An Economical Humidity Jar, Its Design and Use

An economical humidity jar can be made from a 1-gallon pickle jar, a 2-quart plastic bleach bottle, 2 feet of tygon tubing, 6 inches of glass tubing, a No. 10 rubber stopper, a 6 by 6-inch sheet of 1/4-inch plexiglass, one dispersion tube, a squeeze bulb, and bathtub caulk. The top and bottom of the plastic bleach bottle is cut out to allow it to collapse without damage as it passes through the mouth of the jar. This will be the support for the plexiglass disc. Numerous holes are punched out in the side of the plastic bottle to allow free circulation of solution. The plexiglass is cut into a disk slightly smaller than the inside diameter of the jar. A hole large enough for the aerator is cut in the middle of the disk, and enough smaller holes are also cut throughout the, disk. The disc is sawn into halves, which are held together by a slab of bathtub caulk on top of the disk along the cut, so that the caulk, when fully cured, acts as a hinge to allow the disk to be folded for passing through the neck of the jar. Two holes, one in the center for the aerator tube and one for an air outlet, are cut in a rubber stopper. A hole for the rubber stopper is punched in the lid of the jar and the stopper is pressed into the hole and glued securely in the lid with caulk. A squeeze bulb is then attached to the aerator tube and a glass tube for air outlet. Circulation of the air can be maintained by squeezing the bulb. The desired relative humidity is obtained by mixtures of water and glycerine in proper proportions. A small amount of deforming agent is added to decrease formation of air bubbles. Other solutions, such as sulfuric acid and saturated salt solution, can be used, but glycerine solution is recommended here because of its superior ability to maintain relative vapor pressure of water with limited changes in temperature. The relative water vapor pressure inside the jar can be determined by wet- and dry-bulb thermocouples, by refractive index of the solution, or by an electronic hygrometer. Once the relative humidity has been determined, the jar without test specimen should be weighed so that major changes in the condition inside the jar can be detected by subsequent reweighings.

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