5/19/2023 0 Comments Dew point measure wet bulb![]() These are values that you may hear about. ![]() So that is how we calculate the vapor pressure from the wet bulb temperature. So our vapor pressure is about 0.71 kilopascals. So our vapor pressure is going to be this calculation from Tetons formula (Equation 2) and if you plug all those numbers into your calculator (notice our degrees C will cancel) we’re left with kilopascals. We take this standard equation (Equation 4) and insert all these numbers. And just to remind us, this is the constant gamma (6.66 times 10 -4 ℃ -1). If air temperature (T a) is 20 degrees Celsius, the wet bulb temperature (T w) is 11 degrees Celsius, and air pressure (P a) is 100 kilopascals (basically at sea level). Our example problem says find the vapor pressure of the air. So let’s actually put it into a calculation. We can simply take it as a constant for our purposes here as 6.66 times 10 -4 ℃ -1. It’s actually the specific heat of air divided by the latent heat of vaporization, or that 44 kilojoules per mole that I mentioned before. We take the saturation vapor pressure (e s) at the wet bulb temperature (T w) and subtract, the gamma (Ɣ), which is the psychrometer constant 6.66 times 10 -4 ℃ -1 times the pressure of the air (P a), multiplied by the difference between the air temperature (T a) or that dry bulb that I mentioned earlier, and the wet bulb temperature (T w). Here’s the calculation we need to estimate vapor pressure from the wet bulb temperature. This wet bulb temperature is not quite as simple as our dew point temperature to use in a calculation. If we wait till we’ve reached that maximum temperature decrease, we can take that as our wet bulb temperature, or T w. When that happens, it decreases the temperature of this bulb. That’s actually quite a bit of energy used for changing liquid water into water vapor. That’s water as it changes from liquid into water vapor, and it actually takes energy for this to happen (44 kilojoules per mole). You feel some cooling on your skin as water begins to evaporate when you climb out of a pool on a dry, warm summer day. You can imagine what happens during this evaporation by thinking about climbing out of the pool. This would help the water evaporate at an optimum level. I don’t know how you’d ever estimate that speed, but that was the goal. ![]() For example, some instruments had a small fan inside that would blow water across this wick, or more commonly, two temperature sensors were attached on a rotating handle, so they could spin them in the air at about one meter per second (or two miles an hour). Once that wick is saturated with water, the water begins to evaporate, and they would use wind to enhance that evaporation. This was just a fabric with water dripped onto it. Researchers made this wet bulb temperature by putting a cotton wick around the bulb of the thermometer. To calculate vapor pressure from our dew point temperature, we’ll call vapor pressure of the air, e a which is equal to the saturation vapor pressure (e s) at the dew point temperature (T d) (Equation 1). The beautiful thing about dew point temperature is that if you know this value, you can easily calculate vapor pressure and even go on to calculate relative humidity, as I talked about in another lecture. We call this T d or dew point temperature. So we can say that the dew point temperature is the point at which the air is saturated and water begins to condense out. But it was a good approximation and a great way to demonstrate what dew point temperature is. This experiment wasn’t perfect because there is certainly a temperature difference between the inside of our glass where we’re stirring with the thermometer and the outer surface of the glass. At the point the film began to form, we looked at the temperature to get the dew point temperature, which means exactly what it says: the point at which dew begins to form. So we watched the temperature go down, and at some point, we observed a thin film form onto that glass. The thermometers were rotating around in the glass, and our job was to look carefully and find out when a thin film of dew began to form around on the glass. The professor had us take a beaker of water and a thermometer and put ice in the beaker and start to stir it.
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