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AABC Test and Balance Engineer (TBE) study materials
AABC Test and Balance Engineer (TBE) study resources

AABC Test and Balance Engineer (TBE) practice questions

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Question 1HVAC System Fundamentals and Psychrometrics

At standard sea-level atmospheric pressure, what is the approximate barometric pressure in inches of mercury (in. Hg)?

  1. 14.70 in. Hg
  2. 29.92 in. Hg Correct answer.
  3. 25.40 in. Hg
  4. 33.90 in. Hg

Explanation

Standard atmosphere at sea level equals 29.92 in. Hg, which is equivalent to 14.696 psia and 101.325 kPa. These reference values anchor psychrometric and duct-pressure calculations.

Question 2HVAC System Fundamentals and Psychrometrics

In a psychrometric chart, the dry-bulb temperature lines are typically:

  1. diagonal enthalpy lines
  2. curved saturation lines
  3. horizontal lines
  4. vertical lines Correct answer.

Explanation

Dry-bulb temperature is plotted on the horizontal axis, so constant dry-bulb lines run vertically. Humidity ratio is on the vertical axis and constant values appear as horizontal lines.

Question 3HVAC System Fundamentals and Psychrometrics

Sensible heat transfer in an air stream changes which property of the air?

  1. its dry-bulb temperature Correct answer.
  2. its latent heat content
  3. its dew-point temperature
  4. its humidity ratio

Explanation

Sensible heat changes the dry-bulb temperature without changing the moisture content. Latent heat, by contrast, changes moisture content and humidity ratio.

Question 4HVAC System Fundamentals and Psychrometrics

Which quantity expresses the mass of water vapor per unit mass of dry air?

  1. humidity ratio Correct answer.
  2. degree of saturation
  3. specific volume
  4. relative humidity

Explanation

Humidity ratio (also called mixing ratio or absolute humidity) is the mass of water vapor divided by the mass of dry air. It is expressed in lb water per lb dry air or kg/kg.

Question 5HVAC System Fundamentals and Psychrometrics

What is the primary function of a centrifugal fan in an air-handling system?

  1. to move air against system static pressure Correct answer.
  2. to filter particulates from the airstream
  3. to remove moisture from the air
  4. to cool the supply air

Explanation

A centrifugal fan converts rotational energy into airflow and static pressure to overcome duct and fitting resistance. Cooling, dehumidification, and filtration are handled by coils and filters, not by the fan itself.

Question 6HVAC System Fundamentals and Psychrometrics

On a psychrometric chart, the wet-bulb temperature lines are approximately:

  1. lines of constant enthalpy Correct answer.
  2. lines of constant humidity ratio
  3. horizontal lines
  4. vertical lines

Explanation

Wet-bulb temperature lines closely follow constant enthalpy lines on the chart. This is why adiabatic saturation and enthalpy calculations are tied to wet-bulb readings.

Question 7HVAC System Fundamentals and Psychrometrics

Which value best represents the density of standard air used in HVAC calculations?

  1. 0.050 lb/ft³
  2. 0.075 lb/ft³ Correct answer.
  3. 0.062 lb/ft³
  4. 0.100 lb/ft³

Explanation

Standard air at 70°F and sea level has a density of about 0.075 lb per cubic foot. This value underlies airflow, sensible heat, and fan power equations.

Question 8HVAC System Fundamentals and Psychrometrics

The sensible heat equation Qs = 1.08 × cfm × ΔT assumes standard air conditions and is used to calculate what?

  1. latent heat in Btu/h
  2. sensible heat in Btu/h Correct answer.
  3. total heat in Btu/h
  4. airflow in cfm

Explanation

The 1.08 constant incorporates standard air density, specific heat, and time conversion to yield sensible heat in Btu/h from cfm and temperature difference. Latent and total heat use different constants (0.68 and 4.5).

Question 9HVAC System Fundamentals and Psychrometrics

Air at 75°F dry-bulb and 50% relative humidity is cooled to 55°F dry-bulb with no moisture added. What happens to its relative humidity?

  1. It increases. Correct answer.
  2. It drops to zero.
  3. It decreases.
  4. It stays the same.

Explanation

Cooling air at constant humidity ratio moves the state point toward the saturation curve, so relative humidity rises. Relative humidity is inversely related to temperature for a fixed moisture content.

