Measuring Relative Humidity



Humidity is generally measured either by a hair hygrometer or by a relatively complicated device used by most weather services which relies on the hygroscopic properties of Lithium Chloride (LiCl). When LiCl adsorbs water, the resistance changes and is read as the LiCl coating of the instrument is heated and cooled. The current measured is used to determine the dew point.

Home-made hair hygrometers are generally not satisfactory for use in the classroom. The preparation of the hair and the construction of the lever system, which magnifies the minute changes in the length of the hair with changes in humidity, are rather tedious. There are several other methods by means of which rough estimates of the relative humidity can be obtained. In all cases, a sling-psychrometer is used for calibration.



Cobalt chloride is a hygroscopic chemical that changes its color with changes in humidity. Cobalt Chloride is generally available in elementary chemistry sets. Make a small amount of concentrated aqueous solution and dip a piece of cotton cloth or blotting paper into it. Expose the blotting paper to humid air and dry air and note the change in color. Measure the relative humidity with a psychrometer, whenever you note a change in color from blue to pink to calibrate the blotter paper.

Material: Half a teaspoon full of cobalt chloride

White cotton cloth or white blotting paper, 2 x 6"

Water

Several food stuffs are known to be hygroscopic. Flour, ground rock salt, melting salt (calcium chloride), salt with magnesium chloride in it, and brown sugar are all good candidates as are others found in the pantry. These can be weighed on the balance. Make a number of dishes by attaching three 6" pieces of stout sewing thread to the rim of the cut off bottom 1/4" of paper cups. Cover the bottom of the dish with the material you want to test. Hang it on one side of the balance and balance the dish with counter-weights made of paper clips on the other arm of the balance beam. Note the position of the balance pointer and measure the relative humidity with a psychrometer. Now, cover the entire balance with cardboard box, the inside of which should have been thoroughly wetted. Place a dish with hot water under the box and wait for hour, before read the balance again. The relative humidity inside the box will be about 95%.

If you find a material which absorbs humidity you can make a calibration chart on graph paper, on which the ordinates are relative humidity and balance-scale readings, respectively. Enter the two points of our two measurements and draw a straight line through them, assuming that the relationship is linear. You can also wait for a few days and measure the relative humidity with a psychrometer several times each day and thereby obtain more calibration points, through which we draw the best-fitting curve.

Material: One balance

Small paper cups

One yard of heavy sewing thread

Small amounts of flour, brown sugar, rock salt, etc.

Big cardboard box to fit over balance

Small pan and hot water