Hygrometer

The term hygrometer is composed of the Greek words hygrós and métron, which mean wet and measure. According to the origin of the word, a hygrometer is a measuring device for determining the amount of moisture. However, the term hygrometer is not generally used for moisture measuring devices, but specifically for devices for determining the relative humidity of the air. Because humidity plays a decisive role in many biological, physical and chemical processes, it is measured and, if necessary, specifically controlled. Almost everyone is familiar with hygrometers for displaying the air quality in homes and offices, in warehouses and production rooms, and at weather stations for measuring the outdoor air. Different abbreviations are commonly used for relative humidity, which are often also used on the display to identify the measured values. These include rel. F., r.F. and, following the English "relative humidity", r.H. and RH.

Absolute humidity (water vapour content of the air)
Absolute humidity is expressed in grams of water vapour per cubic metre of dry air. It can be increased by inputs from the environment such as respiration and evaporation, but it can also be reduced by the sorption capacity of the materials in the environment. The specification of the water vapour content in grams per kilogram of dry air is called the degree of humidity.

Relative humidity
A relative value always refers to the comparison of two quantities and is often given as a percentage. With relative humidity, the actual amount of humidity is not compared with the amount of dry or humid air, but with the maximum amount of water vapour that the air can contain under the respective conditions. This results in the maximum value of the relative humidity to be measured being 100 percent. In contrast, much higher values of relative humidity are possible and measurable for material humidity where the water content is related to the dry substance of the material. The maximum amount of water vapour that can be absorbed by air is called the saturation amount.

Since this saturation quantity is dependent on temperature and pressure, the relative humidity changes with changes in temperature or pressure even without water vapour being added or removed. Water vapour quantities above the saturation quantity are released as liquid water. The fluctuations in atmospheric air pressure have a relatively small influence, but should be taken into account, especially for highly sensitive measurements and when calibrating hygrometers. In compressed air applications, however, the relationship between humidity and air pressure plays an important role.

Dew point or dew point temperature
Warm air can absorb more water vapour than cold air and more at low air pressure than at high air pressure. If air containing water vapour is cooled down to such an extent that the relative humidity reaches 100 percent, any further reduction in temperature causes liquid water to be formed from the excessive water vapour. This water is called condensation. The temperature at which 100 percent relative humidity is reached is called the dew point temperature. Since warm air is also cooled on cold surfaces, condensation can also form on cold surfaces in warm rooms. This effect is easy to observe when spectacle wearers come from the cold into warm rooms and condensation forms on their glasses. The surface temperature at which condensation forms depends on the current air temperature and humidity. Many hygrometers can also indicate the corresponding dew point for the measured air temperature and humidity.

Table 1: Dew point at normal air pressure depending on air temperature and air humidity.

Because of the large number of models available, the minimum requirements that the hygrometer must meet should first be defined. With these minimum requirements, it is often useful to give the priority to the purpose for which the hygrometer is to be used.

Purpose and conditions of use for the hygrometer
Depending on the intended use, there can be completely different requirements for the hygrometer. Examples of different applications are:

• Measuring the current air humidity indoors
  The air humidity in living and working spaces has an effect on well-being and performance. Therefore, hygrometers are often used to detect too high or too low humidity values and to ventilate as needed. These hygrometers should have an easy-to-read display for the user and possibly additional optical or acoustic warning functions.
  The hygrometers are also used in cases of moisture damage indoors to clarify whether the humidity is one of the causes. The hygrometers that can also be used to determine the dew point and the surface temperature of the building components are particularly suitable for this purpose.
• Measuring air humidity outdoors
  The air humidity in outdoor areas is not only important for precipitation forecasts, but should also be taken into account for the assessment of physical strain during sports and outdoor work. That is why the hygrometer belongs to the measuring instruments of almost every weather station. The sensor's measurement data is often transmitted to the indoor display units or other devices and displayed, stored and evaluated on them.
• Measuring and storing the humidity over long periods of time
  The hygrometers are often used as data loggers, for example to document heating and ventilation behaviour in dwellings with moisture damage or to record humidity and temperature during transport or in the difficult-to-access locations.
• Measuring and controlling humidity in production and storage rooms
  For some products, the correct air humidity during production and storage plays a decisive role for the quality. Both too low and too high air humidity can render products or materials unusable. Therefore, hygrometers are used in these areas to continuously monitor the humidity and display or transmit the data.

Measuring range and accuracy of the hygrometers
Not all hygrometers can completely cover the range from 0 to 100 percent relative humidity. For most purposes, however, this is not necessary. It is important to note, however, that the accuracy of the hygrometers is usually not constant over the entire measuring range. Pay attention to how the accuracy is stated and whether it is sufficient for your application.

Additional measured variables and calculated values in the hygrometer
Almost all digital hygrometers measure not only the relative humidity but also air temperature, i.e. they are thermo-hygrometers. Many models also calculate and display the dew point from the relative humidity and air temperature. Some hygrometers also display the absolute humidity in grams per cubic metre or the wet bulb temperature. The hygrometer with an additional infrared sensor or surface temperature sensor can also directly measure the temperatures of the component surfaces. Other types of devices have additional sensors with which the air pressure or additional air quality values such as the CO2 content, the particle quantity or the formaldehyde content can also be determined.