Question 10HVAC System Fundamentals and Psychrometrics

A coil handles 2,000 cfm with a 20°F temperature drop across it. Using the sensible heat equation, what is the sensible capacity?

  1. 21,600 Btu/h
  2. 43,200 Btu/h Correct answer.
  3. 86,400 Btu/h
  4. 4,000 Btu/h

Explanation

Qs = 1.08 × cfm × ΔT = 1.08 × 2,000 × 20 = 43,200 Btu/h. Substituting the airflow and temperature difference directly yields the sensible load.

Question 11HVAC System Fundamentals and Psychrometrics

The dew-point temperature of an air sample is best described as:

  1. the dry-bulb temperature minus the wet-bulb temperature
  2. the temperature at which the air reaches 50% relative humidity
  3. the temperature at which the air becomes saturated at constant moisture Correct answer.
  4. the temperature of the cooling coil surface

Explanation

Dew point is the temperature to which air must be cooled, at constant humidity ratio, for it to reach saturation. Below that temperature, condensation begins.

Question 12HVAC System Fundamentals and Psychrometrics

Air at 80°F dry-bulb and 60°F wet-bulb is passed over a coil whose surface temperature is 45°F. Which process occurs?

  1. cooling and dehumidification Correct answer.
  2. heating and humidification
  3. evaporative cooling
  4. sensible cooling only

Explanation

When the coil surface is below the entering air dew point, moisture condenses, so both sensible cooling and dehumidification occur. The leaving air follows the coil condition line toward the apparatus dew point.

Question 13HVAC System Fundamentals and Psychrometrics

What does the apparatus dew point (ADP) of a cooling coil represent?

  1. the effective surface temperature the air approaches Correct answer.
  2. the leaving air dry-bulb
  3. the entering air dew point
  4. the coil refrigerant boiling point

Explanation

The ADP is the effective coil surface temperature that the leaving air condition line points toward on the chart. It reflects the average wetted surface the air contacts.

Question 14HVAC System Fundamentals and Psychrometrics

A space has a sensible load of 30,000 Btu/h and a latent load of 10,000 Btu/h. What is the sensible heat ratio (SHR)?

  1. 0.75 Correct answer.
  2. 0.25
  3. 0.33
  4. 1.33

Explanation

SHR equals sensible load divided by total load: 30,000 / (30,000 + 10,000) = 0.75. The coil selection must match this ratio to hold design conditions.

Question 15HVAC System Fundamentals and Psychrometrics

Which instrument most directly measures the wet-bulb temperature of an airstream?

  1. a differential pressure transducer
  2. an inclined manometer
  3. a sling psychrometer with a wetted wick Correct answer.
  4. a hot-wire anemometer

Explanation

A sling psychrometer uses a wetted-bulb thermometer exposed to moving air so evaporation cools the bulb to the wet-bulb temperature. Manometers and anemometers measure pressure and velocity, not wet-bulb.

Question 16HVAC System Fundamentals and Psychrometrics

As air is heated at constant humidity ratio, which property remains unchanged?

  1. relative humidity
  2. dew-point temperature Correct answer.
  3. wet-bulb temperature
  4. specific volume

Explanation

Heating at constant moisture leaves the humidity ratio and therefore the dew point unchanged. Relative humidity, wet-bulb temperature, and specific volume all increase with temperature.

Question 17HVAC System Fundamentals and Psychrometrics

A duct carries 1,200 cfm at a velocity of 1,000 ft/min. What is the duct cross-sectional area?

  1. 0.83 ft²
  2. 12 ft²
  3. 0.12 ft²
  4. 1.2 ft² Correct answer.

Explanation

Area equals airflow divided by velocity: 1,200 / 1,000 = 1.2 ft². This relationship links cfm, velocity, and duct size in system design.

Question 18HVAC System Fundamentals and Psychrometrics

Which statement correctly describes the relationship between total pressure, static pressure, and velocity pressure in a duct?

  1. total pressure equals static plus velocity pressure Correct answer.
  2. velocity pressure equals total plus static pressure
  3. total pressure equals static minus velocity pressure
  4. static pressure equals total plus velocity pressure

Explanation

Total pressure is the algebraic sum of static and velocity pressure. Velocity pressure is always positive, while static pressure may be positive or negative.

Question 19HVAC System Fundamentals and Psychrometrics

An air sample has a humidity ratio of 0.010 lb/lb and a mass flow of 5,000 lb of dry air per hour. What mass of water vapor is carried per hour?

