Data Logger for Temperature and Humidity

The term “log” stands for logging or recording. An electronic device called a logger records the respective measured variables with the integrated or connected sensors at regular intervals and saves them for later evaluation. Humidity and temperature loggers measure values ​​for temperature and humidity at fixed time intervals and saves them or forwards them directly to external storage. It is customary to specify the humidity as relative humidity, i.e. in relation to the maximum humidity content at the current temperature.

Types of humidity and temperature data loggers for mobile or stationary use
There are many different types of humidity and temperature loggers. Mobile versions without a display that cannot show any values ​​are usually very small and can record data at the measuring point over many days. Power is often supplied by a battery that can be recharged via a Mini-USB interface. Humidity and temperature loggers for stationary use are often permanently installed and supplied via mains voltage. For use at particularly high or low temperatures, humidity and temperature loggers are usually equipped with external sensors. As a result, the electronics of the evaluation unit do not have to be protected separately and the device is more easily accessible for the user. The spatial separation of the sensor and evaluation unit also enables the values ​​to be read out easily outside of the monitored area. On the one hand, this increases the ease of use when querying data. On the other hand, it prevents the measurement data in sensitive areas from being influenced by the presence of people. Humidity and temperature loggers that record material parameters, for example of bulk goods, also work with external sensors. If the loggers are used outdoors or in other areas where they can come into contact with splash water, a high degree of IP protection must be ensured.

Measuring range, resolution and accuracy
As with all measuring devices, care must be taken with the humidity and temperature logger that the measuring range, resolution and accuracy meet the requirements of the measuring task. For a data logger for living rooms, offices or archives, a temperature range of 0 to +50 ° Celsius and a humidity range of 10 to 90 percent relative humidity are generally sufficient. For monitoring other areas, e.g. cold stores, drying chambers, production lines, there are humidity and temperature loggers with a more extensive measuring range, which record temperatures from -40 to +85 ° Celsius and humidity values ​​from 0 to 100 percent, for example. Even if the measured values ​​are usually output with a decimal point, it must be ensured that the accuracy for temperature and humidity is specified in the specifications of the devices. When it comes to temperature measurement, the accuracies of the individual loggers differ considerably. With conventional devices, the bandwidth ranges from 0.3 °C to 0.5 °C, 0.6 °C, 0.8 °C up to 1.0 °C and worse. When it comes to the accuracy for the humidity values, there are only a few devices where the accuracy is specified as better than 3 percent. One of these humidity and temperature loggers with higher accuracy offers 0.2 ° Celsius for the temperature and 1.8 percent for the humidity with a measuring range of 10 to 95 percent. The devices can be calibrated to ensure that the specified accuracies are still achieved even after a long period of operation. If the accuracy deviates from the specified specifications, the device-specific accuracy can be achieved again for many devices by adjustment.

Storage rate, storage space, recording duration
Almost all humidity and temperature loggers allow the storage interval to be set. Depending on the purpose of the recording, storage rates of every second, hour, daily and sometimes even longer intervals can be useful. The shorter the selected time interval, the faster the available memory locations are occupied. Some humidity and temperature loggers for which the setting is made on the computer immediately display the maximum recording duration for this time interval when the storage rate is selected. The user can see immediately whether the value is correct or needs to be changed. Special humidity and temperature loggers offer the possibility of saving the measurement data not at fixed time intervals, but only when the value changes. With these, for example, the data record is only automatically saved if the temperature changes by ± 1 ° Celsius or the relative humidity by ± 1 percent. The maximum recording time with a battery-operated humidity and temperature logger is not only limited by the storage locations, but also by the battery life. Particularly in the case of a mobile data logger that is not always accessible for reading, it must be ensured that the storage rate, storage space and battery life are sufficient for the intended measurements.

Humidity and temperature logger with dew point determination
Some of the data loggers also determine the current dew point temperature from the measured temperature and humidity. On surfaces with or below this temperature, the air cools down and water vapor becomes liquid water. This happens because cool air can hold less water vapor than warm air. This fogging with moisture can be seen particularly well on surfaces that do not absorb moisture such as glass, metal, tiles. Humidity and temperature loggers with displays often show the value for the current dew point directly. For all devices with dew point determination, however, the dew point is specified for each data record consisting of temperature and humidity. During the data evaluation, the dew point curve can be shown in a graphic together with the temperature and humidity curve.

Alarm functions for a humidity and temperature logger
Many stationary data loggers, but also some mobile devices, can issue alarms when limit values ​​are reached. The upper and lower limit values ​​for temperature and humidity must be set beforehand by the user according to his/her requirements. In the event of an alarm, room users present in living or working rooms can take countermeasures immediately if a value is outside the defined range. Acoustic alarms require less attention than optical ones. For the effectiveness of a visual alarm via LED lights or displays, the humidity and temperature logger must be placed so that it can be seen and also checked. More complex devices, for example for process monitoring or for cold storage, have interfaces via which signals can be passed on to other devices if limit values ​​are exceeded.

Additional measured variables for humidity and temperature loggers
For certain applications it is advisable to record other parameters in addition to temperature and humidity, or to calculate additional values. In addition to the dew point, special humidity and temperature loggers calculate, for example, the absolute humidity content, the cooling limit temperature and the enthalpy. The additional parameters that are recorded by some climate loggers include the air pressure, the illuminance, the amount of precipitation and the intensity of vibrations.

Options for data transmission with a humidity and temperature logger
Many mobile humidity and temperature loggers can be read out via a USB interface. Other models save the measured values ​​on an exchangeable SD card. There are also loggers available that can transfer the measurement data directly via USB or RS232 interface, Ethernet, WiFi or Bluetooth, so that they are directly available elsewhere. These data loggers with remote transmission are particularly suitable for monitoring sensitive areas that are only entered when necessary or controlled from outside. Since acoustic or visual alarms do not work directly on the logger, there is a special type of notification by e-mail to specified recipients when limit values ​​are reached.