Storage options for the hygrometer
With the hygrometer that is to be used as a data logger, it is often important over what period of time the data can be recorded. The number of the measured values that can be stored is limited, among other things, by the storage capacity. For long-term measurements, it therefore plays a major role which time cycles can be set for the individual measurements. In the case of the hygrometer that is battery-operated, the battery life also plays a role in determining the maximum period of time for which the data can be recorded. Some hygrometers store data on the exchangeable micro SD cards, the others are small and compact like a USB stick and can be connected to the USB port of a PC or laptop directly to read out the memory.

Hygrometer interfaces
Hygrometers can be equipped with interfaces to transfer currently measured or stored data to a computer. Often there is special software for the device that simplifies the evaluation of the measurement data and the configuration for the next measurement. The hygrometers for sensitive areas in industry or logistics should also often have analogue or switching interfaces so that the processes can be controlled automatically.

The functional principle and construction of a hygrometer are not only decisive for the temperatures and humidity values at which the devices can be used. They also have a decisive influence on whether the hygrometer can be used to determine other climatic values in addition to the relative humidity.

The hair hygrometer as an absorption hygrometer
Many materials can absorb moisture as water vapour from their environment or release it to it until humidity equilibrium is reached. If they change their properties in the process, this can be used to measure the relative humidity indirectly. Depending on the structure of the material, the change in humidity can, for example, change its expansion to a greater or lesser extent. Degreased hair is well suited for measuring humidity because the change in length due to humidity is not significantly influenced by temperature.

In order to be able to read the humidity conveniently, this type of analogue hygrometer is equipped with a pointer and scale. While human, horse or sheep hair was always used for these hygrometers in the beginning, many are now equipped with synthetic material. Hygrometers with real hair react quickly and can be used at temperatures from -35 to 65 degrees Celsius. However, if they are primarily used in dry indoor rooms, they must be regularly regenerated at high humidity levels. Otherwise the hair will dry out too much. With the common hair hygrometer, only the relative humidity is recorded.

Absorption hygrometers with polymer sensors
Some plastics also react in a measurable way to the relative humidity of their environment. With the polymer sensors that are widely spread today, the air humidity can be determined directly via the change in electrical resistance or electrical capacitance. In most cases, these compact digital hygrometers are equipped with an additional component for measuring temperature. From the measured values for temperature and relative humidity, the appropriately equipped hygrometer can also determine the absolute humidity and the dew point.

The centrifugal hygrometer as an aspiration hygrometer
The ambient temperature drops when water evaporates. You can feel this on your own body when you are out and about with wet clothes for a long time. Sweating in the summer heat and in the sauna is an active cooling process to regulate the body temperature. The drier the ambient air, the more effective the cooling process. If the air enriched with moisture is carried away from the source of moisture by wind, dry air can quickly flow in and take up the moisture again. To determine the cooling effect of evaporation, the temperature can be measured on a dry and a humidified thermometer respectively.

Since the temperature difference is directly related to the relative humidity of the environment, the humidity can be determined from the wet and dry temperature. Hygrometers that work according to this principle are also called psychrometers. When used correctly, these hygrometers have an accuracy of 0.5 to 1.0 percent and are thus very precise. However, this requires the humid thermometer to be constantly surrounded by air. The most common types of the hygrometers in which the humid air is removed include the aspiration psychrometer according to Assmann and the centrifugal psychrometer. While in the Assmann hygrometer the air change is generated by an integrated fan, the centrifugal hygrometer is actually centrifuged.

Hygrometers that use the evaporation effect to measure relative humidity always also determine the air temperature as the temperature of the dry thermometer and the wet bulb temperature as the temperature of the humidified thermometer. This value indicates, for example, how far air with the current values can be cooled down because of the evaporation.

The dew point mirror hygrometer
If the humid air is cooled, the relative humidity rises although no water is added from outside. Since the saturation amount is lower at low temperatures, the cold air can hold less water vapour and releases the excess water as mist, rain or dew. This effect can be used to measure the relative humidity. To do this, a small mirror is cooled until dew water settles on the cold surface. Then the relative humidity is determined from the temperature difference between the mirror surface and the ambient air. With the hygrometer of this type, not only the air humidity but also the air temperature and the dew point temperature are determined at the same time.

Spectrometer as hygrometer
Spectrometers measure light. If a spectrometer uses a wavelength that causes water molecules to vibrate, a part of the light is absorbed and less light reaches the photocell. From the strength of the attenuation of the light intensity by the water molecules, the absolute humidity of the gas can be determined with this type of the hygrometer. With the suitable spectrometers it is also possible to measure very low humidity values in gases, for example, concentrations of only 2 to 10,000 ppm - i.e. less than 10 millilitres of water per litre of air.

The hygrometers whose measurement values serve as a basis for regulating the indoor climate or for monitoring and controlling technical processes should be calibrated regularly. If during the calibration it comes out that the hygrometer does not achieve the desired accuracy in the measuring range relevant to the application, it must be adjusted or replaced.