  1. 0.05 lb/h
  2. 500 lb/h
  3. 5 lb/h
  4. 50 lb/h Correct answer.

Explanation

Water vapor mass equals humidity ratio times dry air mass: 0.010 × 5,000 = 50 lb/h. This is the basis for latent load calculations.

Question 20HVAC System Fundamentals and Psychrometrics

Which process is represented by a line sloping down and to the right along a constant wet-bulb line on the psychrometric chart?

  1. adiabatic saturation Correct answer.
  2. reheat
  3. sensible heating
  4. mechanical dehumidification

Explanation

Adiabatic saturation follows a constant wet-bulb (approximately constant enthalpy) line as moisture evaporates and the air cools. The process moves toward saturation without external heat exchange.

Question 21HVAC System Fundamentals and Psychrometrics

A cooling coil is selected with a bypass factor of 0.20. If the entering air dry-bulb is 80°F and the ADP is 50°F, what is the leaving air dry-bulb?

  1. 56°F Correct answer.
  2. 50°F
  3. 74°F
  4. 60°F

Explanation

Leaving dry-bulb equals ADP plus bypass factor times the difference between entering dry-bulb and ADP: 50 + 0.20 × (80 − 50) = 56°F. The bypass fraction of air passes untreated.

Question 22HVAC System Fundamentals and Psychrometrics

At a fixed humidity ratio, raising the dry-bulb temperature of air causes the enthalpy to:

  1. decrease
  2. remain constant
  3. increase Correct answer.
  4. drop to zero

Explanation

Enthalpy of moist air includes sensible heat proportional to dry-bulb temperature plus latent heat from moisture. At constant humidity ratio, higher dry-bulb means higher enthalpy.

Question 23HVAC System Fundamentals and Psychrometrics

Two air streams are mixed adiabatically: 3,000 cfm at 60°F and 1,000 cfm at 90°F. Neglecting density differences, what is the mixed dry-bulb temperature?

  1. 75.0°F
  2. 80.0°F
  3. 70.0°F
  4. 67.5°F Correct answer.

Explanation

The mixed temperature is the flow-weighted average: (3,000 × 60 + 1,000 × 90) / 4,000 = 67.5°F. The mixed state lies on a straight line between the two entering states.

Question 24HVAC System Fundamentals and Psychrometrics

Why does a fan's required brake horsepower increase when air density increases at constant airflow?

  1. power is independent of density
  2. power is proportional to density Correct answer.
  3. power is inversely proportional to density
  4. power depends only on motor speed

Explanation

Fan power equals airflow times total pressure divided by a constant and efficiency, and pressure rises with density at constant speed. Therefore brake horsepower scales with air density.

Question 25HVAC System Fundamentals and Psychrometrics

A space requires 12,000 Btu/h of latent cooling with a humidity ratio difference of 0.010 lb/lb across the coil. Using the latent heat equation QL = 0.68 × cfm × ΔW, what airflow is required?

  1. 680 cfm
  2. 12,000 cfm
  3. 1,765 cfm Correct answer.
  4. 8,160 cfm

Explanation

Rearranging, cfm = QL / (0.68 × ΔW) = 12,000 / (0.68 × 0.010) ≈ 1,765 cfm. The 0.68 constant converts humidity ratio and airflow to latent Btu/h.

Question 26HVAC System Fundamentals and Psychrometrics

Which psychrometric property is held constant during a sensible heating process?

  1. Relative humidity
  2. Dew-point temperature
  3. Humidity ratio Correct answer.
  4. Wet-bulb temperature

Explanation

Sensible heating adds heat without adding or removing moisture, so the humidity ratio remains constant. Relative humidity decreases, and both wet-bulb and dew-point temperatures remain unchanged only if no moisture is added, but the defining constant property is humidity ratio.

Question 27HVAC System Fundamentals and Psychrometrics

What does a sensible heat ratio (SHR) of 0.75 indicate about a space's cooling load?

  1. The space requires 75% outdoor air.
  2. 75% of the total heat load is sensible, and 25% is latent. Correct answer.
  3. 75% of the total heat load is latent, and 25% is sensible.
  4. The coil's apparatus dew point is 75°F.

