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Author: [[User:Hschwarz|Hans-Jürgen Schwarz]]<br>


<br> back to [[Air Humidity Measurement|Air Humidity Measurement]] <br>
<br />
The table lists the different measuring principles, evaluates them and shows the different criteria for application.
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{|border="2" cellspacing="0" cellpadding="4" width="100%" align="left" class="wikitable sortable"
|+''Table 1: Comparison of moisture measuring methods (acceding to Weber 1995) ''                   
|-
|bgcolor = "#F0F0F0" | '''Name'''
|bgcolor = "#F0F0F0" | '''Principle'''
|bgcolor = "#F0F0F0" | &nbsp;
|bgcolor = "#F0F0F0" | &nbsp;
|bgcolor = "#F0F0F0" | '''Properties'''
|bgcolor = "#F0F0F0" | '''Measuring range'''
|bgcolor = "#F0F0F0" | '''Temperature range'''
|-
|bgcolor = "#F7F7F7" | '''Chilled mirror hygrometer'''
|bgcolor = "#FFFFEO" | condensation on a chilled mirror
|bgcolor = "#FFFFEO" | a
|bgcolor = "#FFFFEO" | g
|bgcolor = "#FFFFEO" | + very accurate<br>- expensive; prone to damage by pollution
|bgcolor = "#FFFFEO" | -40 - +50 °C<br>dew point
|bgcolor = "#FFFFEO" | 0 - 40 °C
|-
|bgcolor = "#F7F7F7" | '''capacitive <br>(Al-Oxide)'''
|bgcolor = "#FFFFEO" | change in capacitance
|bgcolor = "#FFFFEO" | a
|bgcolor = "#FFFFEO" | g,l
|bgcolor = "#FFFFEO" | + small, low cost, fast<br>- drift, chemically sensitive,<br> calibration
|bgcolor = "#FFFFEO" | -100 - +60 °C<br>dew point
|bgcolor = "#FFFFEO" | 0 - 70 °C
|-
|bgcolor = "#F7F7F7" | '''LiCl'''
|bgcolor = "#FFFFEO" | LiCl Equilibrium temperature
|bgcolor = "#FFFFEO" | a
|bgcolor = "#FFFFEO" | g
|bgcolor = "#FFFFEO" | + robust, grease resistant<br>- continuous operation required
|bgcolor = "#FFFFEO" | 12 - 100% RH
|bgcolor = "#FFFFEO" | -10 - 50 °C
|-
|bgcolor = "#F7F7F7"| '''Infraret'''
|bgcolor = "#FFFFEO" | absorption of infrared light
|bgcolor = "#FFFFEO" | a
|bgcolor = "#FFFFEO" | g,l,s
|bgcolor = "#FFFFEO" |+ solid, contact free<br>- installation, expensive
|bgcolor = "#FFFFEO" | 0 - 30%<br>water content
|bgcolor = "#FFFFEO" | -
|-
|bgcolor = "#F7F7F7" | '''Microwaves'''
|bgcolor = "#FFFFEO" | absorption of microwaves
|bgcolor = "#FFFFEO" | a
|bgcolor = "#FFFFEO" | g,l,s
|bgcolor = "#FFFFEO" | + solids, contact free<br>- installation, expensive
|bgcolor = "#FFFFEO" | 1 - 1000 g/m² 
|bgcolor = "#FFFFEO" | -
|-
|bgcolor = "#F7F7F7" | '''Conductivity'''
|bgcolor = "#FFFFEO" | measuring the electrical conductivity
|bgcolor = "#FFFFEO" | a
|bgcolor = "#FFFFEO" | s
|bgcolor = "#FFFFEO" | + solids (wood, concrete), hand held<br>- accuracy
|bgcolor = "#FFFFEO" | 6 - 100%<br>water content
|bgcolor = "#FFFFEO" | -
|-
|bgcolor = "#F7F7F7" | '''Psychrometer'''
|bgcolor = "#FFFFEO" | temperature difference wet/dry
|bgcolor = "#FFFFEO" | r
|bgcolor = "#FFFFEO" | g
|bgcolor = "#FFFFEO" | + robust<br>- needs maintenance
|bgcolor = "#FFFFEO" | 0 - 100% RH
|bgcolor = "#FFFFEO" | 0 - 50 °C
|-
|bgcolor = "#F7F7F7" | '''Hygrometer'''
|bgcolor = "#FFFFEO" | change in length of hair or synthetic fiber
|bgcolor = "#FFFFEO" | r
|bgcolor = "#FFFFEO" | g
|bgcolor = "#FFFFEO" | + direct display, risible<br>- inaccurate, sensitive to dust
|bgcolor = "#FFFFEO" | 0 - 100% RH
|bgcolor = "#FFFFEO" | -30 - 110 °C
|-
|bgcolor = "#F7F7F7" | '''Capacitive'''
|bgcolor = "#FFFFEO" | polymers change in capacitance
