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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Principles of Humidity and Moisture Measurement. . . . . . . . . . . . 254
Percent Relative Humidity Equation . . . . . . . . . . . . . . . . . . . . . . . . 254
Daltons Law of Partial Pressures . . . . . . . . . . . . . . . . . . . . . . . . . . 255
Humidity and Moisture Conversion Table . . . . . . . . . . . . . . . . . . . 256
Psychrometric Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
9Humidity Measurement
HB humidity chap9.qxd 3/2/2006 9:07 AM Page 251
Introduction
Popular devices for humidity measurement include the hygrometer, adevice which measures only relative humidity; thermohygrometer, adevice which measures both temperature and humidity; psychometer,which measures humidity and dew point through water evaporationrate interpretation; and dew point meter, which measures the tempera-ture at which moisture will form in the sampled environment. Inprocess control, moisture and temperature often need to be measuredin combination.
Thermohygrometers are available in wall mount, dial meter type unitsthat do not require electrical power and digital models. Battery oper-ated digital units are popular. Dial meter type thermohygrometers gen-erally use a cellulose sponge type sensor for humidity and a springor glass bulb thermometer for temperature. As moisture increases, thesponge expands and the lever mechanism moves the indicating needle.Accuracies are typically in the +/- 3% range, and response time is slow.Electronic thermohygrometers generally use either a capacitance orresistance sensor. As the humidity rises, the circuit resistance or capac-itance changes a digital display reading.
When portability is needed, a psychometer is often used. It typically hastwo thermometers-a normal dry bulb thermometer, plus anothercalled the wet bulb, featuring a wick moistened with water. As airpasses over the two thermometers, two temperatures (wet and drybulb) are generated. Using a table, the humidity can be calculated.
Chapter 9/Humidity 253
HB humidity chap9.qxd 3/2/2006 9:07 AM Page 253
Principles of Humidity and Moisture Measurement
Percent Relative Humidity
whereRH = percent relative humiditywvpa = absolute water vapor pressurewvps = saturated water vapor pressure
RHwvpwvp
a
s= 100
254 ISA Handbook of Measurement Equations and Tables
Measurement Factor
Measurement Description
Units of Measure
Wet Bulb Thermometer
The temperature of a wetted thermometer in a stream of air.
F or C
Percent RelativeHumidity
The ratio of actual vapor pressure tosaturation vapor pressure.
0-100%
Dew Point The temperature that air must be cooledto achieve saturation.
F or C
Volume or Mass Parts per million by volume or weight. ppmv or ppmw
HB humidity chap9.qxd 3/2/2006 9:07 AM Page 254
Daltons Law of Partial Pressures
John Daltons law: The Total Pressure of a gas mixture is the sum of the pressures of eachgas component.
P = Pn2 + Po2 + Par + . . .
Partial Pressure is defined as the pressure of a single gas in the mix-ture as if that gas alone occupied the container.
Water in its gaseous state (vapor) is an additional gas component of air,and also appears in Daltons law as:
P = Pn2 + PO2 + PAr + PCO2 + e = Pda + e
wheree = partial pressure of (water) vapor [mbar]Pda = partial pressure of dry air
Chapter 9/Humidity 255
Main Gas Components in Air
Gas % Volume % Weight
Nitrogen N2 78.03 75.47
Oxygen O2 20.99 23.20
Argon Ar 0.93 1.28
Carbon Dioxide CO2 0.03 0.04
All others: H2, He, Ne, Kr etc. 0.02 0.01
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256 ISA Handbook of Measurement Equations and Tables
Humidity and Moisture Conversion Table
To Convert from To Multiply by:
Atmosphere Millibar 1013.25
Atmosphere mm Mercury 760.0
cm Mercury Millibar 13.3322
cm Mercury mm water 135.951
cm water Millibar 0.980665
cm water mm Mercury 0.735559
cm3 in3 0.06102374
cm3 m3 0.000001
cm3 mm3 1000
cm3 gallon 0.00026417
cm3 Milliliter 1
cm3-Atmosphere Joule 0.101325
ft3 cm3 28316.847
ft3 in3 1728
ft3 gallon 7.480519
ft3 liter 28.316847
ft3-Atmosphere liter-Atmosphere 28.316847
in3 cm3 16.387064
in3 ft3 0.0005787
in3 gallon 0.0043290
in3 liter 0.016387064
mm3 in3 0.0000610237
C-temp. interval F 1.8
HB humidity chap9.qxd 3/2/2006 9:07 AM Page 256
Chapter 9/Humidity 257
Humidity and Moisture Conversion Table (cont.)
To Convert from To Multiply by:
C-temp. interval Kelvin 1.0
F-temp. interval C or Kelvin 0.555556
ft water Atmosphere 0.0294998
ft water Bar 0.0298907
ft/F m/C 0.54864
gallon cm3 3785.412
gallon ft3 0.13368
gallon in3 231
gallon liter 3.785412
in Mercury Millibar 33.8639
in Mercury Atmosphere 0.0334211
in water Millibar 2.49089
in/F mm/C 45.72
liter ft3 0.03532467
liter in3 61.02374
liter gallon 0.26417205
liter-Atmosphere ft3-Atmosphere 0.0353147
liter-bar Joule 100
mm Mercury Atmosphere 0.001315789
mm Mercury Millibar 1.333224
mm water Atmosphere 0.000096784
mm water Millibar 0.098665
part per million Milligram/Kilogram 1
part per million Milliliter/m3 1
HB humidity chap9.qxd 3/2/2006 9:07 AM Page 257
258 ISA Handbook of Measurement Equations and Tables
Psychrometric Chart
For applications such as air conditioning, the psychrometric chart is agood analysis tool to assess the thermal comfort conditions throughoutthe year. Atmospheric factors such as air temperature and moisture inthe air are key to thermal comfort. The psychrometric chart representsthe state of a given atmosphere by a point which gives dry-bulb, wet-bulb, relative humidity, specific volume and saturation temperature.Relative humidity (RH) is an expression of the moisture content of agiven atmosphere as a percentage of the saturation humidity at thesame temperature:
Wet bulb temperature (WBT) is measured by a hygrometer (or psy-chrometer), which consists of two thermometers one measuring thedry bulb temperature (DBT), the other having its bulb enclosed in a wetwick. Web bulb depression is a term meaning the difference in thetemperatures between the wet wick thermometer and the DBT, as hap-pens when the wet wick thermometer is cooled down by the evapora-tion on the wick. The amount of evaporation is a direct indication of themoisture carrying capacity of the atmospheric air at that temperature.When the air is saturated, there is no evaporation, and DBT and WBTreadings are identical. The status point is determined at the intersec-tion of the vertical DBT line and the WBT slope on the psychrometricchart.
Dry Bulb Temperature (F)
40 50 60 70 80 90 100 110 120
40
50
60
70
80
90
Wet
Bul
b (F
)
80%
60%
40%
20%
.028
.024
.020
.016
.012
.008
.004
Hu
mid
ity Ratio
= mass o
f water vap
or
(Lbv)/m
ass of d
ry air (Lba)
HB humidity chap9.qxd 3/2/2006 9:07 AM Page 258
Front MatterTable of Contents9. Humidity Measurement9.1 Introduction9.2 Principles of Humidity and Moisture Measurement9.3 Percent Relative Humidity Equation9.4 Dalton's Law of Partial Pressures9.5 Humidity and Moisture Conversion Table9.6 Psychrometric Chart
Index