With a hygrometer, you measure the humidity in a room. Most models also indicate the temperature in the room in addition to the humidity. You can use a humidifier to increase the humidity in a room. With a dehumidifier, you can reduce the humidity in a room. These products don't always have a hygrometer, so you need a separate hygrometer. You simply place a separate hygrometer in the room. This way, you can see at a glance whether you need to turn the humidifier or dehumidifier up or down to achieve the correct humidity.