Characterize and describe hygrometer types with special regard to Psychrometers and chilled mirror dew point hygrometers (slide 01 – 44 - 49 )

A hygrometer /haɪˈɡrɒmɨtər/ is an instrument used for measuring the moisture content in the atmosphere. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation (the dew point), or changes in electrical capacitance or resistance to measure humidity differences.

Hair tension hygrometers use a human or animal hair under tension. The hair is hygroscopic (tending toward retaining moisture); its length changes with humidity, and the length change may be magnified by a mechanism and indicated on a dial or scale.

 

A psychrometer, or wet-and-dry-bulb thermometer, consists of two thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick. The two thermometers are thus called the dry-bulb and the wet-bulb. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer. When the air temperature is below freezing, however, the wet-bulb is covered with a thin coating of ice and may be warmer than the dry bulb.

 

Испарение воды приводит к её охлаждению, тем большему, чем меньше влажность воздуха, контактирующего с водой. По разнице температур воздуха (называемой в психрометрии температурой сухого термометра) и поверхностного слоя воды (называемой температурой влажного термометра, или температурой смоченного термометра[2], или температурой мокрого термометра[3]) можно определить влажность воздуха.

 

Dewpoint is the temperature at which a sample of moist air (or any other water vapor) at constant pressure reaches water vapor saturation. At this saturation temperature, further cooling results in condensation of water. Chilled mirror dewpoint hygrometers are some of the most precise instruments commonly available. These use a chilled mirror and optoelectronic mechanism to detect condensation on the mirror surface. The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation on the mirror, thus closely measuring the dew point temperature. An accuracy of 0.2 °C is attainable with these devices, which correlates at typical office environments to a relative humidity accuracy of about ±1.2%. These devices need frequent cleaning, a skilled operator and periodic calibration to attain these levels of accuracy. Even so, they are prone to heavy drifting in environments where smoke or otherwise impure air may be present.

 

Конденсационный гигрометр определяет точку росы по температуре охлаждаемого металлического зеркальца в момент появления на нём следов воды (или льда), конденсирующейся из окружающего воздуха, и состоит из устройства для охлаждения зеркальца, оптического или электрического устройства, фиксирующего момент конденсации, и термометра, измеряющего температуру зеркальца. В современных конденсационных гигрометрах для охлаждения зеркальца пользуются полупроводниковым элементом, принцип действия которого основан на эффекте Пельтье, а температура зеркальца измеряется вмонтированным в него проволочным сопротивлением или полупроводниковым микро-термометром. Конденсационные гигрометры используются для определения точки росы в газовых средах (не только в воздухе).

 

5. Briefly describe the role of pyrheliometer, pyranometer and Campbell-Stokes sunshine recorder

 

A pyrheliometer (перпендикулярно + гелиус )is an instrument for measurement of direct beam solar irradiance.[1] Sunlight enters the instrument through a window and is directed onto a thermopile which converts heat to an electrical signal that can be recorded. The signal voltage is converted via a formula to measure watts per square metre.[2] It is used with a solar tracking system to keep the instrument aimed at the sun. A pyrheliometer is often used in the same setup with a pyranometer.

 

A pyranometer (пироман=огонь=солнце) is used to measure broadband solar irradiance on a planar surface and is a sensor that is designed to measure the solar radiation flux density (W/m2) from a field of view of 180 degrees.

Sunshine is traditionally measured using a Campbell-Stokes sunshine recorder. It is made of glass sphere set into a wooden bowl with the sun burning a trace on the bowl. It collects sunshine rays into one place where it burns or leaves trace.

 

 

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6. What is infiltration? Describe the process of infiltration! ( slide 02 – 2-14)

Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is most often measured in millimetres per hour or inches per hour. The rate decreases as the soil becomes saturated. If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier. It is related to the saturated hydraulic conductivity of the near-surface soil. The rate of infiltration can be measured using an infiltrometer.

Infiltration is caused by two forces: gravity and capillary action. While smaller pores offer greater resistance to gravity, very small pores pull water through capillary action in addition to and even against the force of gravity. The rate of infiltration is determined by soil characteristics including ease of entry, storage capacity, and transmission rate through the soil. The soil texture and structure, vegetation types and cover, water content of the soil, soil temperature, and rainfall intensity all play a role in controlling infiltration rate and capacity. For example, coarse-grained sandy soils have large spaces between each grain and allow water to infiltrate quickly. Vegetation creates more porous soils by protecting the soil from raindrop impact, which can close natural gaps between soil particles, loosening soil through root action and enhancing the presence of soil organism like termites, worms and small mammals that have a direct impact on soil bulk densities. This is why forested areas have the highest infiltration rates of any vegetative types.

 

The process of infiltration can continue only if there is room available for additional water at the soil surface. The available volume for additional water in the soil depends on the porosity of the soil[1] and the rate at which previously infiltrated water can move away from the surface through the soil. The maximum rate that water can enter a soil in a given condition is the infiltration capacity. If the arrival of the water at the soil surface is less than the infiltration capacity, it is sometimes analyzed using hydrology transport models, mathematical models that consider infiltration, runoff and channel flow to predict river flow rates and stream water quality.

 

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