A thermometer measures air temperature, and needs to be accurate. Your chicks' life depends upon it. A hygrometer measures the humidity and gives you a direct reading of the percentage of relative humidity in the incubator.

A wet and dry bulb thermometer combination can be used to measure the relative humidity. This is more complicated than using a hygrometer, as a conversion chart to find relative humidity is required. A wet bulb thermometer is a thermometer with a wick on the bottom sitting in demineralized water, measuring the temperature of the water in the incubator. A dry bulb thermometer is a regular thermometer, measuring the air temperature in the incubator. The wet bulb/ dry bulb temperature readings can be converted to actual humidity with the aid of the following chart.

Dry bulb reading in Fahrenheit (the actual temperature in the incubator), read left to right, 97 degrees to 102 degrees, on the top line of the chart. Wet bulb reading in Fahrenheit read top to bottom of first column of chart, 56 degrees to 102 degrees. Relative Humidity Reading is calculated by reading down the wet bulb column and across to the dry bulb column.