Instrumentation and control

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

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


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


Dalton’s Law of Partial Pressures

John Dalton’s 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 Dalton’s law as:

P = Pn2+ PO2

+ PAr + PCO2…… + e = Pda + e

wheree = partial pressure of (water) vapor [mbar]Pda = partial pressure of dry air


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



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



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



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

Bulb

(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)


Engineering

Instrumentation and control