Relative Humidity Meter

Humidity meters, also known as hygrometers, determine the current moisture content in the surrounding air. The amount of water vapor contained in the air not only affects human well-being and performance, but also the durability and functionality of many products. Since many materials absorb moisture from the ambient air and can release it to it, the air humidity also influences the material moisture. Therefore, when it goes about materials and devices, there are exact values for the air temperature and for the air humidity that are specified for storage and application. Too much moisture can cause chipping, mold or bacteria, and corrosion. Too low moisture values can make processing more difficult and cause drying cracks in some materials. The humidity can be determined quickly and easily with an air humidity measuring device. If there are deviations from the desired values, it is possible to take targeted measures to change the air humidity.

Humidity meters show the humidity as an absolute or relative value. The absolute air humidity is usually given in g / m³, so it shows how many grams of water vapor are contained in one cubic meter of air. The maximum amount of water vapor that can be absorbed from one cubic meter of air increases with the temperature. Therefore, the relative air humidity always depends on the current temperature. It specifies the ratio of the available water vapor content to the maximum possible water vapor content (saturation amount) under the current temperature and pressure conditions in percent. The relative humidity is often abbreviated as rH or RH. A water vapor quantity of 8 g / m³ as absolute humidity corresponds to:
at 10 degrees Celsius 8 g / m³: 9.4 g / m³ = 0.8511 = 85.1% relative humidity,
at 15 degrees Celsius 8 g / m³: 12.8 g / m³ = 0.6250 = 62.5% relative humidity,
at 20 degrees Celsius 8 g / m³: 17.5 g / m³ = 0.4571 = 45.7% relative humidity,
at 25 degrees Celsius 8 g / m³: 23.0 g / m³ = 0.3478 = 34.8% relative humidity.

To prevent any damage to the moisture-sensitive materials, the formation of condensation on these materials must be avoided. Since the warm air can absorb more water vapor than the cold one, condensation occurs on cold surfaces when warm air cools down. This effect can be often seen on mirrors after a long hot shower and on glasses when someone enters a warm humid room after being in a cold environment outside. Even in tents and pavilions, where warm air rises to the cold tent roof, condensation often forms on cool evenings. From which surface temperature there is a risk of condensation formation depends on the current air temperature and humidity. Many humidity meters can indicate the corresponding dew point for the measured temperature and humidity. The dew point is the temperature below which the water vapor becomes dew or fog.

Humidity meter in living rooms
Humidity meters are often used for living rooms, in which the measured values are read directly by the people present in the room. If the measured humidity values are too low or too high, the humidity can be increased or removed by airing or ventilation. If there are moisture problems on the exterior components of the apartments, the heating and ventilation behavior of the residents is often checked with the help of the humidity measuring device that can record the data. These humidity meters store the measurement time, temperature and humidity at fixed intervals. The data show when which fluctuations occurred. To evaluate the measurement data, the statements of the residents regarding the use of the space and the outside climate should also be taken into account when the problem arises. As a rule, moisturizing caused by the condensation failures can first be identified on the geometric or material-related thermal bridges. To avoid this, the component temperature must be kept above the dew point. The dew point can be determined in the humidity measuring device from the values for temperature and humidity. In addition, some devices have an additional sensor for measuring the surface temperatures, with which the wall or ceiling temperature and the dew point temperature can be compared directly.

Humidity meter in rooms with high water vapor
The ventilation of the rooms with high water vapor such as bathrooms, shower rooms, kitchens can also be controlled depending on the existing air humidity. For example, there are fans with the integrated air humidity sensors for venting the indoor bathrooms, which extract the air from the room only when the selected limit value is exceeded.

Humidity meter in transport containers
Goods are often transported over long distances and under fluctuating climatic conditions. A thermo-hygrometer with storage function placed next to the goods to be transported records the climatic conditions during the transportation. The changes in temperature and humidity values during the transportation can be detected and evaluated. If the goods show damage at the destination, the data stored in the humidity measuring device can be used to check whether the climatic conditions during the transport were the cause for the damage. If necessary, the goods can be protected by a more suitable packaging during the next transport.

Humidity meter in greenhouses
The optimal air humidity in greenhouses has a major impact on the healthy growth and yield of the crops. Pest infestation may occur if the moisture content is too high or too low. The growth stops when the humidity is too low and if the humidity is too high, no fruit can be formed because the pollen clumps, and no fertilization takes place. The values can be easily monitored with an air humidity measuring device. If you choose the humidity measuring device with a signal output, it is possible to control ventilation and irrigation systems depending on the measured humidity values.

Humidity meter in museums and archives
Irreplaceable cultural assets and files are endangered if stored incorrectly. To avoid the damage caused by moisture and to ensure the desired indoor climate conditions, museums and archives are continuously monitored by means of the humidity meters. The air humidity measuring device either records the measurement data, which can be checked by the responsible for the task persons, or it passes the data on further directly to the devices for climate control.

Measuring range and accuracy
Many of the available humidity meters cover the range from 0 to 100 percent relative humidity. Just a few also specify the humidity as an absolute value in grams per cubic meter. The accuracy when it goes about the relative air humidity in the peripheral areas is often worse than in the central area. Depending on the application, the attention much be paid, that the humidity measuring device provides the desired accuracy in the climatic area to be monitored and that it can be adjusted to the correct value in the event of deviations.

