Innovative Measurement Solutions
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What is H2O and its Measurement Effects
David Hailey, Ph.D.
June 20th, 2013
Yes, I am “that” old 😆
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My late 1980’s Midwest Measurement Society badge, I am wearing it today ☺
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Moisture Basics
Water exists in many forms…
NOTE: Steam is typically regarded as a liquid, all gasses are invisible & if you can
see it, then it's not a gas…
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What properties does water (H20) have?
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Moisture Basics: Water Molecules
Active molecules which can migrate
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Moisture Basics: Water Molecule
A water molecule consists of one oxygen and two hydrogen atoms.
QuickTime™ and a
TIFF (Uncompressed) decompressor
ar e needed to see this picture.
Van der Waal radius of water: 2.82 Å to 3.2Å
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Moisture Basics: Water Molecules
Size compared to:
Molecular
Weight
Size
Å
Water
H2O
18
1.3 - 1.77
Methane
CH4
16
2.6
Benzene
C6H6
78
5.68
Methanol
CH3OH
32
2.9
Relatively a small molecule
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Moisture Basics: States of Water
Vapor
Van der Waal Forces
Liquid Water
Hydrogen bonding
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Moisture Basics: Saturated Vapor Pressure
The process of evaporation in a closed container will proceed until there are as many
molecules returning to the liquid as there are escaping. At this point the vapor is said to
be saturated, and the pressure of that vapor (usually expressed in mmHg) is called the
saturated vapor pressure.
Since the molecular kinetic energy is greater at higher
temperature, more molecules can escape the surface and
the saturated vapor pressure is correspondingly higher. If
the liquid is open to the air, then the vapor pressure is seen
as a partial pressure along with the other constituents of the
air. The temperature at which the vapor pressure is equal to
the atmospheric pressure is called the boiling point.
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Moisture Basics: A Simpler View: Equilibrium
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Rates of evaporation and condensation are equal
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Moisture Basics: Its about energy…….
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A molecule evaporates when sufficient kinetic energy is gained
through collisions with its neighbors for it to overcome the
attractive forces between the liquid molecules. These forces
include hydrogen bonds and Van der Waals force. As the
molecule escapes it takes with it kinetic energy, leaving the
water surface with a diminished total kinetic energy.
A molecule condenses when it is moving slowly enough and is
pulled back to the water surface by the attractive forces, i.e. its
velocity is insufficient to resist the pull of the various forces of
attraction. The molecule plunges into the water, transferring
energy to the molecules near where it hit the surface and is
once again liquid
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Moisture Basics: Water Vapor
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Water vapor is water in pure gas phase
It is not to be mistaken with steam, clouds, fog, mist, etc.
where water is present in the form of droplets
In gas form, behaves like an ideal gas:
PV=nRT
P ≡ Pressure
V ≡ Volume
n ≡ Number of moles of gas
R ≡ Gas Constant
T ≡ Absolute Temperature
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Moisture Basics: Water Vapor
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Moisture Basics: Boyle’s Law
.
Animated graphics from NASA 2009, J. David Hailey Ph.D
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Moisture Basics: Charle’s Law
.
Animated graphics from NASA 2009, J. David Hailey Ph.D
Moisture Basics: Dalton’s Law
John Dalton (1766 - 1844)
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If there are n gases, the total pressure is the sum of the partial
pressures of the n gases
P(t) = P(1) + P(2) + ….. +P(n)
All the n gases act independently from each other, as if the other
gases did not exist!
So in the atmosphere (14.7 psia) made up of primary nitrogen and
oxygen
P(atm) = P(nitrogen) + P(oxygen) + P(H2O) ….. +P(n)
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Moisture Basics: Practical Application of Daltons Law
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Partial Water Vapor Pressure
of Environment is higher than
that of Pipeline.
Pipeline absolute pressure is
greater than the environment.
Water will ingress the pipeline
where ever it will find an
opening.
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Moisture Basics: Absorption into materials
Water is everywhere……. and is either part of or absorbed by almost
all materials:
• Plastics
• Steel
• Glass
Any change in the energy level changes the equilibrium between
evaporation and condensation. Taking this concept further, any
change in energy (normally denoted by temperature) changes the
amount of water absorbed in a material.
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Moisture Basics: The Diurnal Effect
As the atmosphere heats during the day, the moisture equilibrium in all
materials changes as increased thermal energy increases evaporation
rates. This occurs indoors as well as in external systems. The extent of
the fluctuations depends on the wetted materials.
Moisture Basics: Take Away
•  Moisture is everywhere.
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•  Water is a small mobile molecule.
•  Water behaves as a ideal gas; if you can see it, it is already liquid.
•  Water is absorbed in almost every material.
•  Any change in temperature shifts the condensation/evaporation
equilibrium.
•  Gas phase water is typically higher in the ambient environment than
in the process being measured.
•  Dalton’s Law applies being one of the least understood but most
troublesome issues in moisture measurement.
Moisture Measurement: Units of Measure
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Moisture is measured in various different engineering units, that
are suitable for particular applications:
§  Water Vapor Pressure (mbar, mm Hg, etc.)
§  Dewpoint (°C, °F)
§  Parts per million by volume (ppmv)
§  Parts per million by weight (ppmw)
§  Lbs of water per million standard cubic feet (at temp and pressure)
§  Grams per cubic meter (g/m3)
§  Relative humidity (% @ temperature)
§  Ug/ml (absolute measurement)
These are not linear conversions; ppm to dew point is logarithmic, lbs H2O/
MMCF is a density measurement, dew point is pressure dependent etc. This
means that the accuracy statements between units needs to be recalcualted
based on dew point.
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190
10.00
180
9.00
8.00
170
7.00
160
6.00
150
ppmv
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Moisture Measurement: Units Comparison
5.00
140
4.00
130
3.00
120
110
100
ppmv is a concentration
measurement whereas lbs
H2O/MMCF is density
2.00
1.00
0.00
l
b
s
/
M
M
C
F
ppmV
LBS MMCF
Moisture Measurement: M-Calc training
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Free conversion software from COSA Xentaur at www.cosaxentaur.com
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Certified Moisture Dew Point Calibration Standard
The calibration gas is supplied in
specifically lined high pressure aluminum
cylinders, which will discharge a constant
water concentration from 1800 psi to
200 psi for nitrogen/water blends from
1 to 150 ppmv.
NIST Traceable.
Blending accuracy is
±10% and analytical accuracy is ±2%.
The concentration is guaranteed for two
years.
NOTES:
1.  Use N2 standards if under 70°F (21°FC), as CH4 standards will need a 4-stage regulator to
keep Joule-Thomson effect from condensing out he H2O in the standard and you get a low
(bad) calibration or reference measurement value.
2.  TDLAS based measurement will read incorrectly (apx. -40% low) with N2 background in
calibration standard.
3.  HTF based will measure correctly regardless of background gas.
Can Calibrate these units,
and check others
ESS: When things go bad…..
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Joule Thomson
effect in PA; 2012.
Note insulation and
the regulator still
froze up.
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ESS: When things go bad…..
Same site, again:
A brass regulator
at the sample take
off point.
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ESS: When things go bad…..
Brass regulators,
Also consider the
diurnal effects on
the standards and
the stream
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Thank you for your attention, and your time!