The properties of moist air that were just presented can be illustrated in tables or maybe more conveniently in graphical form. This includes both the sensible heat of the dry air and the latent and sensible heat of the water vapor. The enthalpy of the air is actually the sum of the individual enthalpies of the water vapor plus the enthalpy of the dry air. Specific enthalpy on the psychrometric chart is often referred to as heat content of the air per unit weight of air. In the imperial system of units, shown on the psychrometric chart earlier, it is in units of cubic feet per pound of dry air. Specific Volume is the inverse of density, or the volume of air per unit weight of dry air. Specific volume was presented earlier in lesson 1.1:review of fundamentals. If only three ounces of water were in the six ounce container, the container would be half full, or fifty percent. If the container contains six ounces of water, the container is full and therefore we could say it is saturated. To illustrate relative humidity, consider a six ounce container. In other words, relative humidity is the amount of moisture the air contains relative to the amount of moisture it could contain, expressed as a percentage. The maximum amount of water vapor that the air can hold is dependent on the air temperature at a given atmospheric pressure. Saturated air means that the condition of the air when it contains the maximum amount of water vapor that in can hold. Relative humidity is the ratio of the actual water vapor pressure in the air relative to the water vapor present if the air were saturated. Relative humidity is discussed more often in everyday life than humidity ratio, and is a little easier for most people to understand. There are 7,000 grains of water in a pound. Since air has typically a relatively small amount of water vapor, in terms of pounds, another expression for humidity ratio is commonly used called grains of moisture. Presented another way, if we could take a pound sample of air and squeeze out all of the water, the humidity ratio would be the amount of water in pounds relative to the amount of dry air in pounds. Humidity ratio gives us an indication of the actual weight of the water in the air. Basically, humidity ratio is simply the weight of the water vapor per pound of dry air. The formula for humidity ratio is shown here and looks pretty complicated. Humidity ratio is also known as moisture content. This temperature is referred to as the dew point. Evidence of this change is seen by moisture forming on surfaces. That is, change from a vapor to a liquid state. If air is cooled at a constant pressure, there is a point at which the water vapor in the air will condense. You can see other examples of sling psychrometers online. Shown here is an example of a simple and very easy to use sling psychrometer that is manufactured by forestry suppliers. The psychrometer is rotated through the air to get the readings. It consists of two thermometers side by side, where one bulb is covered by a wet cloth and the other is dry. A tool called a sling psychrometer or sling thermometer is used to measure both dry bulb and wet bulb temperature of an air sample. The difference between the dry bulb reading and the wet bulb readings of the same air is referred to as wet bulb depression. If the bulb is surrounded by a water soaked wick and moving through the air, then the temperature will be different than the dry bulb temperature and that is what is called wet bulb temperature. If the thermometer is measuring the air temperature and the bulb of the thermometer is dry and open to the air, the temperature rating is called the dry bulb temperature. So, let’s take a look at a basic thermometer. These properties are: dry bulb temperature, wet bulb temperature, dew point, humidity ratio, relative humidity, specific volume, and specific enthalpy. Seven properties of atmospheric air are shown on the psychrometric chart. Psychrometrics is the name given to the study of the air and water vapor mixture. It is this mixture that is referred to as moist air. Atmospheric air is the mixture of both dry air and water vapor.
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