|bgcolor = "#FFFFEO" | r
|bgcolor = "#FFFFEO" | g
|bgcolor = "#FFFFEO" | +low price, small, quick<br>- Drift, chemically sensitive,<br>calibration
|bgcolor = "#FFFFEO" | 0 - 100% RH
|bgcolor = "#FFFFEO" | -40 - 180 °C
|-
|bgcolor = "#F7F7F7" | '''Electrolytic'''
|bgcolor = "#FFFFEO" | current flow through phosphorous oxide
|bgcolor = "#FFFFEO" | a
|bgcolor = "#FFFFEO" | g
|bgcolor = "#FFFFEO" | + no calibration, ppm range<br>- flow rate, measuring range
|bgcolor = "#FFFFEO" | 5 - 1000 ppm
|bgcolor = "#FFFFEO" | 0 - 40 °C
|-
|bgcolor = "#F7F7F7" | '''Resistive'''
|bgcolor = "#FFFFEO" | resistance of a hygroscopic layer
|bgcolor = "#FFFFEO" | r
|bgcolor = "#FFFFEO" | g
|bgcolor = "#FFFFEO" | + low cost, good measuring effect, hysteresis<br>- condensation, temperature influence
|bgcolor = "#FFFFEO" | 30 - 90% RH
|bgcolor = "#FFFFEO" | 10 - 40 °C
|}
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a - absolute, r - relative, g - in gas, l - in liquids, s - in solids
==Time response ==
Criteria for the usefulness of sensors are: <br>
*the accuracy of the measurement,
*the transition function,
but also the
*'''''time response.'''''
The '''''time response''''' for most sensors is mainly determined by the amount of water needed for equilibrium. However, the time the water takes to diffuse onto the sensor needs to considered, too. Since this amount of water can be provided or discharged by the ambient air, the amount of air that flows onto a sensor plays an important role.
Because the diffusion rate is a function of temperature, a sensor will always have a slower reaction the lower the temperature. Other factors apply for sensors with different measuring principles, e.g. chilled mirror hygrometer.
In addition, filters complicate the exchange of gases. When using filter sizes of below 5 - 8 µm no more direct gas exchange takes place, but only gas diffusion.
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{|border="2" cellspacing="0" cellpadding="4" width="70%" align="left" class="wikitable sortable"
|+''Table 2: Time response of different measurement methods (according to Weber 1995)'' 
|-
|bgcolor = "#F0F0F0" | '''Type'''
|bgcolor = "#F0F0F0" | '''T<sub>50</sub>-time'''
|bgcolor = "#F0F0F0" | '''Interval skip???????'''
|-
|bgcolor = "#F7F7F7" | '''resistive'''
|bgcolor = "#FFFFEO" | <nowiki><</nowiki> 60 s
|bgcolor = "#FFFFEO" | 30 / 80&nbsp;% RH
|-
|bgcolor = "#F7F7F7" | '''capacitive'''
|bgcolor = "#FFFFEO" | <nowiki><</nowiki> 5 min
|bgcolor = "#FFFFEO" | 30 / 80&nbsp;% R
|-
|bgcolor = "#F7F7F7"  | '''LiCl'''
|bgcolor = "#FFFFEO" | <nowiki><</nowiki> 5 min
|bgcolor = "#FFFFEO" | <nowiki>-</nowiki>1 / 10 °C TP
|-
|bgcolor = "#F7F7F7" | '''Chilled mirror hygrometer'''
|bgcolor = "#FFFFEO"| <nowiki><</nowiki> 60 s
|bgcolor = "#FFFFEO" | <nowiki>-</nowiki>20 / 0 °C TP
|-
|bgcolor = "#F7F7F7"  | '''elektrolytic'''
|bgcolor = "#FFFFEO" | <nowiki><</nowiki> 30 min
|bgcolor = "#FFFFEO"| 0 / 100 ppm
|-
|bgcolor = "#F7F7F7"  | '''Fiber'''
|bgcolor = "#FFFFEO" | <nowiki><</nowiki> 10 min
|bgcolor = "#FFFFEO" | 10 / 90&nbsp;% RH
|}
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[[Category:Humidity Measurement]] [[Category:R-HSchwarz]] [[Category:R-SLaue]] [[Category:Review]]

Latest revision as of 17:50, 9 January 2013

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