Psychrometrics

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Psychrometrics is the measurement of the heat and water vapor properties of air.

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Psychrometry

Psychrometry is the science and practice of air mixtures and their control. The science deals mainly with dry air, water vapor mixtures, with the specific heat of dry air and its volume. It also deals with the heat of water heat of vaporization or condensation and the specific heat of steam in reference to moisture mixed with dry air. Psychrometry is a specialized area of thermodynamics.

Psychrometrics chart

Missing image
Psychrometric_chart_simplified.png
A simple Psychrometric chart.

The psychometric chart is a graph of the properties (temperature, relative humidity, etc.) of air. It is used to determine how these properties vary as the amount of moisture (water vapor) in the air changes.

Dry-bulb temperature is the air temperature determined by an ordinary thermometer.
Wet-bulb temperature reflects the cooling effect of evaporating water.
Dew point temperature is the temperature below which moisture will condense out of air.
Relative Humidity is a measure of how much moisture is present compared to how much moisture the air could hold at that temperature.

The versatility of the psychrometric chart lies in the fact that by knowing just two properties of moist air, the other properties can be determined.

Dry-bulb temperature

Common thermometers measure what is known as the dry-bulb temperature.

Wet-bulb temperature

A hygrometer is an instrument used to measure the amount of moisture in the air. If a moist wick is placed over a thermometer bulb the evaporation of moisture from the wick will lower the thermometer reading (temperature). If the air surrounding a wet-bulb thermometer is dry, evaporation from the moist wick will be more rapid than if the air is moist. When the air is saturated no water will evaporate from the cloth wick and the temperature of the wet-bulb thermometer will be the same as the reading on the dry-bulb temperature. However, if the air is not saturated water will evaporate from the wick causing the temperature reading to be lower. The accuracy of the wet-bulb temperature depends on how fast air passes over the bulb. Speeds up to 5,000 ft/min (60mph) are best but dangerous to move a thermometer at that speed. Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (sunlight, for example).

A wet bulb temperature taken with air moving at about 1-2 m/s is referred to as a screen temperature, whereas a temperature taken with air moving about 3.5 m/s or more is referred to as sling temperature.

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