To make your own wet/dry hygrometer, you'll need a pair of bulb thermometers---the kind that have a reservoir of liquid at the bottom, which flows into a narrow, graduated glass tube. Ideally, they should be a matched set, so there as to eliminate the chance that differences in the designs will affect the readings. Make sure the bulbs have enough room around them to let the air circulate freely and, if the incubator has a fan to circulate the air, that they are placed outside the direct flow of air from the fan blades.

Make the holder out of any nonporous material such as wood, metal or plastic. One easy way to do that is to cut one side and the bottom from a plastic milk jug so that the base holds up the side at a 90-degree angle. Then, punch a pair of holes in the side about a half-inch to an inch from the bottom just large enough to put the thermometer bulbs through without forcing them. To hold the thermometers even with the base, use a small block of wood to rest the ends on or cut a second piece of plastic from the jug and cut a set of holes in it, then tape the plastic upright with a strong, waterproof tape like duct tape.

Once you have the thermometers mounted, put a snug fitting 3- to 6-inch length of cotton tubing over one of the thermometer bulbs. Dip the tube in water to wet it before installing it, or dribble some water on it when it's in place so that it is uniformly damp but not dripping wet. Hang the loose end of the tube into a container of water. A small bowl or a jar lid will do for the water reservoir but it should be large enough to not run dry or get too low. Its sides should be close to the height of the thermometer bulbs so the tube is not kinked, otherwise it might not wick up the water as well and the bulb end might not stay wet enough for the hygrometer to remain accurate.