Kobold AFH-G Operating Instruction

Operating Instruction

for

Humidity/Temperature- Measuring

Instrument

Model: AFH-G

AFH-G

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1. Note

Please read and take note of these operating instructions before unpacking and
setting the unit for operation, and follow the instructions precisely as described
herein.
The devices are only to be used, maintained and serviced by persons familiar
with these operating instructions and with the prevailing regulation applying to
procedural safety and the prevention of accidents.

2. Contents

1. Note.................................................................................... 2

2. Contents ............................................................................. 2

3. Specific Application ............................................................ 3

4. Operating Principles ........................................................... 3

5. Instrument Inspection ......................................................... 5

6. Mechanical connection ....................................................... 5

7. Electrical connection........................................................... 7

8. Technical Data.................................................................... 8

9. Order Codes....................................................................... 9

10. Maintence and Instruction for use ...................................... 9

11. Calibration ........................................................................ 10

12. Dimensions....................................................................... 11

13. Declaration of Conformance............................................. 12

Manufactured and sold by:

Kobold Messring GmbH

Nordring 22-24

D-65719 Hofheim

Tel.: [phone]

Fax: [phone]

E-Mail: [e-mail]

Internet: www.kobold.com

AFH-G

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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 dam­ages resulting thereform are not the responsibility of the manufacturer. The user
assumes all risk for such usage. The application specifications include the in­stallation, start-up and service requirements specified by the manufacturer.

4. Operating Principles

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 ., 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 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 pre­processing into standardised signals 0..20mA or 4..20mA or 0...10V.

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The fan-shaped measuring element, which faces outward from the housing, is
protected by a perforated sensor tube. The sensors are designed for pressure­less systems. The unit should be installed in a location where condensation can­not enter into the housing. A preferred position would be ”sensor vertically down”
or ”sensor horizontal”. In these positions, a cover plate with a 0.8 mm diameter
hole will prevent water from entering.
The AFH-G range of sensors have built-in temperature sensors (mainly Pt100)
for simultaneous measurement of temperature. Temperature readings are con­verted likewise into standardised signals 0..20mA or 4..20mA or 0..10V.

4.2. Ageing

In order to maintain their long-term stability, it is important that the measuring
elements undergo a special ageing process, details of which cannot be given
here.

4.3. Reaction of the sensor

Due to the law of diffusion, there is a time delay before the fibres are saturated
during water absorption. This is a decisive factor when determining the reaction
time. Thus, for one individual fibre with a diameter of 3 µm, a short saturation
time (several seconds) can be measured. Empirical investigations show that
bundled or woven fibres, as are used here in the Kobold sensor, give rise to a
longer period prior to saturation. This is because the individual fibres impede
each other during water absorption and/or water loss, and the ensuing humidity
does not register until later. Measurements have shown that, at a wind speed of
2m / sec. the half-life period is 1.2 mins. This represents an effective period of
approx. 30 - 40 mins.