature of the air leaving the register
   • C.The surface temperature of the vanes and duct it can see✓ Answer
   • D.The dew point of the air passing through the register face

   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 readings require a wetted wick over the sensor tip
   • B.The wall is the right spot; the probe just needs taping down
   • C.The tip reads the duct wall and coil radiation, not the air✓ Answer
   • D.The number is fine but must be corrected for static pressure

   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 boiling tap water, a fixed point anywhere
   • B.Compare it with the reading on the customer's thermostat
   • C.Compare it with the outdoor temperature given on the radio
   • D.Immerse the probe in a stirred bath of crushed ice and water✓ 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 stirred ice bath and use the proven one✓ Answer
   • B.Trust the digital one, since it displays tenths of a degree
   • C.Trust the older one, since it has been in service longest
   • D.Average the two readings and diagnose from that number

   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 first space is drier; the second one is near saturation✓ Answer
   • B.Neither reading shows humidity without a dew-point meter
   • C.Both are near saturation; the gap only tracks sling speed
   • D.The second space is drier; the first one is near saturation

   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.Whirl it close to your body so the wick stays warm and damp
   • B.Wet the wick with distilled water and read the wet bulb first✓ Answer
   • C.Read the dry bulb first, then wait a few minutes for wet bulb
   • D.Soak the wick with tap water and hang it still in the airstream

   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. Whirl the psychrometer, pausing to check it, until the wet-bulb reading stops falling — that steady low point is the true wet bulb. 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 holds steady and the dew point rises
   • B.The relative humidity rises and so does the dew point
   • C.The relative humidity falls and the dew point stays put✓ Answer
   • D.The relative humidity falls and the dew point falls too

   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 nothing left inside
   • B.The gauge is defective; an open system always reads above zero
   • C.The system is at atmospheric pressure, which reads as zero✓ Answer
   • D.Gauge pressure and absolute pressure are always the same value

   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 need conversion by chart
    • B.It reads only in psi and cannot show any vacuum at all
    • C.Its scale is far too coarse to resolve micron pressures✓ Answer
    • D.It reads vacuum only while the vacuum pump is 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 off the pump and seeing the micron reading hold✓ Answer
    • C.Hearing the vacuum pump change sound, then shutting it off
    • D.Seeing the micron gauge dip low once while the pump runs

    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.In parallel across the load with the disconnect pulled open
    • B.Clamped around a single conductor feeding the load
    • C.In series with the load, with the circuit de-energized
    • D.In parallel across the load's terminals, circuit energized✓ 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.An ohmmeter always reads low across an inductive coil
    • B.The meter leads are too short for an accurate reading
    • C.The circuit was dead, and resistance needs a live circuit
    • D.Parallel components still wired in offer other current paths✓ 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.Those motors run on direct current, which meters cannot read
    • B.Only a true-RMS meter may be used on energized circuits
    • C.True-RMS meters are the only ones that never need calibration
    • D.The waveform is not the sine wave averaging meters assume✓ 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 draws no current at all and must be defective
    • B.The meter must be set to volts to read through the jacket
    • C.A clamp-on meter cannot measure alternating current at all
    • D.The opposing fields of the two conductors cancel each other✓ 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 in front of it
    • B.An infrared thermometer aimed at its face
    • C.A manometer connected across the register
    • D.A flow hood sealed over the register face✓ 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 joints with an electronic leak detector✓ Answer
    • B.Release the rest of the charge to hear the escaping gas
    • C.Call it leak-free and top off the charge each season
    • D.Replace the compressor, since slow loss means it is worn

    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, so no oxide scale forms✓ Answer
    • B.Nitrogen lowers the melting point of the filler metal
    • C.Nitrogen is needed on soldered joints, not on brazed ones
    • D.Nitrogen cools the joint so the filler metal sets 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; the cylinder's weight can be judged by hand
    • B.Nothing beyond switching it on, since scales do not drift
    • C.That it sit level, be zeroed, and be checked periodically✓ Answer
    • D.That it go back to the factory for calibration after a 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

Practice questions based on the published NATE knowledge areas and standard HVACR theory and service practice. NATE is a mark of North American Technician Excellence; this site is not affiliated with or endorsed by NATE. NATE certification is not a licence — HVAC licensing is set by your state and locality. Confirm current exam requirements with NATE and current code requirements with the authority having jurisdiction. About NATE certification →