Hygrometry  The science of moisture measurement including evaporation, evapotranspiration (atmometry), condensation and precipitation (hytemometry), and water vapor content of the air (humidity).  Less that 10% of the water vapor content within an air mass can be detected from a fixed sampling station.  The variability of mixing ratios vary from to 100 g/kg at surface pressures and temperatures over the Earth, a variation of 5 orders of magnitude.

Classification of Humidity Sensors  (1) Removal of water vapor from the air  (2) Addition of water vapor to the air.  (3) Equilibrium sorption of water vapor by the sensor. l Adsorption - Adhesion of water molecules to the surface of the sensor. l Absorption - taking in of water molecules into the sensor material.

 (4) Measurement of physical properties of the air which are changed by moisture.  (5) Attainment of vapor-liquid or vapor- solid equilibrium (condensation or solidification of water).  (6) Chemical processes.  They may also be classified as recording or non-recording or as fast, medium or slow response types.

(1) Removal of Water Vapor  A. Weighing (gravimetric) Method - Utilizes a desiccant material to absorb moisture from the air. Desiccant is weighed before and after exposure to air.

 B. Volumetric Method - A volume of a sample of air is measured before and after removal of water by a desiccant or by a cyrogenic bath (cooling the air until water vapor precipitates out).

 Coulometric (Electrolytic) Method - Water vapor in an air stream is electrolyzed into hydrogen and oxygen by a current flowing between wires inside a tube through which the air passes. The voltage is maintained at a constant value. The electrolysis current is measured, giving a representation of the amount of water vapor being electrolyzed.  Laboratory device. Accurate to 10 parts per million.

 Electrolytic Method - Measures change in current through a desiccant before and after absorption.  Pressure Method - Measures difference in pressure between ambient air and air which has been dried with a desiccant.

(2) Addition of Water Vapor  Can use Weighing, Volumetric, or Pressure methods by measuring the difference between the values, first, when the air is not saturated and second, after the air is saturated by evaporating water into the air.

 Psychrometers - Measure change in temperature due to cooling effect of evaporating water into the air. l Heat loss by the environment equals the heat gain by water required to evaporate sufficient water to saturate a volume of air. Heat loss by Heat gain by environment water l m 1 = mass of air from which heat is lost l m 2 = mass of water to which heat is added T = Air temperaure, T w = wet-bulb temperature, C p = specific heat of air (1000J/kg o K, e s = saturation vapor pressure, e = actual vapor pressure,  = M v /M a = 0.622, L= latent heat of vaporization, h 1 and h 2 = ventilation coefficients.

 Then,  This can be written as: where “A” is the psychrometric constant.  When airflow past the thermometers is greater than 3 m/s, then  Near sea level,

 Lab conditions for developing psychrometric tables are: l P = 1000 mb l Wind speed >3 m/s l Distilled water l Clean, muslin wick l Elimination of radiation onto thermometers l Elimination of conduction of heat into or out of thermometers.

 Mercury-in-glass Type Psychrometers l Sling Psychrometers Inexpensive Easy to use Portable Accuracy: 3-4% Not good for very low humidity Accuracy decreases for near freezing temperatures Thermometers should be matched. Response time about 1 minute

 Rotor Psychrometer l Same as Sling Psychrometer except mounted on rotating shaft.  Hand-held Electric Psychrometer l More accurate - constant ventilation rate.

 Assmann Psychrometer l Protected mercury-in-glass thermometers. Electric temperature sensors may be used. l Spring powerd fan provides constant air flow rate. l Response time about 1 minute, dependent on type thermometer. l Was once used as a standard. l Accuracy: about ±0.1 o C

Commonwealth Scientific and Industrial Research Organization Psychrometer  Adopted by WMO in 1978 as a reference standard.  Accuracy: ±0.4%

 Electric Temperature Sensors for Psychrometers l Thermocouples, thermistors or positive resistance sensors are used. l More accurate than mercury-in-glass. Does not depend on visual estimation of observer. l Faster time response l More expensive.

 Errors in Psychrometric instruments l Temperature calibration errors. l Ventilation across sensor different than that used to prepare psychrometric tables. l Psychrometric tables usually prepared using 1000 mb. Errors if pressure different. (For the same T - T w, the air is closer to being saturated at low pressures than at higher pressures.) l Radiation/Conduction errors. l Impure water, dirty wick, unsaturated wick changes evaporation rate. l Time Constant. l T w never reached, even in laboratory.

Sorption Hygrometry  Mechanical Method - Changes in length, volume, weight, hardness, and electrical conductivity.  Hair Hygrometer

 Bundle of hair is used as the sensor.  Treated hair expands about 2.5% of its original length from 0 to 100% RH.  Response time: about 10 seconds.  Hair must be treated by removing fatty material and then rolled to obtain an elliptical cross section. First perfected by Dr. E. Frankenberger.

 Problems: l Contamination by dust, oil, pollutants, rain, fog. l Hair must be cleaned often. l Hair must be replaced at least every few years. l Different expansion coefficients for each hair in bundle. l Hysteresis is a problem. l Response time increases as Temperature and Relative Humidity decreases. l Changes in zero drift can be significant.

 Advantages: l Inexpensive l Accuracy: ±3% at 50% RH

 Polytherm thread - Synthetic material whose length increases with absorption of water. l Almost exclusively used in place of hair.

Colorimetric Humidity Sensors  Typically, cobaltous chloride or cobaltous bromide is used which changes color as humidity changes.  Generally can indicate humidity from about 8% to 80%.  Accuracy: ± 5%  Response time: 30 minutes to 2 hours  Advantage: Inexpensive

 When temperature is lower than the calibration temperature (usually 75 o F) the card or plug will appear slightly bluer than it would at calibration temperature.  When warmer, sensor appears more pink.  Correction typically about 2.5% RH for each 20 o F away from 75 o F