AFH-G
AFH-G 07/02
Seite 3
3. Specific Application
The AFH-G is to be installed only in the specified applications. Every usage
which exceeds the specifications is considered to be non-specified. Any dam-
ages resulting thereform are not the responsibility of the manufacturer. The user
assumes all risk for such usage. The application specifications include the in-
stallation, start-up and service requirements specified by the manufacturer.
4. Operating Principles
The type AFH-G measuring instrument serves to measure relative humidity in-
doors and in air ducts. Auxiliary temperature measurement is available as an op-
tion.
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 spe-
cial process to acquire hygroscopic properties, this means that they absorb and
release moisture. The molecular structure of the fibres changes when they ab-
sorb 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
.
, 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 exter-
nal 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 tempera-
ture measurement. The temperatures are also converted to the standardized
signals 0-20 mA, 0-10 V , and 4-20 mA.
4.1. Design of the sensor
The expanding action (predominantly lengthways) of the fibres is picked up by
means of an electronic sensing system and converted by integrated signal pre-
processing into standardised signals
0..20mA or 4..20mA or 0...10V.
