I am going to describe here how to use a psychrometric chart, which is also known as a humidity chart,
and what I am going to do is show you how to get things like dry bulb temperature, relative
humidity, moisture content, the fraction, or the ratio of kilograms of water per kilograms
of dry air, dew point temperature, enthalpy, the humid air volume, specific volume, and
wet bulb temperature. To make this clear what I am going to do is take advantage of an online
interactive simulation for psychrometric charts, and I've given the location here, so what
I am going to do now is switch over and look at that interactive simulation and then describe
these various properties on the chart. So here is what a psychrometric chart typically
looks like, though we don't usually see if with various colors but it looks complicate.
So what i want to do it remove a number of things and add these back in as we discuss
them. Let's remove the grid lines, and normally we don't display those temperature values
that are shown, let's remove the specific volume, and let's remove enthalpy. So here
is the real basis of the psychrometric chart. We're showing temperature on the x-axis and
moisture content, kilograms of water, or kilograms of dry air. In this particular simulation
we can change the temperature, and this is the dry bulb temperature, and that means the
actual temperature of the air if we had a thermometer in the air, and I can move the
nose along a relative humidity line and I can change the relative humidity. So saturation
line is here, that's as much moisture as we can have in the air at equilibrium, and then
this is 90, 80, 70, etc. percent of saturation. So now what we want to do is we want to look
at relative humidity at about 28%, and the temperature is about 35 degrees C, so I am going to display,
it 's showing three temperatures. This is the dry bulb temperature, this is the dew
point temperature, and the dew point temperature corresponds to how much do I have to cool
down this air, I am not changing the moisture content, so whatever moisture is in there
stays there, until I get to saturation, and this means if I cool any lower I will start
condensing out water, that's the dew point. Here is the wet bulb temperature. This is
the temperature if I were to wrap, for example cloth around a thermometer and move it through
the air I would get evaporative cooling. Water would evaporate because we are not a saturation
conditions. Water would evaporate, this would cause cooling because energy of evaporation
comes from lowering the temperature, and that temperature I've measured would be the, let
me display the values, would be the wet bulb temperature, so wet bulb temperature is 21
degrees, that's something I can measure. Dew point is 14 degrees, and the wet bulb temperature
corresponds fairly closely, not exactly, but here that's what we're going to approximate
it as, as along a constant enthalpy line, so I've added in the enthalpy line so we can
read the enthalpy now, 59 kilojoules per kilogram. this is of course for some reference conditions,
we're typically saying for the charts you're going to find in books. So I can read enthalpy
because everything along this blue line has the same enthalpy. Let's remove this so it's
not too complicated, and let's look at specific volume. So for this point specific volume,
or humid volume is probably a better term, is the number of cubic meter per kilogram
of dry air. So 0.94 cubic meters per kilogram of dry air. I can look again as I change relative
humidity I can see how these various value change, so let me add these in so now I'll
explain the values, and this simulation I can change the relative humidity or I can
change the temperature and observe changes in the various values that can be of course
read off the chart, and of course this is more typically how the chart would look. So
hopefully this gives you good introduction of how to use a psychrometric chart to get information
about air that has moisture that's below saturation.