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HP-GTS – the solution for the paper and printing industry

ROTRONIC relative humidity and temperature meter - for the paper industry

From our partner ROTRONIC we present HP-GTS – the solution for the paper and printing industry, for the precise measurement of humidity and temperature, and the related equilibrium relative humidity in the paper and printing industry.


Discussions on environmental impacts during paper storage, transport, printing and finishing have existed since the beginning of industrial paper production. Generations of professionals have had to cope with major negative impacts in order to monitor and control temperature and humidity. Despite years of testing, some relationships are still not sufficiently clarified, if at all. There is still a lot of controversy in the practice of paper converting, in particular about the relationship between the atmosphere, paper and printing. In modern production facilities, paper intended for offset printing is prepared at a relative humidity of 50 % ± 5 % and at 40 % ± 5 % for digital printing. These humidity characteristics must be continuously monitored at all stages of the production process.

To keep the paper in optimal condition for the printing process, it is wrapped in a special packaging material that protects it from environmental influences and changes. The paper manufacturer has minimal influence on how the paper bundles or fibre gaps will react to the environmental conditions in the premises of the printers, bookbinders or end-customers.


Under normal printing conditions, there are problems related to direct atmospheric conditions. When printing problems occur, such as duplication of dots, incorrect entries, overlapping, curling, edge bleed, wrong dimensions, these atmospheric effects become significant. This is more likely to happen in seasons with extreme weather conditions – hot summers and cold winters. The latter causes poor print quality, especially in the levelling process. This has a negative impact on production processes, such as incorrect entries or interruptions due to breaks.

Room temperature and humidity measurement

The value describing the evolution of relative humidity as a function of temperature depends on the accuracy of the measurements made. Here, the temperature measurement must be very precise. This means using thermometers that read in tenths of degrees.

Although accurate temperature measurements are relatively simple, the same cannot be said for reliable measurements of absolute or relative humidity. The practical problem with all humidity measurements is that they are made on relatively small amounts of moisture.

At a temperature of about 20 °C, an increase in moisture content of about 2 g/m³ causes an increase in relative humidity of about 10%.

Measuring the equilibrium relative humidity of paper

As a hygroscopic material, paper strives to achieve equilibrium with the humidity of its surroundings. For this reason, the humidity of the air inside the paper structure must be equal to the humidity of the paper bundle. This value indicates the degree of equilibrium between the relative humidity of the paper and the ambient air. The equilibrium relative humidity ( ERH ) can be defined as the value of the relative humidity of the atmosphere at a given temperature at which the paper will neither gain nor lose moisture. If there is a difference between the relative humidity (% RH) of the atmosphere where the paper is located and the ERH of the paper, water vapour exchange can occur. This results in a change in the ERH and moisture content of the paper, which affects the quality of the paper. If the % RV and ERH are equal, no exchange will occur and all parameters will remain stable. In this respect, ERH is a very important factor in ensuring paper capacity and quality.

To determine the relative humidity of the air or the equilibrium relative humidity of the paper, it is necessary to measure the change in a measurable humidity-dependent parameter. Such parameters affected by moisture fluctuations include, for example, changes in the length of paper fibres, the conductivity of electrolytes and changes in the resistivity of semiconductors. In the practice of paper production and converting, these methods are widely used in measurement and control systems. The methods used to calibrate humidity instruments are very accurate and are based on the principle of measuring the dew point and determining psychrometric differences.

Interaction between moisture and the atmosphere

By measuring the relative humidity and the atmospheric temperature, it is possible to determine whether the quality of the paper will change after unpacking, thereby adversely affecting the planned printing. A difference between ERH and external humidity of up to ± 5 % RH is acceptable. When the humidity difference is too large (> 8-10% RH), water vapour is exchanged too quickly, resulting in deformation of the paper edges. The latter leads to poorer print quality and affects the amount of paper wasted.

Paper moisture content of more than 60% ERH can extend the drying time of the ink up to three times longer than usual. The optimum atmospheric humidity for the storage and transport of paper or cardboard is approximately 45 % RH (for temperatures between 10 °C and 30 °C). By observing the above storage conditions, ink drying can be optimised when the paper bundle is collected (to avoid major temperature changes in humidity). The length of time bundles of paper or cardboard can be stored depends on the temperature difference between the bundle and the ambient conditions, and the size of the bundle. As a general rule, the storage time is between one and two days, with a difference of 10 °C and a volume of 1m3. If the paper humidity is too low (<42% RH), it can lead to static, which affects the performance of the printing machine. This takes multiple sheets at once, leading to maintenance costs and delays.

HP-GTS – the solution for the paper and printing industry

Unwanted paper changes and printing delays can be avoided by measuring the ERH of the bundles and the temperature, using a relative humidity and temperature meter. From the Rotronic range, we offer the Hygropalm HP-GTS kit with portable instruments for measuring equilibrium relative humidity and temperature (paper bundles) with state-of-the-art technology.

HP-GTS replaces the long-standing GTS and GTS-set products.

Image: HP-GTS relative humidity and temperature meter for the paper industry

Next to the meter Hygropalm HP-GTS we offer a number of sensors of different models and other instruments:

– Sword-shaped sensors for measurements in paper bundles
– sensors for measuring relative humidity and temperature with high accuracy
– sensors for measuring humidity, temperature and dew point
– mini relative humidity and temperature sensors
– calibration devices for relative humidity sensors


  • direct measurement of equilibrium relative humidity (ERH)
  • robust and stable humidity probes
  • combined measurement of % RH and °C
  • state-of-the-art ergonomically designed handheld equipment
  • large screen
  • computer connection and battery power


  • fast, accurate and reliable measurements
  • low maintenance
  • both values are readily available
  • comfortable and easy to use
  • results are easy to read on the spot

Areas of use, applications:

measuring humidity in bundles of paper, cardboard and textiles for paper and textile machinery and printers.


A major improvement is the HW4 software, which allows a 2-point temperature setting in the negative temperature range. This can be used for any HC2(A) sensor and/or HC2-PT100-B4 interface.


At the ELPRO Calibration Laboratory, which is accredited to SIST EN ISO/IEC 17025:2017, we also calibrate your measuring instruments to the best of our ability.

Author Katarina Žunko, source ROTRONIC: ”Hygropalm HP-GTS-Measuring device for paper industry”
January 2021

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