Moisture and theory

 

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Moisture and theoryIn recent years & egrave; witnessed a growing interest toward the sensors moisture for monitoring and control not only in traditional sectors of interest, such as meteorological and environmental conditioning, But even in strictly industrial field.
Think of the many control processes in drying systems, storage and production, but also to the increased number of agro-food applications, museums, air forces.
For this purpose we have developed different types of sensors to meet different conditions of operability requirements in each field of application.
Present then a quick discussion about methods, the recognition and definition of ’ humidity described in very simplified way which is intended to form the ’ applicator on a topic yet not too known.

Needs

The measurement of moisture in the air and in the flue gas is becoming increasingly important. The continuous improvement of technological processes and the ever-increasing demand for quality and energy savings, require a precise procedure, stable and cheap to measure moisture.

The different methods of measurement

We give a descriptive indication of the most used and popular methods for the detection of ’ humidity, intentionally omitting techniques and technologies that are designed solely for industrial fields, military, laboratory or precision.

Hair hygrometer

The hair hygrometer is one of the oldest methods and perhaps even more used to measure moisture.
The length of the hair can vary in function of ambient humidity.
This variation is referred to as relative humidity by simple mechanical means.

Advantages
– method of measurement is easy to use and with low installation costs
– Economic applications

Disadvantages
– High maintenance costs
– Frequent hair regeneration
– Can only be used by 15 % at 85 % And UR up to max. 50 °(C)
– High inaccuracy, often not quantifiable
– Very slow steps

Psychrometer

A temperature sensor covered with a moist cotton gauze cools due to evaporation.
A second temperature sensor measures the ambient temperature.
The ambient humidity can be determined on the basis of the difference between the two temperatures.

Advantages
– If used meticulously and systematically, You can get a very precise measure from 2 al 3 %UR

Disadvantages
– Cannot be used for measurements at several points
– Long measurement times (must be moistened with distilled water prior to each measurement)
– Before every important measure, You must bring the system to the room temperature and replace the gauze

Dew point mirror

A mirror is cooled until & eacute; He begins to form condensation on its surface.
Monitoring the formation of condensation is possible to measure the dew point.

Advantages
– Wide measuring range
– High accuracy

Disadvantages
– Slow and expensive method
– Battery not power
– Heavy (non-portable measuring instrument)
– And’ request a measurement of high-precision temperature
– Slow adaptation time
– Big bench tools

Capacitive humidity sensor

A capacitor varies its ability as a function of ambient humidity present.

Advantages
– Economic measures, precise and quick (up to ±1%UR)
– Wide measuring range (by 0 to 100 %UR, by -40 a +180°(C))
– Stable over time
– Small measuring instruments and laptops

Disadvantages
In the past, capacitive sensors were considered unreliable and unstable.
Today the capacitive sensors have been adopted at international level, establishing itself in industrial measurement technology (even in internal laboratory tests).

Humidity measurements

Absolute humidity (Unit: g/m3)
The absolute humidity indicates how many grams of water are in a cubic meter of air or gas.

Relative humidity (Unit: %UR )
The relative humidity is the percentage of the maximum amount of water vapour contained in the air.
The maximum amount depends mainly on the temperature. The relative humidity always refers to a temperature.

The wet-bulb psychrometer temperature (Unit: °(C))
Evaporation cause cooling.
The temperature measured with a thermometer is lowered if it is wrapped with a wet gauze due to evaporation of the heat.
Evaporation depends on the relative humidity and the surrounding air speed.
A second thermometer, kept in a dry place, can measure the temperature difference.

Some definitions related to moisture ’

Degree of humidity (Unit: g/kg)
The moisture level x is defined as the mass ratio between water and air (dry gas).

Dew point td (Unit: °(C))
The dew point & egrave; a temperature value expressed in °(C).
Gradually as the temperature drops, It also reduces the ability of air or water-holding gas.
The dew point is the temperature at which the water in the air condenses.

Partial pressure of water vapor pas (Unit: mbar, hPa)
Total pressure level in a room that can be also determined by the water vapour.

Enthalpy, heat content the
The heat content is stored heat energy from the moist air.
The energy is fixed on 0 a 0°(C).
The enthalpy is important for calculating the heating and cooling capacity.
Differential measurements, e.g.. those made before or after the heat exchangers, are of particular interest.

Value aw
The aw value contains information about water linked chemically.
The measure is based on the equilibrium moisture content.
In a closed room, where the air is proportionally lower than solids, free water contained in solids determines the relative humidity of the ambient air.
Water activity is practically equal to the equilibrium moisture content in a closed room. However, is not measured in the field by 0 to 100% RH, but is expressed by 0 to 1 aw (aw = 0 indicates the anhydrous substance, aw = 1 indicates the pure water).
Because the activity of the water depends on the temperature, You must always indicate the reference temperature.

Equilibrium moisture
Substances that absorb, restrain or transpire water are hygroscopic defined.
They try to establish an equilibrium with the humidity of the ambient air.
In hygroscopic substances, the water causes an increase in steam pressure on the surface of the substance.
When the water vapour pressure on the surface of the substance is equal to that of the ambient atmosphere, We reach equilibrium moisture.
Any difference between the two pressure causes an exchange of water.

Water content
For water content means the quantity of water contained in a substance in relation to its solid part.
The unit is expressed as percentage weight. And’ important to know the d & rsquo content; water usually when a certain substance is sold by weight (as grain) but often, in the masonry, is an estimate of d ’ water content to evaluate the coefficients and the best conditions for evaporation l ’ addove there are ideal conditions for drying (as poor or inadequate ventilation air).

 

A good reference text
CopertinaGuerraFor the more interested and curious to know, also recommend a good text, written by Edgardo Pinto War, one of the most distinguished experts on issues concerning the wall moisture ’ in historical buildings and modern.

Moisture as a symptom and not a cause of the disease. The author reveals the misunderstanding that has been handed down for years about the damage found on plaster and masonry in the presence of water ingress.
After over thirty years of experience in designing, construction and practice of deepening the theme, Edgardo Pinto war is able to expose the most difficult topics so informative, making them easily understandable even for those who have never dealt with the matter.

The volume is, from a completely new point of view, the issues related to the effects of moisture on the walls ’, by placing the center of ’ analysis, the real cause of the problem: that & egrave; l & rsquo; action sali soluble transported from water ingress through the masonry and not of the & rsquo; s water & eacute; (the same methodology of rehabilitation that offers IgroDry).
L ’ author seeks the goal of ben ’ inform architects, engineers, Surveyors, designers, managers and grooming and help them manage more correctly the moisture problem to allow them a more critical and conscious assessment with respect to technical and commercial proposals, most recently in vogue.
The goal is to treat our art at its best, buildings and historical works of our heritage and to put anyone in a position to do more & ugrave choices; weighted to avoid treating false sun causes moisture & agrave; that the practice and usage of trade we unconsciously imposes.

A wonderful thesis

SimoLombTesiA doctoral thesis in architecture for the preservation of environmental and cultural assets of ’ University Federico II of Naples made by Simona architect Lombardi and ’ followed by ’ Ing. Pietro Mazzei.

Are over 400 pages very well structured and very detailed describing all topics concerning the deterioration resulted from moisture in the masonry ’, the possible diagnosis and recovery.
Follow many pages of products divided into categories from formulated for chemical barriers and pressure infusion, evaporating and finishing plasters, elettrosmotici systems, protective paints and much more, then the data sheets of the products and the producers with its regulations.
A few years ago thesis (When igrodry still did not exist), but that highlights a problematic and difficult to solve.

The PDF file you will download from here is integral and contains the entire thesis ’..

 

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