Data storage and data transmission options in a humidity measuring device
The continuous recording of the values for temperature and humidity with the air humidity meter is important for the proof of compliance with storage and operating conditions. The heating and ventilation behavior in the apartments and in the workplace can also be verified with the stored measurement data from a humidity measuring device. There are data loggers with built-in internal memory that are read out via a USB interface or devices with storage on an SD card. Some humidity meters transmit the measured values via interfaces for Ethernet, RS485 or Modbus RTU directly to the other devices. Also humidity meters with data output via analog interfaces with 4 to 20 milliamps or 0 to 10 volts are available. Some humidity meters offer connection options for visual or acoustic alarm devices or have already built-in alarm devices.

Additional measuring and calculation functions in the humidity measuring device
Since the relative air humidity is always related to the current air temperature, all air humidity meters must also record the current temperature. The humidity measuring device that also shows the temperature is called a thermo-hygrometer. In many devices, due to the stored functions, the dew point can be determined from the temperature and humidity. If the air humidity meter also has an infrared temperature sensor or an external temperature sensor, it can be measured directly whether the dew point on the endangered surfaces has been reached or has fallen below. For climatic studies there are devices that also calculate the wet bulb temperature and the enthalpy.

Humidity meters for air humidity are used wherever it goes about the monitoring of climatic conditions in the environment. Humidity meters for this application are available in various designs and types, depending on the case and area of application. Moisture meters are therefore widely used.

Depending on the application, the right moisture meter is selected. There are humidity meters with memory such as the PCE-313A or the PCE-HT 110. Then portable devices with lower accuracy and no memory are available, such as as the PCE-555 humidity meter. For accurate applications in research or calibration, the PCE-330 is used.
Furthermore, various types of additional equipment, such as pyrometer or connector for another sensor are available.

In addition to humidity meters for mobile and temporary use, the humidity meters or sensors for stationary applications can be found in the range of our products. These sensors transmit the measured value by bus or standard signal to a process control unit or data logger. The meters of this type are often mounted on the wall. The aim of this sensor system is to monitor the climatic conditions, for example, of a warehouse.
For many humidity meters, there are various filters available which are used to protect the sensor element. There is a sintered metal filter, which lets the humidity pass through itself, but prevents liquid from coming onto the sensor and destroying it. There are also PTFE filters which protect against chemical contamination.
For industrial applications, humidity meters for special applications are available. These have heated sensor elements that protect against condensation and make the sensor usable in the high humidity range.

The monitoring of the relative humidity in the storage of goods plays an important role. Certain goods and products may only be stored under particular environmental conditions. An air humidity meter helps the warehouse operator and the quality assurance contractor to ensure safe and correct warehousing. This air humidity measuring device can also be a portable hand-held meter as well as a permanently installable precision instrument. A humidity meter for fixed installation in the warehouse is installed at certain points. Display of the air humidity, detected by the humidity meter, can be carried out in various ways:

- Display of the humidity on a small installed display, in the control cabinet or on site
- Display the relative humidity on a large display
- Placement of the measured value in the intranet of the company

Furthermore, the measured values can be stored. Particularly in the companies which have to document the storage conditions due to ISO certifications, a permanently installed humidity meter in the warehouse with data logger is absolutely requisite. Besides the visualization of the measured values, a humidity meter also provides the user with the possibility to have a complete documentation of the conditions in the warehouse. After storage, the measured values can be converted into various data formats such as Excel, CSV, etc.

All humidity meters are delivered calibrated. However, since the humidity sensors tend to drift in time, they should be recalibrated continuously in order to always measure exact values. You can either carry out the recalibration with the optional calibration tools and standards yourself, or you can send the humidity meters for ISO calibration.

Below you may see two pictures of the humidity meters in practical use. In the picture on the left there is one of the measuring devices during the humidity and temperature measurement in the officer, on the right – a picture of one of the humidity measuring devices during the long-term recording of the room climate (measuring instrument mounted on a tripod)

About Relative Humidity: Relative Humidity (RH) is the ratio of absolute (actual) humidity to saturation. The relative air humidity is measured with a humidity meter with the display from 0% (completely dry air) to 100% (completely saturated air, such as fog, clouds or water vapor bath) (not all models of humidity meters, some have got a narrow measuring range). The physiological wellbeing range is in the range of 40 ... 65% RH.
Since warm air has the property of absorbing more water vapor than cold air, the air in winter is too dry for a person, and in summer – sultry. If the air is heated, the amount of water vapor which it would have at saturation is increased. This has the consequence that the % value of the RH is lower, even if the actual amount of water remains constant. Similarly, when the air is cooled, the amount of water vapor decreases. This increases the percentage RH value. Condensate forms with an excess of water in the saturated ambient air. Some of our humidity meters offer, besides the pure measurement of the relative humidity and temperature, the calculation of the dew point or the dew point temperature.

Dew point temperature is defined as the temperature at which the current water vapor content in the air is at a maximum level (100% relative humidity). The dew point temperature is therefore a variable independent on the current temperature. One way to measure the dew point temperature is to cool the metal down, till the water vapor covers the surface. Then, the temperature of the metal is the dew point temperature. As already mentioned, many of our humidity meters show the dew point directly on the display.