Some hygrometers can be calibrated with humidity references from saturated salt solutions. In the containers with the salt solution, characteristic humidity values appear in each case as equilibrium humidity. With these references it is possible to check the hygrometers whose capacitive, resistive or electrolytic sensors can be inserted into the measuring chamber. The inserted sensor must be sealed for the measurement in such a way that no ambient air enters the container. These references can only be used at temperatures of 20 to 40 °C and are not suitable for the hygrometers with thermal measuring principles.

In calibration laboratories, the hygrometers are often calibrated together with a tested reference instrument of higher accuracy in the climatic cabinets. The reference instrument provides the nominal values to which the measured values of the instrument to be tested are compared. When the hygrometers are certified and laboratory calibrated, a test certificate is issued with the customer's address data so that the customer can include the calibrated instruments in his in-house ISO test equipment pool. The ISO calibration certificate confirms the determined accuracy and describes the conditions of the calibration. Here is some more information on terms related to the calibration:

Calibration
Checking the accuracy of the measurements of the hygrometer without interfering with the measuring system or determining the systematic deviation of the reading of the measuring instrument from the true value of the measurand.

Calibration certificate
The calibration certificate documents the metrological properties of the measuring instruments as well as the traceability to the national standard.

Calibration interval
In order to be sure that the determined measured values are permanently correct, the measuring instruments used must be monitored or calibrated at regular intervals. The period between calibrations is called the calibration interval. There is no generally binding definition of when a measuring device must be recalibrated. Important points in determining the interval are:

- the measurand and the effects of incorrect measurements
- the stress on the measuring instrument
- the frequency of use
- the environmental conditions
- the stability of the previous calibrations
- the required measurement accuracy
- specifications of the company's quality assurance system

This means that the interval between two calibrations must ultimately be determined and monitored by the user. When purchasing new hygrometers, care should be taken to ensure that they are as easy to calibrate as possible. High-quality hygrometers that are used for important measurements should also be able to be adjusted with the help of the calibration values.

Change in relative humidity depending on air temperature
At normal air pressure, a water vapour quantity of 10 g/m³ leads to the following relative humidity values:

If the air temperature and humidity are known, this diagram shows how far the air can cool down before condensation forms. The temperature of the room air, the relative humidity and the temperature on cold surfaces are measured. Some hygrometers can measure both the air temperature and the temperature of the surfaces via contact sensors or infrared sensors in addition to the air humidity. With these measured values, it is possible to read-out from the diagram how far the air humidity rises when it cools down.

Situation 1:
Room temperature 20 °C, humidity 40 %, lowest surface temperature 14 °C
Starting from the point of intersection of the 40 % humidity curve with the horizontal 20 °C line, a vertical line is drawn up to the horizontal 14 °C line. The intersection of these lines is between the 50% and 60% humidity curves. The vertical line can be drawn further up until it meets the curve for 100 % humidity. With this point, the dew point temperature of about 6 °C can be read on the temperature scale.

Situation 2:
Room temperature 20 °C, humidity 60 %, surface temperature 11 °C
The humidity rises above 100 % on the colder surface and condensation forms. The dew point is about 12 °C.

Change in relative humidity depending on air pressure
If the air temperature remains constant, the relative humidity and the dew point change with an increase in pressure. Here are example values for an air temperature of 25 °C:

The term hygrometer comes from the Greek words hygro (meaning “wet, moist; moisture”) and metron (meaning “a measure”). Thus, a hygrometer is a device used to measure moisture in the air, i.e., humidity. In modern measuring technology, a hygrometer often is used in combination with a temperature measuring instrument. The measured values for relative humidity and temperature can be used to in tandem to measure dew point or water vapor content. Frequently, a hygrometer also is mistakenly referred to as a hydrometer. However, these two types of testing devices do not have much in common. In contrast to the hygrometer, which measures humidity, a hydrometer measures the density or a specific weight of a liquid.

Humidity, also called relative humidity or air humidity, is defined as the amount of water vapor in the air. However, humidity is relative. It is normal and even important that air contains water vapor. The amount of water vapor in the air depends on several factors, including the temperature of the air. Warm air is generally more humid than cold air. This means that when air temperature decreases, the maximum amount of water the air can hold also decreases. A hygrometer displays readings in a percentage of relative humidity (% RH). So, if a hygrometer displays a measuring result of 20% RH, then the air holds 20% of the maximum amount of water possible at that temperature.

When the air is humid outdoors, water and sweat evaporate more slowly. Fog, dew and precipitation become more likely. Indoors, high humidity can trigger symptoms for those suffering from allergies, asthma and chronic obstructive pulmonary disease (COPD). Excessive condensation also can form in overly humid indoor environments, resulting in the growth of mold and bacteria. However, when the indoor air is too dry, skin becomes itchy and dry and can even crack. Some people also experience problems with nose bleeds in very dry climates.

When the air is too humid indoors, it can sometimes be enough to simply open some windows and doors. Other times, the performance of the HVAC system should be evaluated by a trained professional. In extreme cases, a more complex measure must be taken, like the installation of an extractor fan. In many instances, a reliable hygrometer can be used to identify the root of the problem.