Transcript Hygrometry - Texas A&M University

Hygrometry Part 2
Electrical Humidity Sensors
Dew Cell
A
Wick of
A
fiberglass
a
Cross-section
A-A
wool or
dacron is saturated with Lithium Chloride
and wrapped about a tube.
Double winding of wire is wrapped about the
tube such that current passes from one wire,
through the LiCl to the other wire.
c
e
When the wick is wet, the resistance between
wires is low and higher current flow causes wires
to warm.
Heated wires evaporate water which causes
drying of LiCL wick which increases the
resistance which reduces the current.
Eventually a temperature equilibrium is reached.
At the equilibrium temperature, the vapor
pressure of LiCl is in equilibrium with vapor
pressure of water in air.
At this equilibrium, the dew point (water) is
a function of the LiCl dew point.
Td  23.99905  0.69356Td(L iCl ) , Td (Li Cl ) > 12.5o C
Td = -20.86781 + 0.478537 Td (Li Cl ) , Td( Li Cl ) < 12.5o C
Time Response: about 5 minutes
Accuracy: ±1.5oC
Semiconductors
Thin-film Capacitor Element
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Capacitance between two plates changes as
moisture in the film (dielectric) between the plates
d
a
changes.
e
HUMICAP sensor:
Time Response:
300 milliseconds
Accuracy: ±0.5% RH
c
e
e
c
d
a
e
e
b
b
a Glass substrate
d Upper Electrode
b Lower Electrode
e Leads
c Polymer film
Some reported errors in Humicap sensors
are due to:
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Chemical contamination. Due to packaging
materials. 2-4%
• New packaging expected to eliminate this error.
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Temperature-dependence error. Using a linear
dependence of temperature to sensor output
rather than an actual non-linear dependence.
Errors mainly below temperatures of -20oC and
increase with colder temperatures.
Sensor arm heating. Due to radiational heating.
Capacitive sensors have a slightly non-linear
transfer function.
Can have a slight drift due to dirt on the sensor.
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A fine pore filter will prevent dirt accumulation but
will also increase the time constant.
Sometimes the contamination is permanent (e.g. SO2)
More stable than resistive sensors.
Relatively inexpensive if bought in quantity.
Used on some radiosondes (Vaisala)
Carbon Hygrometer (Hygrister)(resistance type)
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Flat glass plate or other resistor material (plastic) is
coated with matrix of carbon particles and binding
material.
Carbon absorbs water.
Carbon particles separate and resistance increases.
One of most common humidity sensors in radiosondes.
Cheap
Not accurate below -40oC
Time Response: 10 - 20 seconds
Accuracy: ± 5% RH
Piezoelectric Hygrometer
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A special cut quartz crystal with hygroscopic
material deposited on its surface will change its
resonance frequency as the crystal gains or
loses mass due to absorption or evaporation of
water.
f  Km
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Time Response: about 1 minute
Needs frequent (usually automatic) calibration.
Mass changes when it collects dust, other
particulate matter, or loses hygroscopic
material.
4. Measurement of the Physical
Properties of Air.
Infrared / UV Hygrometers: (Spectroscopic
technique) (Often called Gas Analyzers)
These make use of the property that water vapor
absorbs radiation at certain wavelengths.
If all processes that reduce radiation along a
path, such as spreading, reflection, refraction
were controlled, or known, and only absorption
was left, then the measured reduction along a
path must be due to the amount of water vapor in
that path.
Infrared Type of Gas Analyzer
A primary water vapor absorption band is: 2.59
mm (infrared).
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Water vapor does not absorb at 2.45 mm.
IR radiation is transmitted across a path of
atmospheric air at 2.59 mm and then at 2.45 mm.
Since the frequency of both beams is close, the
reflection, scattering, refraction of both beams is
considered nearly the same and differences can be
ignored.
The difference in intensity of IR radiation
received by a receiver on other side of chamber is
a measure of the amount of moisture in the air.
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2.6 mm instrument
• Expensive
• Slow response
Filter Wheel
M otor
M easurement Path
IR Source
Lens
IR Detector
LI-Cor LI-6262 Differential IR Gas Analyzer
This detector uses two chambers and can detect
CO2 as well as water vapor.
One reference chamber is maintained at zero
concentration of water vapor and CO2.
The second chamber is the sample chamber
through which a stream of air is pumped.
The IR radiation source is a heated sphere, held
at 1250oK.
IR radiation passes through an optical filter and
then down both chambers to detectors.
The filter allows 2.59 mm radiation to pass for
the detection of water vapor.
Then it allows 4.26 mm radiation to pass for the
detection of CO2.
Each beam is sent to different detectors and the
amount of water vapor or CO2 is based on the
difference in absorption of IR radiation in the
sampling chamber as compared to the reference
chamber.
Li-Cor Schematic
Output voltage of detector is the difference
between the voltage from sample cell and
reference cell by: V  k(v s  vr )
The transmittance and absorptance are:
vs

