Thermohygrometer: Features, Functions, Parts, Uses

Why is it important to know the relative humidity of the environment? This is important for the following: if the relative humidity is low, the process of water evaporation will be favored, which can damage stored food, or dehydrate our skin.

A fixed thermohygrometer in the De Fundatie Museum. Source: FaceMePLS from The Hague, The Netherlands / CC BY (

On the contrary, if the relative humidity is high, the water vapor condensation process is facilitated. The water produced accumulates on fruits and other foods, favoring the growth of mold, with the consequent damage to them.

Historical background

Leonardo Da Vinci is credited with creating the first hygrometer in 1480. It took two centuries for the French physicist, Guillaume Amontos, to achieve the creation of a hygrometer that was later improved by Fahrenheit. In 1775, Joahnn Heinrich Lambert named a device designed to measure humidity as a hygrometer.

Thermohygrometer Features

Rotronic DV-2 thermohygrometer. Source: Harke / Public domain


Thermohygrometers are devices designed to determine the content of water vapor per m 3 in the air, in a given environment, and express it as relative humidity. In addition, they have a temperature measurement system, which makes it possible to relate the relative humidity of an environment with the existing temperature in it.

Thermohygrometers can be analog and digital. Analogs are simple devices in which your measurements are displayed on graduated spheres and needles. Its advantage is its simplicity, coupled with its precision and the ability to operate without electricity.

However, they have several disadvantages, among them: they only present current measurements, since they do not keep a record of past measurements that allow a sequence of changes to be established. And besides, these devices have few functions.


Digital thermohygrometers are electronic devices that are powered by batteries and information about relative humidity and temperature is displayed on a display that can be illuminated. Relative humidity is expressed as a percentage (%) of the maximum humidity, and the temperature is given in degrees Celsius or Fahrenheit.

The batteries of these devices can last up to 3 years and when changing them, the records made are not affected.

Thermohygrometer functions

Thermo-hygrometers allow you to measure the relative humidity and also the ambient temperature. The temperature is expressed in degrees Celsius or Fahrenheit, while the relative humidity in percentage (%).

Modern appliances have information about the dew point or temperature. This corresponds to 100% saturation with water vapor and therefore condensation in liquid water occurs. This information is important because the occurrence of this phenomenon in a closed environment must be avoided.

Some thermohygrometers work as barometers, that is, they have the ability to measure ambient pressure as well. This is important as pressure is inversely related to relative humidity.

Modern thermohygrometers have a built-in memory system that allows the storage of temperature and relative humidity records. This makes it possible to establish how these parameters vary as a function of time, and then this information is transferred to a computer for data processing.


A digital thermohygrometer. Source: Leandro / Public domain

In general, a thermohygrometer consists of the following parts: an LSD display, a device for measuring relative humidity, a device for measuring temperature, an alarm system, and a time-measuring system.

LSD display

The LSD display has information on temperature, relative humidity, time and date, etc.

Thermal and humidity sensors

The devices for measuring relative humidity and temperature are electronic systems equipped with sensors and integrators. These transform the stimuli into readings of temperature and relative humidity, expressed respectively in degrees Celsius or Fahrenheit, and in percentages (%).

Alarm system

An alarm system to warn of an exaggerated variation in some of the variables controlled by the thermohygrometer.

Time system

A time measurement system that reports the date and time.


Thermohygrometers, like hygrometers, have multiple uses that can be summarized as follows:


Thermo-hygrometers are used in museums where valuable works of art, old books, wooden musical instruments, etc. are stored, which are sensitive to an increase in relative humidity and the consequent condensation of water vapor. This moisture can destroy a very valuable material for humanity.

That is why in these places they are practically crowded with thermo-hygrometers in all the rooms.

Meteorological stations

Among other information, the thermo-hygrometer also allows to have an approximate knowledge of the heat index or caloric sensation.

For example, a temperature of 32.2ºC with a relative humidity of 70% produces a heat index of 40.5ºC. This is very dangerous for the health of human beings, and knowing this data can take the appropriate protection measures.


In the paint and coating industry, thermo-hygrometers are used to correct for humidity, as the paint or coating application is very sensitive to humidity and dew point.

In the cigarette industry, thermo-hygrometers are used to control humidity, because tobacco is very susceptible to humidity, which can damage it and reduce its quality.

This equipment is also used especially for the protection of the covers of the tablets, which are very sensitive to humidity.


Foods like grains and cereals are stored in large warehouses known as silos. They are equipped with thermo-hygrometers that allow them to control the relative humidity of the air. If this increases beyond a certain limit, condensation of water vapor may occur.

This condensation can lead to the generation of mold, which in turn will seriously damage stored food. Among the measures to be taken is to increase the ventilation of the silos, use dehumidifiers, etc.

Fire protection

By decreasing the relative humidity, the generation of fires is favored, since the materials dry out by increasing the evaporation of the water that forms them. By knowing the existing risk through the use of the thermohygrometer, fire protection measures can be taken, such as the installation of humidifiers.

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