This one’s similar to wet bulb temperature but in the opposite direction. On the psych chart: Wet bulb temperature lines run diagonally downward from the saturation curve toward the right. When the space ( not the air) can’t hold any more water vapor molecules without some of them condensing into liquid water, the wet bulb temperature equals the dry bulb temperature. There’s one condition where the wet bulb and dry bulb temperatures are equal. Well, I say it’s lower, but it’s not always.
It’s lower than the dry bulb temperature because swinging the wet bulb thermometer causes some of the water to evaporate, taking some heat away. It’s sometimes called the temperature of evaporation because you measure it by wrapping a wet material around the bulb of a thermometer, swinging the thermometer in the air at a certain rate, and then recording the temperature reading. Fortunately, if you use the psychrometer properly, the two numbers are very close. On the psych chart: Dry bulb temperature is shown on the horizontal axis along the bottom.Īctually, that should be plural since there are two types of wet bulb temperature: what we measure with a psychrometer and the thermodynamic wet bulb temperature. When we measure temperature of the air, it includes both components, so the dry bulb temperature of the air applies to both. Recall that air - moist air - is made of dry air and water vapor. And that brings us back to air, and the molecules it comprises. It’s not the best definition (because of things like quantum mechanics and zero point energy), but it’s fine for this discussion. One definition of temperature is that it’s a measure of the kinetic energy of the molecules. It can be measured in a number of ways and is one of the three independent quantities we need to define a psychrometric state point. It’s the temperature we’re talking about when we use that word without any descriptors in front. He’s got a separate chapter on each quantity. I’ll give you what I think is the essential knowledge you need about them.
Now, let’s take a look at each one individually. Here’s Gatley’s list of the top 9 psychrometric quantities, sorted by whether we can get them by measurement or have to calculate them. It could be wet bulb temperature, dew point temperature, relative humidity, or any of a number of other quantities.
A quantity that tells us how much water vapor is in the air.Barometric pressure (or altitude, from which we can get the barometric pressure).The majority of the dozens of psychrometric quantities we could work with cannot be measured directly but rather must be calculated from other, measurable quantities.Īnother important thing to understand is that to determine a state point on the psychrometric chart, we need to know the values of three independent quantities: In defining the nine psychrometric quantities below, I’ll give you the basic info about what they are, what might make them confusing, and which ones can be measured. We’ll confine ourselves to the top nine in this article, but I’ll also tell you about a couple of terms it’s best to avoid even though they’re commonly used.
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Don Gatley, in his book Understanding Psychrometrics, says there are more than 50, with one piece of psychrometric chart software allowing you to plot as many as 63. The properties of moist air can be described by dozens of quantities. Here we’ll take a look at the main psychrometric quanitites, define them carefully, and tell which commonly used term you should avoid. Some are easy to understand ( e.g., dry bulb temperature and barometric pressure) others are a bit more abstract ( e.g., enthalpy). To understand how all that works, we need quantities and we need them to be well-defined. Psychrometrics, you may recall, is the science that involves the properties of moist air and the processes in which the temperature or the water vapor content or both are changed.
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