Hygrometry Part 2

Electrical Humidity Sensors Dew Cell Wick of fiberglass 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.

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. Time Response: about 5 minutes Accuracy: ±1.5oC

Semiconductors Thin-film Capacitor Element HUMICAP sensor: Capacitance between two plates changes as moisture in the film (dielectric) between the plates changes. HUMICAP sensor: Time Response: 300 milliseconds Accuracy: ±0.5% RH

Some reported errors in Humicap sensors are due to: Chemical contamination. Due to packaging materials. 2-4% New packaging expected to eliminate this error. 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. 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) 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 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. 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). 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. 2.6 mm instrument Expensive Slow response

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: The transmittance and absorptance are: Substitution and rearranging gives: 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 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 The detector current is related to the absolute humidity in the air by: 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.

Microwave Hygrometer 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.

Light beam reflected off mirrored surface to a photodetector. When the mirror is clean, light reflects off the mirror to receiver #2. The mirror is cooled by a Peltier thermoelectric cooler. A thermometer embedded in the mirror detects the temperature of the mirror.

When the temperature of the mirror is the same as the dew point of the air, moisture begins to condense on the mirror. At that point, some light will be reflected toward sensor #1 and less toward sensor #2. The temperature of the mirror 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.

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 T0 = Ref. Temp. Accuracy: ~0.5oC

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.

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.

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