Humidity Theoretical and practical aspects

2 01.11.2011

Fascination water

Steam is not water in gaseous phase

gaseous

liquid solid

cond

ense

vapo

rize

solidify

melt

sublimateresublimate

State of phaseWater

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Water molecule size ≈ 0,0001 micron

Fascination Water

small, barely visible droplet ≈ 1 to 40 micron

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Fascination Water Dimensions of water droplets

d = diameter v = fall velocity n = concentration of particles

Cloud droplet (typical)

Big cloud droplet

Crossover betweenCloud droplet and raindrop

Condensation nucleus

Raindrop (typical)

d = 100 µmv = 0,7 m/sn = 10.000 /m³

d = 0,1 µmv = 0,0000001 m/sn = 1.000.000.000.000 /m³

d = 10 µmv = 0,02 m/sn = 1.000.000 /m³

d = 50 µmv = 0,27 m/sn = 1.000.000 /m³

d = 1.000 µm (=1 mm)v = 6,5 m/sn = 1.000 /m³

Measuring principles

Our measurement devices and sensors are measuring water

in gaseous phase

Not water-level Not droplet size Not fog density

Not moisture content

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Principal aspects

Relative permitivity for some materials at 18 °C and 50 Hz vacuum εr = 1,0 air εr = 1,00059 water εr = 80,1 Polypropylen εr = 2,1 Plastics εr = 2…4

The signal change of the sensor is effected by the change in concentration of water molecules within the polymer lattice structure of the used dielectric.

capacitor

In case of a humidity sensor: εr = εr Polymer + εr H2O

C = f(%RH)

Ralative permitivity of water

–  When water vapor is the main component of the determined εr-value, you can see from the Tables, the dielectric constant of water is strictly speaking not a constant but a substance-specific base value which is temperature dependent.

–  This results in a direct line to a temperature dependence typical of the entire humidity sensor characteristic.

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C = f(%r.F.)

H O2

H O2H O2

H O2

Temp. (°C) εr (water) 0 87,69

10 83,82 20 80,08 30 76,94 40 73,02 50 69,70 60 66,51 70 63,45 80 60,54 90 57,77

100 55,15

How it works:

Capacitive humidity sensors are capable of detecting water in its vapor phase.

In a broader sense, gas sensors that are highly sensitive to H2O.

This is done by the storing of water vapor molecules in the polymer layer.

The high dielectric constant of water (εr = 80,5), compared to that of plastics (εr = 2 ... 4),

corresponds with a high capacitve signal.

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How it works:

The water vapor exchange between the polymer and ambient air is only done by an existing diffusion gradient.

Only in the case that this gap is equal to zero,

the capacity is in correspondence to the water vapor pressure of the ambient air.

C= εr x S d

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Sensor Technologies

Substrate (ceramic or glass)

Base electrode

Connector pad for cover electrode

Polymer (hygroscopic dielectric) Cover electrode

Thin film sensor (schematic construction)

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Sensor Technologies

Thick film or foil sensor: (schematic construction)

Polymer (hygroscopic dielectric)

Electrodes with sintered structure, permeable

Permeable protective fleece with supporting function

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Layer composition

Protection vleece

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How it works:

Electrodes with double function: •  electrode acts as an

electrically conductive plate

•  Plate also permeable to water molecules

Polymer / dielectric

Noble metal

Noble metal

Impossibile visualizzare l'immagine.

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Sensor Technologies

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air

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Sensor Technologies

Size correlations: Water vapour molecule

0.0001 Mikron

1-40 Mikron

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How it works:

200

205

210

215

220

225

230

235

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Cap

ac

ity

[pF

]

Relative Humidity [%RH]

Characteristic

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Measurement with polymer based capacitive Humidity sensors

The humidity sensor is one of the primary parts of a humidity measurement device

Even though it is just small, it has to do the actual job. Basically, it makes the difference whether a measurement

instrument works well in the field or not. 01.11.2011 17

Sensor and Application Even the best sensor cannot measure correctly, when built in an

non-suitable protective filter or in a non-qualified probe construction, and thus cause measurement failures.

Correct measurement is depending on the interaction of -> sensor

-> protection cage -> construction of the probe

-> electronics

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Basic combinations

IN-1WA-1 ask for

M-1-R sintered

Teflon

sintered steel

steelm

eshPE-HD

sintered

MFDHC-2

industrial 200 °C25 mm dia.inscre

w,

steel

OEM

analo

gAi

r Chip

300

0

sensor

filter

probe design

electr

onic

Probe

standard

small exoticapplication

fast

fine dust

200°C

abrasiveextrem

e air flow

standard

no ventilation,

fine dust

cust

omsp

ecific

pres

sure

standard

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Even if everything seems to be perfect, still keep an eye on the application.

A probe which may be perfect for use in non-ventilated radiation shields cannot automatically be used in similar applications like ventilated radiation shields, cheese cellars or for controlling the

sterilisation of medical equipment.

Sometimes, even normally non-recommended sensors used in well constructed probes and placed at ideal installation points achieve better results than good sensors with standard probes

located in non-qualified places. 01.11.2011 20

Definition Relative Humidity (%RH)

Is defined as the present ratio of water vapor in the air

to the maximum possible water vapor at the same temperature /

pressure.

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Area of application

Dewpoint [°Tp]

Ambient Temperature [°C]

20 40 60 80 100

20

40

60

80

100

-20-40-60-80

-20

-40

-60

-80

100

%RH

50 %

RH

25 %

RH

10 %

RH

Technical GasesPressure Dewpoint

Atmospheric Environment

Weatherballoon

Drying Processes

Cheese Manifactoring

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How to read technical data

Temperature-Humidity-Range HH-1-R

0

10

20

30

40

50

60

70

80

90

100

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Temperature [°C]

Rel

ativ

e H

umid

ity [

%R

H]

ICE

Atmospheric climate

Max. dewpoint 80°C

Theoretical max. dewpoint

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How to read technical data Max. Dewpoint 60°C

Theoretical max. Dewpoint

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Impacts acting on the sensor

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Biological media* Microorganism (germs, molds)* plants, spores* animals (insects, acarians, ...)

Chemical media /environmental influences* Aeorosols* Pollutants (ozone, NOx, SO2, ...)

Natural media* Weathering (Air, Humidity, ...)* Water, temperature* Dust, particles

Light, radiation

Temperature

Mechnanical stress* Static force (pressure)* dynamic force (vibration, impact)* shear strain, elongation, bending

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Changes caused by impacts

Chemical changes* Molecular weight distribution (degration or crosslinking)* Reaction in the polymer strings (Hydrolysis, oxidation)* ...

Changes of technical characteristics* weight* consistency, breaking elongation* Impact resistance* changes of functional characteristics* ...

Influences

Optical changes

* changes of colour* deposits, residues (organic cover) * tension cracks, fractures* matarial decomposition* Deformation, overstarin* ...

Thank you for your attention

Please fire me with questions

?

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Short Break

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