Review: Saturation vapor pressure

Transcription

1 Hygrometry Dr. Christopher M. Gofrey University of North Carolina at Asheville Photo: C. Gofrey Hygrometry refers to the measurement of atmospheric humiity Several variables escribe the quantity of ater vapor in the atmosphere Absolute humiity ( v, i.e., ensity of ater vapor) Specific humiity (q) Mixing ratio () Vapor pressure (e) Relative humiity (e/e s ) Depoint (T ) Wet bulb temperature (T ) Can convert from one variable to another ith knolege of temperature an pressure Revie: Saturation vapor pressure The Clausius-Clapeyron equation gives the saturation vapor pressure over a plane surface of ater as a function of temperature: e e s exp e s = Pa T = K R v = J kg -1 K -1 s L R L = ( J kg -1 ) (234 J kg -1 K -1 )T C v 1 1 T T Saturation vapor pressure is a function of temperature Saturation Vapor Pressure (mb) Saturation Vapor Pressure vs. Temperature Temperature ( C) Saturation vapor pressure over ice The Clausius-Clapeyron equation also gives the saturation vapor pressure over ice as a function of temperature: e si e st Ls exp Rv e s = Pa t T t = K R v = J kg -1 K -1 L s = J kg Tt T Saturation vapor pressure over ater an ice Saturation Vapor Pressure (mb) e s over ater 3 2 e s over ice Temperature ( C) 1

2 Revie: Depoint temperature Revie: Wet-bulb temperature The epoint temperature is the temperature to hich a given air parcel must be coole at constant pressure an ater vapor content in orer for saturation to occur. Substitute e for e s an T for T in the Clausius-Clapeyron equation: The et-bulb temperature is the temperature to hich air may be coole by evaporating ater into it at constant pressure until it becomes saturate. T T is the et-bulb epression e e s exp L R v 1 1 T T T T T Methos of measuring humiity Thermoynamic methos Psychrometer Hygroscopic substance Hair hygrometer Conensation methos Chille mirror Sorption methos Chemical Electrical Diffusion methos Optical methos Psychrometry To temperature sensors One temperature sensor measures the ambient (rybulb) temperature (T) ick The other measures the et-bulb temperature (T ) Psychrometry The psychrometric formula relates the etbulb epression (T T ) to vapor pressure pc e es L Psychrometric constant: pc p mb.65 at stanar SLP L K Experimentally, e e p T T T s T ApT T A=.62 C -1 for ater-covere et bulbs A=.54 C -1 for ice-covere et bulbs Psychrometry: T vs. Relative Humiity Static sensitivity increases ith increasing air temperature Static sensitivity increases ith ecreasing relative humiity 2

3 Image source: Ahrens 6 th e. Image source: Image source: Image source: Image source: Image source: Requirements for a successful psychrometric measurement To ell-matche thermometers Aequate ventilation (> 3 m s -1 ) Raiation shiel Distille ater to moisten the ick Dissolve salts affect the evaporation rate Clean ick Special psychrometer ick ith no hyrophobic chemicals (not cotton) Psychrometers Assman psychrometer Sling psychrometer Psychrometers Equilibrium sorption of ater vapor Psychro-yne Many hygrometers use the process of sorption to measure ater vapor Absorption: uptake of ater into the bulk of the substance Asorption: surface retention of ater molecules Mass of ater is proportional to relative humiity Consequences of sorption processes: Material expans/contracts Resistance or capacitance of material changes Hair hygrometer Bulk polymer resistive sensor Measures resistance of a conuctive polymer Sorbe ater provies alternative conuctive paths Resistance ecreases as RH increases The length of human an animal hair varies nonlinearly as a function of relative humiity Once use iely by the NWS Drabacks: Drift, hysteresis, large lag times Elan HM2 series precision bulk-polymer RH sensor 3

4 Image source: Carbon hygristor Capacitive sensors As relative humiity increases: Linear imension increases Distance beteen carbon particles increases Resistance increases Use only on raiosones Vaisala HMP45C The ielectric constant of the polymer changes ith relative humiity Lockhee Martin Mark II Microsone carbon hygristor Overvie: Functional moel for electric hygrometers Mirror coole via Peltier effect until e/frost forms Thermistor or RTD embee in mirror etects T NWS Moel HO83/188 Mirror coole via Peltier effect until e/frost forms Thermistor or RTD embee in mirror etects T No e/frost etecte De/frost etecte 4

5 Measure the attenuation of certain bans of raiation ue to ater vapor absorption Primary ater vapor absorption bans: nm (.12 m) Ultraviolet Lyman-alpha emission line of atomic hyrogen Fastest response of all humiity sensors Terrible rift (i.e., ithin minutes) 2.6 m Infrare Expensive Slo response Infrare Water Vapor Absorption Beer s La The fraction of incient raiation,, transmitte through an atmospheric path s I exp k v s I k : absorption coefficient (m 2 kg -1 ) v : ater vapor ensity s: measurement path length I: intensity of attenuate raiation I : intensity of source raiation Schematic of an IR ater vapor sensor Requires to filters to compensate for rift or irty lenses: Reference ban Absorbing ban Campbell Scientific KH2 Krypton Hygrometer UV raiation emitte by a krypton lamp at nm an nm 5