CPR Psychrometry & Moisture Mapping 2 — Questions and Answers
Question 1: On a psychrometric chart, what does the curved line at the far left represent?
- The dry-bulb temperature axis
- The 100% relative humidity saturation curve (Correct answer)
- The dew point temperature line
- The wet-bulb depression line
Correct answer: The 100% relative humidity saturation curve
The curved line on the left of a psychrometric chart is the saturation curve, representing 100% relative humidity conditions.
Question 2: Which psychrometric property directly indicates the energy content of moist air and is critical for evaluating dehumidifier performance?
- Relative humidity
- Dew point temperature
- Specific enthalpy (Correct answer)
- Wet-bulb temperature
Correct answer: Specific enthalpy
Specific enthalpy measures the total heat content (sensible + latent) of moist air, which is essential for calculating dehumidifier energy efficiency.
Question 3: A restorer measures a dew point of 55°F in a water-damaged room. What does this value mean?
- The air temperature must be reduced to 55°F before condensation will occur (Correct answer)
- The wet-bulb temperature equals 55°F
- The moisture content equals 55 grains per pound
- The relative humidity is 55%
Correct answer: The air temperature must be reduced to 55°F before condensation will occur
Dew point is the temperature at which air becomes saturated and condensation begins; a 55°F dew point means surfaces below that temperature will accumulate condensation.
Question 4: Vapor pressure deficit (VPD) in drying science is best described as:
- The difference between indoor and outdoor barometric pressure
- The difference between the saturated vapor pressure and the actual vapor pressure of the air (Correct answer)
- The rate at which vapor migrates through building materials
- The total pressure exerted by water vapor at the dew point
Correct answer: The difference between the saturated vapor pressure and the actual vapor pressure of the air
VPD is the gap between how much moisture the air could hold at saturation versus how much it actually holds, driving evaporation from wet materials.
Question 5: When using a sling psychrometer, the technician reads a dry-bulb of 80°F and wet-bulb of 67°F. The difference between these readings is called:
- Sensible heat difference
- Wet-bulb depression (Correct answer)
- Dew point spread
- Specific humidity variance
Correct answer: Wet-bulb depression
The wet-bulb depression is the difference between dry-bulb and wet-bulb temperatures, used with psychrometric tables to determine relative humidity.
Question 6: At higher altitudes, how does reduced atmospheric pressure affect the drying process?
- Evaporation slows because moisture molecules have more mass
- Evaporation accelerates because lower pressure reduces the boiling point of water (Correct answer)
- Relative humidity readings become higher than actual conditions
- Dehumidifiers become more efficient due to reduced load
Correct answer: Evaporation accelerates because lower pressure reduces the boiling point of water
Lower atmospheric pressure at high altitudes lowers water's boiling point, allowing moisture to evaporate more readily from wet materials.
Question 7: A psychrometric chart shows two air conditions plotted. One has a higher humidity ratio but lower dry-bulb temperature than the other. Which statement is correct?
- The cooler air is always at higher relative humidity
- Relative humidity comparison requires reading both conditions against the saturation curve, not just comparing values (Correct answer)
- The warmer air has lower relative humidity since it can hold more moisture
- Both conditions have equal relative humidity if their dew points are the same
Correct answer: Relative humidity comparison requires reading both conditions against the saturation curve, not just comparing values
Relative humidity is the ratio of actual moisture to maximum possible moisture at that temperature, so both dry-bulb and humidity ratio must be evaluated against the saturation curve for accurate comparison.
CPR Psychrometry & Moisture Mapping 2
On a psychrometric chart, what does the curved line at the far left represent?