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The type AFH-G measuring instrument serves to measure relative humidity indoors and in air ducts. Auxiliary temperature measurement is available as an option. The moisture sensing element in the sensor comprises several strips of plastic fabric each with 90 fibres of 3 μm diameter. These plastic fibres undergo a special process to acquire hygroscopic properties, this means that they absorb and release moisture.
The molecular structure of the fibres changes when they absorb water, giving rise to a measurable change in length. The length of the plastic fibres is thus a measure of the relative humidity. The swelling effect, acting primarily in the longitudinal direction, is sensed by an electronic pick-off system and transferred to an integrated signal preprocessing system. The output signal of the passive sensors is 100…138.5 Ω, and 4 – 20 mA for the active sensors.
The special treatment of the measuring element ensures that its hygroscopic properties remain stable, that is sensitivity is maintained until destroyed by external influence. Regeneration found in conventional instruments is not required here, but is also not damaging. The measuring element is protected by a perforated tube and is open to the housing. Measuring instruments with an integrated temperature sensor provide temperature measurement. The temperatures are also converted to the standardized signals 100 – 138.5 Ω, and 4 – 20 mA.
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