How does a sling psychrometer work?

Question

i need to know

Answer 1

It is actually 2 thermometers, one has a water "sock" to measure wet builb, the other is dry, you sling it to make a current of air, that favors water to evaporate, and cool the bulb so the termometer measures colder than the dry bulb one, then you go to a table to find out the humidity content of air, that is all. Find out relative humidity in air

Answer 2

The sling psychrometer has two thermometers.  The purpose of both is to get both the air temperature, and the 'wet-bulb' temperature, which can be used to deduce the dewpoint temp and therefore relative humidity.  

One thermo. simply measures the air temperature and the other is more complicated.  The bulb of the other is wrapped in some gauze, and the gauze is imbued with a little water, or sometimes they use rubbing alcohol.  The rubbing alcohol is used instead if it's below freezing and the water would freeze.

You swing the contraption (some have a little fan instead to blow across the bulbs).  The air flowing over the one with gauze/alcohol will have evaporation going on around the bulb.  The temperature that results on the bulb with the evaporation is called the wet-bulb temperature.  This is NOT the dewpoint.  The wet-bulb temp is in between the dewpoint (the point at which water vapor in the atmosphere can start condensing) and the regular air temperature.  The reason it's not the dewpoint is because as water evaporates, it gives off heat. So the result is that it shows a temp higher than the actual dewpoint.

It's true though that the higher the wet-bulb temp, the more humidity/moisture is in the air.  

Meteorologists and weather observers use a table to look up the corresponding dew point using the dry air temp (dry bulb temp) and the wet-bulb temp .  There are math formulas, often simplified into more lookup tables, for dry bulb temp, DEWPOINT, and the corresponding relative humidity.  And that's what they do to determine the dewpoint and relative humidity using a sling psychrometer.

Answer 3

a sling psychrometer - consists of two thermometers mounted together with a handle attached on a chain. One thermometer is ordinary. The other has a cloth wick over its bulb and is called a wet-bulb thermometer.

When a reading is to be taken, the wick is first dipped in water and then the instrument is whirled around. During the whirling, the water evaporates from the wick, cooling the wet-bulb thermometer. Then the temperatures of both thermometers are read.

If the surrounding air is dry, more moisture evaporates from the wick, cooling the wet-bulb thermometer more so there is a greater difference between the temperatures of the two thermometers. If the surrounding air is holding as much moisture as possible - if the relative humidity is 100% - there is no difference between the two temperatures. Meteorologists have worked out charts of these differences for each degree of temperature so that the observer can find relative humidity easily

You can make a sling psychrometer by using two commercial thermometers. Wrap the bulb of one tightly with a piece of cloth. Attach the thermometers to a narrow, thin board with wire or strong tape. Drill a hole in the top of the board and attach a wooden handle to the board with a short piece of chain.

http://i4weather.net/sling.jpg