Explanation

SHR is the ratio of sensible heat to total heat (sensible plus latent). An SHR of 0.75 means sensible heat accounts for 75% of the total load, leaving 25% as latent.

Question 28HVAC System Fundamentals and Psychrometrics

Air at 85°F dry-bulb and 50% relative humidity is cooled to 60°F dry-bulb. If the humidity ratio remains constant, what is the approximate final relative humidity?

  1. 25%
  2. 75%
  3. 100% Correct answer.
  4. 50%

Explanation

Cooling air at constant humidity ratio raises relative humidity because the saturation vapor pressure decreases with temperature. At 60°F, the air is near saturation, so relative humidity approaches 100%.

Question 29HVAC System Fundamentals and Psychrometrics

A cooling coil has a bypass factor of 0.15. If the entering air dry-bulb is 80°F and the apparatus dew point is 50°F, what is the leaving air dry-bulb temperature?

  1. 75.5°F
  2. 50.0°F
  3. 62.0°F
  4. 54.5°F Correct answer.

Explanation

The leaving dry-bulb is the apparatus dew point plus the bypass factor times the difference between entering dry-bulb and apparatus dew point: 50 + 0.15 × (80 − 50) = 54.5°F.

Question 30HVAC System Fundamentals and Psychrometrics

In a cooling coil, the apparatus dew point (ADP) is best described as which of the following?

  1. The temperature of the chilled water leaving the coil.
  2. The temperature at which the coil surface is perfectly dry.
  3. The effective surface temperature of the coil if all air contacted the surface. Correct answer.
  4. The dew-point temperature of the entering air.

Explanation

The ADP represents the effective coil surface temperature that the air would reach if it were in perfect contact with the entire coil surface. It is the point where the coil's process line intersects the saturation curve.

Question 31Test Instruments, Calibration, and Measurement Uncertainty

A pitot tube used with an electronic micromanometer reads velocity pressure in a duct. Which quantity does velocity pressure directly represent in the standard duct velocity equation V = 4005√VP?

  1. The absolute static pressure in the duct
  2. Total pressure minus static pressure at the same point Correct answer.
  3. The friction loss between two traverse points
  4. Static pressure minus total pressure at the same point

Explanation

Velocity pressure is the dynamic component of airstream energy and equals total pressure minus static pressure at the measurement plane. The constant 4005 converts velocity pressure in inches of water to velocity in feet per minute for standard air.

Question 32Test Instruments, Calibration, and Measurement Uncertainty

An inclined manometer is chosen over a vertical U-tube for low-range duct pressure readings primarily because it:

  1. Eliminates the need to level the instrument
  2. Multiplies the readable leg length for a given pressure Correct answer.
  3. Removes the effect of fluid density on the reading
  4. Measures absolute rather than gauge pressure

Explanation

Inclining the measuring leg spreads a small fluid displacement over a longer scale, improving resolution at low pressures. The reading must still be corrected for the sine of the inclination angle and for fluid density.

Question 33Test Instruments, Calibration, and Measurement Uncertainty

A rotating vane anemometer is being used to measure face velocity at a filter bank. Which practice most improves the accuracy of the result?

  1. Holding the instrument at the exact center of the filter face
  2. Using the largest vane head available regardless of duct size
  3. Taking a timed traverse of multiple points across the face Correct answer.
  4. Reading once with the vane perpendicular to airflow

Explanation

Face velocity is non-uniform, so a multi-point traverse averaged over the area gives a representative value. A single center reading is biased by the velocity profile.

Question 34Test Instruments, Calibration, and Measurement Uncertainty

In a calibration program, traceability means that an instrument's calibration is:

  1. Performed at the same interval every year
  2. Verified by two technicians independently
  3. Documented in the project commissioning report
  4. Related to national standards through an unbroken chain of comparisons Correct answer.

Explanation

Traceability requires an unbroken, documented chain of calibrations linking the instrument to recognized national or international standards, each with stated uncertainty. Interval choice and documentation are separate program elements.

Question 35Test Instruments, Calibration, and Measurement Uncertainty

A differential pressure transmitter has a calibrated span of 0 to 2.0 in. w.c. and an accuracy specification of ±1% of full span. What is the maximum permissible error at a reading of 0.5 in. w.c.?

  1. ±0.010 in. w.c.
  2. ±0.040 in. w.c.
  3. ±0.005 in. w.c.
  4. ±0.020 in. w.c. Correct answer.

