1. 9
Humidity Measurement
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
Principles of Humidity and Moisture Measurement. . . . . . . . . . . . 254
Percent Relative Humidity Equation . . . . . . . . . . . . . . . . . . . . . . . . 254
Dalton’s Law of Partial Pressures . . . . . . . . . . . . . . . . . . . . . . . . . . 255
Humidity and Moisture Conversion Table . . . . . . . . . . . . . . . . . . . 256
Psychrometric Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258

2. Chapter 9/Humidity 253
Introduction
Popular devices for humidity measurement include the hygrometer, a
device which measures only relative humidity; thermohygrometer, a
device which measures both temperature and humidity; psychometer,
which measures humidity and dew point through water evaporation
rate interpretation; and dew point meter, which measures the tempera-ture
at which moisture will form in the sampled environment. In
process control, moisture and temperature often need to be measured
in combination.
Thermohygrometers are available in wall mount, dial meter type units
that 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 spring
or glass bulb thermometer for temperature. As moisture increases, the
sponge 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 or
resistance 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 has
two thermometers-a normal “dry” bulb thermometer, plus another
called the “wet” bulb, featuring a wick moistened with water. As air
passes over the two thermometers, two temperatures (wet and dry
bulb) are generated. Using a table, the humidity can be calculated.

3. 254 ISA Handbook of Measurement Equations and Tables
Principles of Humidity and Moisture Measurement
Measurement
Factor
Percent Relative Humidity
RH
wvp
wvp
a
s
= ×100
Measurement
Description
where
RH = percent relative humidity
wvpa = absolute water vapor pressure
wvps = saturated water vapor pressure
Units of
Measure
Wet Bulb
Thermometer
The temperature of a wetted
thermometer in a stream of air.
°F or °C
Percent Relative
Humidity
The ratio of actual vapor pressure to
saturation vapor pressure.
0-100%
Dew Point The temperature that air must be cooled
to achieve saturation.
°F or °C
Volume or Mass Parts per million by volume or weight. ppmv or ppmw

4. Dalton’s Law of Partial Pressures
John Dalton’s law:
The Total Pressure of a gas mixture is the sum of the pressures of each
gas 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 Dalton’s law as:
P = Pn2
+ PO2
+ PAr + PCO2
…… + e = Pda + e
where
e = 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

5. 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

6. 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

7. 258 ISA Handbook of Measurement Equations and Tables
Psychrometric Chart
40 50 60 70 80 90 100 110 120
For applications such as air conditioning, the psychrometric chart is a
good analysis tool to assess the thermal comfort conditions throughout
the year. Atmospheric factors such as air temperature and moisture in
the air are key to thermal comfort. The psychrometric chart represents
the 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 a
given atmosphere as a percentage of the saturation humidity at the
same temperature:
Wet bulb temperature (WBT) is measured by a hygrometer (or psy-chrometer),
which consists of two thermometers – one measuring the
dry bulb temperature (DBT), the other having its bulb enclosed in a wet
wick. “Web bulb depression” is a term meaning the difference in the
temperatures 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 the
moisture carrying capacity of the atmospheric air at that temperature.
When the air is saturated, there is no evaporation, and DBT and WBT
readings are identical. The “status point” is determined at the intersec-tion
of the vertical DBT line and the WBT slope on the psychrometric
chart.
Dry Bulb Temperature (F)
40
50
60
70
80
90
Wet Bulb (F)
80%
60%
40%
20%
.028
.024
.020
.016
.012
.008
.004
Humidity Ratio = mass of water vapor
(Lbv)/mass of dry air (Lba)