senzorji: vlažnosti: Rotronic HT-1 / temperatura: PT100 1/3 DIN razreda B obseg uporabe: -50 … 100 °C / 0 … 100 % RV točnost pri 23 °C: ±0.8 % RV / ±0.1 K
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Aleksandra Lepenik, expert in temperature measurements
Measurement expert Zoran Lepenik
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By Blaž Tomaž Vertačnik, Aleksandra Lepenik; October 2023
Advanced chilled mirror technology for precise dew point and frost point measurements
ELPRO Lepenik & Co. from our partner Michell Instruments’ Relative Humidity Specialist programme, introduces a new High precision embossing meter (S8000 -100) for measuring low dew points. It is designed to meet the stringent requirements of standards in humidity calibration laboratories. Using the latest chilled mirror technology and in-house development, Michell engineers strive to achieve reliable and repeatable measurements of low dew points down to -100 °C, with an accuracy of ±0.1 °C, without the need for additional cooling.
At the core of the hygrometer is a unique advanced optical system that detects small changes in the moisture condensed on the surface of the glass. This ensures high sensitivity and a fast response to frost point changes, even at low humidity, where measurements are most challenging. The high-precision chilled mirror hygrometer achieves ±0.25 °C stability at -100 °Cfp (freezing point) in less than six hours. This result represents the best speed of any instrument in its class. It also achieves a reproducibility of ±0,15 °Cfp at a repeatability of 100 °Cfp.
S8000 -100 The smallest and lightest instrument that measures frost points down to -100 °C. It weighs just 22 kg and can be mounted on a 19-inch stand or placed on a work surface.
With full automation and remote monitoring software, operators do not need to continuously monitor the instrument. The wide range of communication protocols includes Modbus RTU over USB, RS232, RS485 and Modbus TCP over Ethernet. Data logging to SD card and user-configurable analogue outputs are also available.
Typical applications of this precision instrument include humidity measurement in standardised laboratories (e.g. ISO 17025, ISO 9001, ISO 14001, etc.), humidity calibration/calibration applications, and precision humidity measurements for research and development.
Michell Instruments is a world leader in dew point. Dew point has been in development since 1974 at an initial site in the UK. With four decades of experience, it designs and manufactures a wide range of sensors, instruments and flexible (customisable) systems for measuring dew point, humidity and oxygen. They are used in a variety of applications and industries, such as compressed air, power generation, petrochemicals, oil and/or gas extraction and processing, food processing and pharmaceuticals. Michell’s innovative products have a significant impact on cost savings, cleanliness, energy efficiency and process safety.
Author Katarina Žunko, source. ”High precision chilled mirror hygrometer – Michell S8000 -100”
July 2021
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Precise relative humidity measurement with chilled mirror hygrometers from Michell
The most commonly used measurement technology for monitoring relative humidity is based on polymer capacitive sensors. They are fast responding and generally very cost-effective in the range of 10-98% RV. However, for accurate measurements of relative humidity and dew point, it is preferable and recommended to use chilled mirror hygrometers. From our product range we present chilled mirror relative humidity meters our partner Michell Instruments, part of the PST Group.
The following article discusses relative humidity and the purpose of chilled mirror humidifiers.
Relative humidity (RH) is the ratio of the amount of water vapour present in a gas compared to the maximum amount of water vapour that the gas can hold before condensation occurs at a given temperature. This means that changes in temperature, pressure or humidity will affect the % RH.
Formula for calculating relative humidity:
RV = (water vapour pressure/water vapour saturation pressure)*100
They are commonly used to calculate the % RH in industry. Compared to polymer sensors, chilled mirror hygrometers are very reliable and virtually drift-free. For power generation, the robust (EX) industrial chilled mirror hygrometer Optidew 501 is preferred . The latter is used to protect gas turbine systems as it helps to predict the conditions that cause frost. This improves efficiency and prevents irregularities or damage inside the turbine. Its design allows the chilled mirror sensor to provide a fast response comparable to a polymer sensor in most applications, including environmental monitoring. Precise and repeatable measurements make chilled mirror hygrometers very useful and desirable in many meteorological applications.
Relative humidity meters Optidew 501 and S8000 automatically calculate the % RH. To ensure the best level of accuracy of the instrument, it is necessary to understand how these calculations work under industrial conditions where %RV values are very low. Chilled mirror instruments primarily measure the dew point, or in very dry conditions the frost point.
For Michell chilled mirror products, the calculation of the water vapour pressure and the saturation water vapour pressure are calculated using the vapour pressure formula.
When calculating relative humidity in meteorology, it is important to understand that in the atmosphere below freezing, condensation of vapours occurs, turning them into a liquid state (instead of ice). This process occurs at temperatures below the freezing point, and thus Mpemba phenomenon (so-called ”supercooling”). In technical terms, this is the phenomenon where water with a higher initial temperature freezes faster than water with a lower initial temperature.
Because this effect can occur in clouds and the upper atmosphere, the World Meteorological Organisation(WMO) decided almost half a century ago that the % RH should be calculated by taking into account the saturation of water vapour above water, regardless of temperature. This decision was taken to avoid the possibility of relative humidity values exceeding 100% when the atmosphere is in a super-cooled state (Mpemba effect). Using this calculation method gives different results only when the temperature drops below 0 °C.
This method of calculating % RH is used by the S8000 and Optidew 401/501 chilled mirror relative humidity meters . You can see other chilled mirror hygrometers and their applications in our programme.
Author Katarina Žunko
May 2021
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.
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%.
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.
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.
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.
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
KEY FUNCTIONS:
Benefits:
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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