vr  t ransm it tance= 1 A t he absorpt ance
Substitution and rearranging gives: V  kv r A
Measuring V and vr and knowing k from
calibration, then absorptance can be determined
which is directly related to the amount of
moisture (or CO2) in the air
UV Gas Type Analyzer
Lyman-Alpha Humidiometer
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Uses absorption of 0.12156 mm
radiation by water vapor in the air.
Radiation passes through the air and then to a
detection chamber containing nitric oxide.
The 0.12156 mm radiation ionizes the nitric
oxide.
Nitric oxide is part of an electric circuit.
The more radiation absorbed by water vapor, the less ionization
that occurs and the smaller the current flow in the circuit.
The amount of current flow then becomes a measure of the
water vapor in the air.
Lyman-Alpha Humidiometer
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The detector current is related to the absolute
humidity in the air by:
I  I0 exp kdrv
where:
I0 = dry air current,
d = path length through air. Sensor shown is
1 cm in length.
rv = absolute humidity of the air.
k = constant determined from calibration.
Uses “salt crystal” windows since other materials block the UV
radiation.
The device is: small and fast, expensive, the UV lamp decays
quickly, the salt windows deteriorate due to moisture and must
be frequently replaced.
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Microwave Hygrometer
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Two chambers are used, (1) contains a dry gas, (2)
contains atmospheric air
Both subjected to microwave energy of varying
frequency.
Frequency at which resonance occurs is detected by
crystal diodes.
Difference in resonance frequency between dry
chamber and atmospheric air chamber is due to water
vapor in the air.
Accuracy: ±0.2% Response time: 10 seconds
Expensive
5. Condensation or Solidification of
water.
Dew-Point Hygrometer - Uses optical device to
detect formation of dew on a surface.
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Light beam reflected off mirrored surface to a
photodetector.
Receiver
Transmitter
#1
Receiver
Lamp
When the mirror
#2
is clean, light
reflects off the
mirror to receiver
M irror
#2.
Thermoelectric
Heater/Chiller
The mirror is cooled
by a Peltier thermoelectric cooler.
A thermometer embedded in the mirror detects the
temperature of the mirror.
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When the temperature of the mirror is the same as
the dew point of the air, moisture begins to condense
Receiver
on the mirror.
Transmitter
#1
Receiver
Lamp
#2
At that point, some light will
be reflected toward sensor #1
and less toward sensor #2.
M irror
Thermoelectric
The temperature of the mirror
Heater/Chiller
is considered to be the dew
point of the air.
The mirror is then heated to evaporate water on it
and all light should be reflected toward sensor #1.
Then the mirror is cooled again and the process
repeated.
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When dew forms, the vapor pressure over the
liquid dew is in equilibrium with the vapor
pressure of the air.
The saturation vapor pressure related to the
temperature of the mirror at this equilibrium
condition is also the actual vapor pressure of the
air. The saturation vapor pressure, with respect to
water, is related to temperature (the temperature of
the mirror, i.e. the dew point) by the equation:
T = Temp. of mirror = Td
17.67 T 

T0 = Ref. Temp. esat T  esat T0  exp
243.5  T 
o
Accuracy: ~0.5 C
6. Chemical Reaction of Reagent
and Water
Uses chemicals in a laboratory to remove
water from an air sample and then the
removed water is measured.
ASOS Dew Point Sensor
Uses a chilled mirror sensor
The temperature of the mirror is sensed by a
platinum wire sensor embedded in the mirror.
Dew point temperatures are measured every 30
seconds.
A 1-minute mean dew point is determined.
Then, 5-minute running averages are determined.
Once per day the mirror is heated to recalibrate
the reference reflection to compensate for a dirty
mirror.
Parameter
Range
RMSE
Max Error
Resolutio n
Dew Point -34.4oC to -18oC 1.7oC to 4.4oC 2.5oC to 7.7loC
0.05oC
-18oC to 0.0oC 1.0oC to 4.4oC 1.9oC to 7.7oC
0.0oC to 30oC 0.6oC to 2.6oC 1.1oC to 4.4oC
Takes measurement once every 30 seconds.
One-minute average obtained.
Once each minute, a 5-minute average is
obtained.
5-minute average is rounded to nearest whole
degree Celsius.
End