Explanation

Percent-of-span accuracy applies to the full span, not the reading. One percent of 2.0 in. w.c. is 0.020 in. w.c. regardless of the measured value.

Question 36Test Instruments, Calibration, and Measurement Uncertainty

A thermocouple psychrometer is used to determine wet-bulb temperature in an airstream. The wet-bulb reading is most reliable when the sensor wick is:

  1. Immersed in distilled water during the reading
  2. Insulated to prevent evaporative cooling
  3. Dry and shielded from radiation
  4. Clean, wetted, and exposed to moving air Correct answer.

Explanation

Evaporative cooling requires a clean, saturated wick and sufficient air velocity across it to reach the true wet-bulb condition. Contamination or stagnant air biases the reading.

Question 37Test Instruments, Calibration, and Measurement Uncertainty

Before using a handheld digital manometer on a duct, a technician zeros the instrument with both ports open to ambient. The purpose of this step is to:

  1. Establish the zero reference and remove sensor offset Correct answer.
  2. Verify the display resolution equals the accuracy
  3. Confirm the battery voltage is adequate
  4. Convert gauge pressure to absolute pressure

Explanation

Zeroing establishes the reference against which all subsequent differential readings are made and cancels residual sensor offset. Skipping it introduces a constant bias into every reading.

Question 38Test Instruments, Calibration, and Measurement Uncertainty

A duct traverse is performed with a thermal anemometer that has a directional sensitivity error when yawed more than about 20 degrees from the probe axis. Which measurement condition most likely produces a biased result?

  1. Probe inserted near an elbow where the flow is swirling Correct answer.
  2. Probe used in a round duct at a single diameter
  3. Probe aligned with the duct axis in straight, fully developed flow
  4. Probe held steady at each traverse point

Explanation

Thermal anemometers are directionally sensitive, so significant yaw or swirl causes the sensor to under-read or over-read depending on design. Traverse locations should be in straight, fully developed flow away from fittings.

Question 39Test Instruments, Calibration, and Measurement Uncertainty

A pressure gauge reads 12.5 in. w.c. and the true pressure is 12.0 in. w.c. Expressed as a percentage of the reading, the error is approximately:

  1. 2.0%
  2. 4.2% Correct answer.
  3. 0.5%
  4. 5.0%

Explanation

Error is the difference (0.5 in. w.c.) divided by the true or indicated reading, giving about 4.2%. Expressing error as a percentage of reading makes small-span errors look large.

Question 40Test Instruments, Calibration, and Measurement Uncertainty

Two instruments measure the same duct static pressure. Instrument A reads 1.00 in. w.c. with an expanded uncertainty of ±0.02 in. w.c. and Instrument B reads 1.00 in. w.c. with ±0.10 in. w.c. Which statement best describes the comparison?

  1. The readings are consistent within the combined uncertainty Correct answer.
  2. The readings agree within the uncertainty of Instrument A alone
  3. Instrument B is out of calibration because its uncertainty is larger
  4. The difference is significant because the readings are identical

Explanation

When comparing two measurements, the uncertainties combine, typically in quadrature, to give the uncertainty of the difference. Identical readings with overlapping uncertainty bands are consistent.

Question 41Test Instruments, Calibration, and Measurement Uncertainty

A technician records a duct velocity of 1,250 fpm using a pitot tube and micromanometer. The velocity pressure reading is closest to:

  1. 0.312 in. w.c.
  2. 0.097 in. w.c. Correct answer.
  3. 0.560 in. w.c.
  4. 0.780 in. w.c.

Explanation

Rearranging V = 4005√VP gives VP = (V/4005)². For 1,250 fpm, VP = (0.312)² ≈ 0.097 in. w.c.

Question 42Test Instruments, Calibration, and Measurement Uncertainty

A calibration certificate states a coverage factor k = 2 for an expanded uncertainty. This means the reported expanded uncertainty corresponds to approximately:

  1. Two standard deviations Correct answer.
  2. The instrument's resolution limit
  3. A 99.7% confidence interval
  4. One standard deviation

Explanation

Expanded uncertainty is the standard uncertainty multiplied by the coverage factor. For a normal distribution, k = 2 corresponds to about two standard deviations, roughly 95% confidence.

Question 43Test Instruments, Calibration, and Measurement Uncertainty

An electronic balometer (flow hood) is used to measure supply airflow at a diffuser. Which condition most degrades its accuracy?

