Determination of atmospheric pressure icon

Determination of atmospheric pressure




НазваDetermination of atmospheric pressure
Дата10.09.2012
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Тopic 8 methods of hygienic assessment of complex effects of the microclimate on the human heat exchange
Theme: methods of hygienic assessment of the dust and chemical admixtures level in atmospheric air
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Determination of atmospheric pressure. Atmospheric pressure can be measured by mercury barometer or aneroid. For the prolonged measuring of the atmospheric pressure barographs (aneroid with recording device and tape deck) are used. The pressure value expresses in hectopascals (hPa - in-system unit). 1hPa is such pressure that is applied by a solid with mass about 1g on 1 cm2 of surface; 1hPa equals 0,7501mm mercury. For converting pressure value expressed in mm mercury in hectopascals it is needed received value to be multiplied by 4/3. Usual variations of atmospheric pressure are within the limits of 760±20mm mercury, or 1013±26,5hPa.

Determination of atmospheric pressure is carried out according to the aneroid. Before the aneroid reading one should to knock by the finger at its glass for the overcoming the pointer inertia.


Air temperature determination. Air temperature usually is measured by mercury or alcohol thermometers. Thermometer is putted at the measuring place for 5 minutes. The temperature of liquid in thermometer reservoir comes up to the surrounding air temperature, than the temperature is registered. Thereto it is possible to use aspiration psychrometer. Its dry thermometer registers air temperature more precisely because its reservoir protected from radiant heat.

For the purpose of prolonged registration of air temperature (during 24 hours, week) thermographs are used. They are consisted of sensor (curved hollow metallic plate filled with toluol or bimetallic plate), which is connected with recording device, and tape deck.

For the determination of mean air temperature in an apartment it should be made 3 measuring across at the height of 1,5m from the floor (in the middle of the room, in 10cm from the external wall and near the internal wall) and than the calculation of mean value can be made. According to this data temperature evenness in horizontal direction is determined. With the purpose of determination of temperature difference in vertical direction the measuring is made near the floor (at the height of 10cm) and at the height of 1,1m.


Air moisture determination. For the characteristic of air humidity it is used next values: absolute, maximum and relative humidity, humidity deficit and dew-point temperature.

Absolute humidity it is the quantity of steams in grams that is contained at present moment in 1m3 of air. Maximum humidity it is quantity of steams in grams that is contained at the saturation moment in 1m3 of air. Relative humidity it is the ratio of absolute humidity to maximum humidity expressed in percents.

Humidity deficit it is the difference between the maximum humidity and absolute humidity.

Dew point it is the temperature, when maximum humidity equals absolute humidity.

For air humidity estimation the most important value is relative humidity.

For the air humidity determination psychrometers and hygrometer are used. Aspiration psychrometer (pic.3) consists of 2 thermometers. Their sensor parts are contained in metallic tubes through which the air is passed by the ventilator. Such structure of device secures thermometers from the radiant heat and makes the constant air speed. This gives the opportunity to carry out the examination in stationary conditions. The end of one of the thermometers is wrapped round by thin cloth. Before every use it is moistened by distilled water with the help of special pipette. The ventilator is started by the key. The psychrometer reading is doing in 3-4 minutes from the beginning of the ventilator work but after the air speed has become constant.

Calculation of absolute humidity is doing according the formula:


K=F-0,5(t-t1)×B/755


Where K - unknown absolute value, g/m3; F – maximum humidity according the temperature of wet thermometer (determine according the table); t – temperature of dry thermometer; t1 – temperature of wet thermometer; B – barometric pressure at the moment of examination, mm mercury; 755 – mean barometric pressure, mm mercury.

Transferring of found absolute humidity in relative humidity is making according the formula:


R=K×100/F1


Where R – unknown relative humidity, %; K – absolute humidity, g/m3; F1 – maximum humidity according the temperature of dry thermometer (determine according the table).

In addition to calculation according to the formula relative humidity can be determined according the tables with use of aspiration psychrometer readings.

Hygrometers register directly relative humidity of the air. They are consisting of sensor element (flock of degreased hair) that is mechanically connected with recording part (pointer). Continual registration of relative humidity can be done by hygrograph which consists of hygrometer with recording device and tape drive.


Determination of air speed. For determination of low air speeds at an apartment (below 1-2m/s) it is used katathermometers and for high air speeds (below 50m/s) – anemometers.

Katathermometers (pic.4) can be with cylindrical or globe-shaped reservoir that is filled up with stained alcohol. Scale of cylindrical katathermometer has points from 35 to 38°C. If katathermometer is heated up to temperature which will be higher than air temperature, it will loose some calories during cooling, during cooling from 38 to 35°C this number of calories will be constant for device. This heat loss from 1cm2 of reservoir surface is determined by laboratory way and is expressed at every katathermometer in mcal/cm2.

