A thermometer hygrometer is a device that is used to measure both the current air temperature and the relative humidity at a particular location. The temperature is typically measured in degrees Fahrenheit or Celsius, and the unit might show the indoor temperature, outdoor temperature, or both. Relative humidity refers to how saturated with water vapor the air is, and it is usually shown as a percentage. A temperature hygrometer will commonly be either a dial instrument or a digital one.
Air temperature is a measurement of the level of sensible heat in the atmosphere. The higher the temperature, the more heat in the air, and the warmer it feels. A thermometer hygrometer has the ability to quantify how hot or cold it is in that location. Some digital models have a wireless sensor to measure the temperature at a remote position from the main unit. For example, if the main unit is indoors to measure the temperature there, it might also have a wireless sensor that can be placed outdoors to record the temperature outside as well.
A hygrometer shows the amount of relative humidity where the temperature hygrometer is located, and possibly at a remote location if the model includes a wireless sensor. Relative humidity is determined by the amount of moisture in the air versus how much the air is capable of holding at that specific temperature. Since hot air can contain more water vapor than cold air, the same amount of water vapor in the air would cause a higher relative humidity at a low temperature than it would at a high temperature. A relative humidity of 100% would mean the air is fully saturated.
A dial version of a thermometer hygrometer will have one dial for measuring temperature and one for relative humidity. A bimetallic coil is generally used for a dial thermometer. It is sensitive to changes in heat and controls the needle on the face of the dial, which indicates what temperature it measures. The dial hygrometer portion typically uses a hair that grows or shrinks with changes in air moisture. It is attached to a lever that moves the needle on the dial to show the percentage of relative humidity.
Digital models of a thermometer hygrometer are often handheld units that run on batteries and have a screen to display the measurements. The electronic thermometer has a sensor that changes resistance with changes in heat, and a circuit converts the resistance into a temperature. An electronic hygrometer uses a sensor that also changes electrical resistance, but the change is caused by the amount of moisture in the air. Both the recorded temperature and relative humidity are shown as numbers on the screen of the unit, and it usually has the capacity for a greater temperature range than the dial models. It might display both the indoor and outdoor measurements if it has a wireless remote sensor that is placed in the opposite location.
A thermometer hygrometer is useful for various tasks. For example, it is used by mold remediation technicians and home inspectors to determine air quality. Also, people who grow plants in a greenhouse will often use one to make sure that the temperature and humidity is ideal for what they are trying to grow.