  1. The diffuser has a strongly asymmetric throw pattern Correct answer.
  2. The hood is sized to fully cover the diffuser face
  3. The instrument is zeroed before each reading
  4. The hood skirt is sealed against the ceiling

Explanation

Flow hoods are calibrated against a known uniform flow pattern; highly asymmetric or swirling discharge can cause backpressure and non-uniform flow across the sensing grid. Proper sizing, sealing, and zeroing all support accuracy.

Question 44Test Instruments, Calibration, and Measurement Uncertainty

A calibration interval for a field manometer is being set. Which approach is most consistent with accepted calibration practice?

  1. Use the manufacturer's maximum interval for all applications
  2. Calibrate only after a reading is questioned in the field
  3. Base the interval on observed drift, usage, and required accuracy Correct answer.
  4. Set the interval once and never revise it

Explanation

Intervals should be established from history of drift, severity of use, and the accuracy needed for the application, then adjusted as data accumulate. Fixed or reactive approaches risk out-of-tolerance use.

Question 45Test Instruments, Calibration, and Measurement Uncertainty

A technician measures dry-bulb at 75°F and wet-bulb at 62°F. As the wet-bulb depression increases at constant dry-bulb, the relative humidity:

  1. Increases
  2. Approaches saturation
  3. Decreases Correct answer.
  4. Remains unchanged

Explanation

A larger difference between dry-bulb and wet-bulb indicates more evaporative capacity in the air, which corresponds to lower relative humidity. Saturation occurs when the depression is zero.

Question 46Test Instruments, Calibration, and Measurement Uncertainty

A pitot-static probe is connected backward so that the total-pressure port feeds the low side of the manometer. The resulting velocity pressure reading will be:

  1. Identical to the correct reading
  2. Negative, indicating reversed connection Correct answer.
  3. Zero because the ports cancel
  4. Doubled because both ports contribute

Explanation

Velocity pressure is total minus static. Reversing the connections makes the manometer compute static minus total, producing a negative value of the same magnitude.

Question 47Test Instruments, Calibration, and Measurement Uncertainty

A digital thermometer with a resolution of 0.1°F and an accuracy of ±0.5°F is used to record temperatures. Which statement about these two specifications is correct?

  1. Accuracy is always numerically smaller than resolution
  2. Resolution limits how finely a value can be displayed, while accuracy bounds its error Correct answer.
  3. Resolution determines the calibration interval
  4. Resolution and accuracy are the same thing

Explanation

Resolution is the smallest display increment; accuracy describes how close the displayed value is to the true value. A fine resolution does not guarantee small error.

Question 48Test Instruments, Calibration, and Measurement Uncertainty

A hot-wire anemometer is used to measure low velocities near a supply diffuser. Which environmental factor most affects its reading accuracy?

  1. Ambient temperature variation at the sensor Correct answer.
  2. Duct static pressure at the fan
  3. Duct insulation thickness
  4. Barometric pressure at the building site

Explanation

Thermal anemometers infer velocity from heat transfer, which depends on the temperature difference between the heated sensor and the air. Significant air-temperature changes require compensation or correction.

Question 49Test Instruments, Calibration, and Measurement Uncertainty

A technician must measure the static pressure across a coil with an expected drop of 0.15 in. w.c. using a gauge with ±2% of full span accuracy and a 0 to 10 in. w.c. span. The uncertainty of the reading is approximately:

  1. ±0.020 in. w.c.
  2. ±0.200 in. w.c. Correct answer.
  3. ±0.015 in. w.c.
  4. ±0.003 in. w.c.

Explanation

Two percent of a 10 in. w.c. span is ±0.200 in. w.c., which exceeds the entire expected reading. A gauge with a span closer to the measured value should be selected.

Question 50Test Instruments, Calibration, and Measurement Uncertainty

During a duct traverse, one of eight velocity pressure readings is anomalously high compared with the others. The most defensible action is to:

  1. Replace it with the average of the other seven
  2. Use the highest reading to be conservative
  3. Repeat the reading and investigate the cause before deciding Correct answer.
  4. Discard it and average the remaining seven

Explanation

An outlier may indicate a real local condition or a measurement error, so it should be rechecked and the cause investigated before any data handling decision. Arbitrary deletion or substitution distorts the traverse.