For determination of air cooling capability katathermometer is heated on water bath as long as alcohol will fill up to ½ - 2/3 of upper expansion of reservoir, than katathermometer is wipe dried, hung on support at place where it is need to be air speed determined. By stop-watch it is registered the time of alcohol column motion from 38 to 35°C. The value of cooling capability katathermometer H that characterizes air cooling capability is calculated according to the formula:

H=F/a

Where F – factor of katathermometer, mcal/cm2; a – time in seconds of alcohol column motion from 38 to 35°C.

Globe-shaped katathermometer unlike cylindrical has temperature scale from 33 to 40°C. The work algorithm with Globe-shaped katathermometer is the same as with cylindrical katathermometer. During controlling the katathermometer cooling within different intervals of temperature it is need to be keeping next conditions: arithmetical mean of the highest (T1) and the lowest (T2) temperatures must equal 36,5°C i.e. intervals can be select from 40 to 33°C, from 39 to 34°C and from 38 to 35°C.

For calculating of the value H in such case it is used the formula:


H=Ф (T1-T2)/a


Where Ф – constant of katathermometer that is expressed in mcal/(cm2×°); a – time of cooling katathermometer from temperature T1 to T2, °C.

Air speed can be calculated according to the formulas with use of dry katathermometer cooling value (H) and temperature of air:


For air speeds less then 1m/s – формула

For air speeds more then 1m/s – формула


At formulas it has been taken next values: V – unknown air speed, m/s; H – dry katathermometer cooling value, mcal/(cm2×c); Q – difference between mean body temperature 36,5°C and air temperature in degrees; 0,20; 0,40; 0,13; 0,47 – empirically determined coefficient.

For determination of high air speeds 2 types of anemometers are used: cap and vane (pic.5). By the first it can be measured air speed within 1 to 50 m/s, by the second – 0,5 to 15 m/s.

During working with anemometer blades should idle during first 1-2 minutes to accept them constant speed of revolving. It is necessary to control a direction of the air currents, it needs to be perpendicular to revolving surface of anemometer blades. Than the counter is started by the help of starting-lever, that is located on one side with anemometer-dial. The big pointer of anemometer-dial is showing units and tens conventional points, small pointers are showing hundreds and thousands of conventional points. Time of observation is registered according to stop-watch with synchronous starting and stopping anemometer and stop-watch. It is determined the number of points in 1 second according to the differences in anemometer counter readings at the begging and at the end of observation. One can determine air speed knowing the number of conventional points in 1 second and with using of certificate that goes with cap-anemometer and graphic that goes with vane-anemometer. For example:


Indications of pointers




Before the observation

After 10 minutes from the beginning of the observation

Big pointer

40

0

First small pointer

100×3

1

Second small pointer

1000×1

5




1340

5100

Difference in indications 5100-1340=3760. The number of points in 1 second 3700/600=6,27. Air speed according to the certificate equals 6,4 m/s.


Determination of air changes per hour. Air changes per hour (K) it is quotient of division inlet air quantity during 1 hour to cubic capacity of apartment. When air speed was determined by anemometer in ventiduct by the way of equal travel of anemometer along its perimeter and in the center of vent, it can be calculated air changes per hour according to the formula:


K=a×b×c/V


Where a – ventiduct area, m2; b – air speed, m/s; c – time of ventilation, s; V – apartment volume, m3.

For estimation of received air changes per hour it is usually determined necessary volume of ventilation for given apartment. In living and public quarters this volume depends on number of people in apartment and is calculated according to the maximum permissible concentration of carbon dioxide.

Calculation of ventilation volume according to the carbon dioxide is making by formula:


L=k×n/(P-P1)


Where L – ventilation volume, m3; k – carbon dioxide amount that is breathe in by a human during 1 hour (22,6l); n – number of people in apartment; P – maximum permissible concentration of carbon dioxide in apartment (1,0 l/m3 that is in line with 0,1%); P1 – carbon dioxide amount in the air (0,4 l/m3 that is in line with 0,04%).

When division of received ventilation volume to cubic capacity of given apartment is carried, it could be determined necessary forgiven apartment air changes per hour.


Example. In ward with cubic capacity 60 m3 where are 3 people ventilation is provided by ventlight that is opened every hour for 10 min. air speed – 1m/s, area of ventlight is 0,15 m2. Give the estimation of apartment ventilation.

Answer. During 1s 1×0,15=0,15 m3 of air comes in ward, during 10 min – 90m3.

Air changes per hour equals: 90m3/50m3=1,5. Needed ventilation volume is 22,6×3/(1-0,4)=113m3. Needed air changes per hour is 113m3/60m3=1,8.

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