![]() ![]() Absolute Humidity vs Humidity RatioĪt this point it is probably worth noting the issues associated with using the term absolute humidity to describe the vertical axis. The hope was that by simply animating back and forth between the two chart types, most of that explanation would become pretty obvious as you get to see clearly almost all of the psychrometric processes change from straight lines under the absolute humidity axis to complex curvilinear lines under relative humidity.Ī by-product of this transition is that bioclimatic regions, when viewed against relative humidity, become a reasonably recognisable 90° rotated version of the original Olgyay bioclimatic chart, which helps in understanding the contribution of Givoni and Milne. Thus, one of the reasons for developing this tool was to see if I could morph seamlessly between the standard psychrometric chart and the much simpler relative humidity chart. Having previously taught psychrometrics to architects, I always found it challenging to convincingly explain why the psychrometric chart used humidity ratio or absolute humidity in the vertical axis instead of the more familiar metric of relative humidity, and why it was so important to understand the various metric lines. As is true of the weight, or density, the pressure from the water vapor is usually only about 1/2 to 1–1/2 per cent of the total pressure.Figure 7b: This can also be done in the relative humidity chart and mapping the same data. Most of the total pressure is from the air, but some is from the water vapor. The pressure exerted by the air, or barometric pressure, is caused by both the dry air and the water vapor. Standard barometric pressure at sea level is 29.921 inches of mercury, which is equivalent to 14.696 pounds per square inch absolute. Air, in addition to having weight, also exerts a pressure, called barometric pressure, which is usually measured in inches of mercury above a perfect vacuum. At ordinary room temperatures, it constitutes only about 1/2 to 1–1/2 per cent of the total weight of the mixture which we call air. However, their densities are far different because the water vapor is a very light gas. Each fill the room, so their volumes are equal. It is filled with a mixture of dry air and water vapor. Therefore, problems connected with cooling or heating dry air are rather simple, for when we cool it we remove its sensible heat of about 1/4 of a Btu per pound of dry air per degree, and when we heat dry air, we add that same amount of sensible heat. Moreover, the temperatures, densities, volumes, and pressures all vary proportionately. Conversely, if we cool dry air, it becomes heavier per cubic foot, again provided that its pressure remains constant. Dry air obeys the gas laws of Charles, and Boyle, which state that if a gas, such as air, is heated, it expands and its density or weight per cubic foot becomes less, provided the pressure remains constant. Thus, when we warm or cool dry air, we add or remove sensible heat only, and are not concerned with latent heat. In our atmosphere it is superheated three or four hundred degrees, or far above its condensing or boiling temperature. It can be liquefied, but at very low temperatures, about 300☏ below zero. Dry air is itself a mixture of several gases, mainly nitrogen and oxygen, but these do not change in their proportions, and they behave so much the same, that we can consider dry air as if it were a single gas. In this chapter, when dry air is referred to, it will mean the air part when water vapor or moisture is referred to, it will mean the water part and when referring to just air, it will mean the mixture of dry air and water vapor. ![]() ![]() It is best to think of dry air and water vapor separately, for they behave independently of one another. First, atmospheric air is a mixture consisting of dry air and water vapor in varying relative amounts. Before we start to explain the psychrometric chart, let us review a few of the principles on which it is based. The tables are somewhat more accurate, but the chart is accurate enough for all practical purposes and is much easier to use. This can be accomplished through use of psychrometric tables or a psychrometric chart. In broader terms it is the science and practices associated with atmospheric air mixtures, their control, and the effect on materials and human comfort. Psychrometry is an impressive word which is defined as the measurement of the moisture content of air. ![]()
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