{"id":45124,"date":"2025-07-09T21:04:43","date_gmt":"2025-07-09T21:04:43","guid":{"rendered":"https:\/\/www.alicat.com\/?post_type=support&p=45124"},"modified":"2026-01-23T17:53:38","modified_gmt":"2026-01-23T17:53:38","slug":"messung-der-relativen-luftfeuchtigkeit-mit-alicat","status":"publish","type":"support","link":"https:\/\/www.alicat.com\/de\/support\/relative-humidity-sensing-with-alicat\/","title":{"rendered":"Messung der relativen Luftfeuchtigkeit mit Alicat"},"content":{"rendered":"\n[et_pb_section fb_built=”1″ _builder_version=”4.27.4″ _module_preset=”default” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_row _builder_version=”4.27.4″ _module_preset=”default” custom_padding=”2rem||0px||false|false” locked=”off” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_column type=”4_4″ _builder_version=”4.27.4″ _module_preset=”default” global_colors_info=”{}” theme_builder_area=”post_content”][et_pb_heading title=”Relative Humidity Sensing with Alicat Devices” _builder_version=”4.27.4″ _module_preset=”default” custom_margin=”||1rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”][\/et_pb_heading][et_pb_text _builder_version=”4.27.4″ _module_preset=”default” custom_margin=”||2rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”]When we think about air, we often picture a clean, dry mixture of gases flowing invisibly around us. However, air is almost never truly dry. Due to the presence of water vapor, humidity plays a crucial role in everything from chemical engineering to industrial processes.\n\nAlicat\u00ae<\/sup> mass flow meters and controllers<\/a> can be equipped with relative humidity (RH) sensors that enable precise monitoring of humidity within the gas stream.\n\nThis option ensures accurate control of process conditions by eliminating the need for manual corrections for moisture content. Humidity affects all materials, including the physical properties of gases. When water vapor enters a gas stream, it displaces part of the gas volume and alters viscosity and compressibility due to its different molecular weight and thermodynamic properties compared to dry gas. As a result, both mass flow and volumetric flow measurements become inaccurate unless moisture is accounted for, since water vapor occupies space and has a different mass than the gas itself.[\/et_pb_text][et_pb_heading title=”What is relative humidity, and how do we measure it?” _builder_version=”4.27.4″ _module_preset=”default” title_level=”h2″ global_colors_info=”{}” theme_builder_area=”post_content”][\/et_pb_heading][et_pb_text _builder_version=”4.27.4″ _module_preset=”default” custom_margin=”||2rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”]Relative humidity is the percentage of water vapor in the air compared to the maximum amount the air can hold at a given temperature. It depends on both the air temperature and the partial pressure of water vapor in the atmosphere.\n\nAccording to the NIST database<\/a>, the amount of water vapor a gas mixture can hold — that is, its saturation point — depends strongly on temperature. For example, at 38\u202f\u00b0C, a gas can hold about twice as much water vapor as it can at 25\u202f\u00b0C; at 14\u202f\u00b0C, it holds roughly half as much, and at 0\u202f\u00b0C, only about one‑fifth as much.\n\nAs such, Alicat specifies 90% humidity as the maximum limit for its devices, as even a small change in temperature (around 1 \u00b0C) at higher temperatures can shift RH by more than 5%.\n\nAlicat devices use RH sensors that measure using a capacitive sensing method. These sensors typically consist of a thin polymer layer sandwiched between a solid substrate and a porous protective layer. As this polymer absorbs or releases moisture from the surrounding air, its dielectric constant changes—since water has a high dielectric constant compared to air. This change in dielectric constant alters the capacitance of the sensor, which is measured using a precision oscillator circuit. The result is an extremely sensitive, accurate reading of relative humidity.[\/et_pb_text][et_pb_heading title=”Alicat\u2019s approach to humid gas measurement” _builder_version=”4.27.4″ _module_preset=”default” title_level=”h2″ global_colors_info=”{}” theme_builder_area=”post_content”][\/et_pb_heading][et_pb_text _builder_version=”4.27.4″ _module_preset=”default” custom_margin=”||2rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”]

In high\u2011humidity environments or applications where water vapor content can fluctuate, Alicat devices can normalize readings to reflect the flow as if no water vapor were present. This automatic correction removes the need for manual calculations, saving time and reducing the risk of error.<\/p>\n

Upon factory calibration, the COMPOSER\u2122 feature<\/a> allows devices calculate the percentage of water vapor of the set gas and adjusts for viscosity and compressibility\u2014delivering accurate dry gas flow measurements with response times faster than 10 ms.<\/p>\n

Accurately relative humidity sensing also helps maintain proper stoichiometry in gas\u2011phase chemical reactions by ensuring that only the true reactant gases are counted toward reaction ratios. This prevents excess or insufficient dosing, reduces byproducts, and keeps reactions balanced and efficient, which is critical for maintaining precision in chemical dosing, gas mixing, and other sensitive processes.<\/p>\n

In Alicat devices equipped with RH sensors, measurement uncertainty remains low under the following conditions: 25 \u00b0C, 1 atmosphere pressure, and 20 \u00b0C dew point.<\/p>\n