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Vaisala Humidity 101 – Humidity Sensor Technology - Tutorial
Bruce McDuffee Vaisala Humidity Instructor
Yumi Shakya Vaisala Application Engineer
Humidity 101 \ webinar series
Humidity Sensor Technology - Tutorial
Sensor Technologies ƒMechanical/Organic ƒsychPtersrome ƒChilled mirror ƒElectronic
Vaisala HUMICAP®Training ƒHeated probe ƒChemical purge
Vaisala DRYCAP®Training
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Exploit the expansion and contraction of organic materials with changes in humidity; changes are amplified through the use of levers that move a needle or pointer.
hair, catgut, goldbeater’s skin, textile, or plastichorse hair, human
hair element
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+cheap to buy +no power needed
-20…90% best range -Uncertainty from +/-5% up to +/-15% -calibrated with a wet rag to 100%;  no dry end calibration -considerable hysteresis -responds to changes in humidity very slowly
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hair element
Psychrometer (wet-dry bulb)
Two matched thermometers over which air is drawn. One thermometer has the bulb covered by a wet ‘cloth’ or ‘wick’. This is the ‘wet-bulb’.
dry bulb
wet bulb
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+simple & fundamental measurement +relatively cheap ($35 to $80 for sling) +can have good stability +tolerates condensation without damage
1 fa n m o tor he a t intro duc e d 5 p oor w a te r q ua lity 2 turbula nc e w a te r te m pe ra ture c ha ng e s a ffe c t d e p re ssion 3 thic k w ic k 6 w a te r c o nta ine r m a te ria l non- pla stic c onta m ina te d w ic k 7 tw o- w ire sy ste m c a use s e rro rs w ic k q ua lity 8 ta ble s, a c c ura c y o f re a d ing po rous ( c e ra m ic ) sle e v e 4 no n- se a le d the rm om e te rs no n- id e ntic a l the rm o m e te rs po or re solution instrum e nts low a c c ura c y instrum e nts
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1 2 4 HY G RO M ETER 3
Psychrometer (wet-dry bulb)
uqerserimos ks el il uto &seai mgi hnuectriatn y(+/- 2% to 5%)-  H-sie avopret ah t) by wat(alteredfidi deieb lmuh lempil wlempsa-  ras eiaalgrer sequid- rlatealcuc eb tsum yllaus utsules rn-aintEHURROYGTRMEICNESOLCwickanetatimtr y/didteecff aon cbyd  -gnitarycarucca
Condensation Hygrometer
Chilled Mirror
ligh t
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c h ille d m irro r
d e te c to r
lig h t
c h ille d m irro r
to r
Condensation Hygrometer
+precise measurement (+/-.1C) +wide measurement range (-100..+100C) +very good long term performance  fundamental measurement +
--usually requires some skill/training to operate -mirror cannot tell ice or dew -contamination on mirror causes errors -requires frequent maintenance -complex in construction
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Electrical Sensors
Capacitive – thin film polymer
Capacitive – aluminum oxide
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Resistive Humidity Sensors
Measures the electrical impedance or resistance of a hygroscopic medium such as conductive polymer, salt, or a treated substrate.
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The resistance changes inversely with humidity.
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