Speed and Direction of Air Movement

The normal air speed is 0.2-0.4 m/sec (according to sensations of a person), in a day nursery — up to 0.1 m/sec (prevention of colds).

Air speed is measured by an anemometer if the speed is more than 1 m/sec, or by a catathermometer if is less than 1 m/sec according to Hill’s formula.

For investigation of direction of air movement in open places weather-vanes are used. The construction of wind rose, i.e. a graphic representation of primary direction of air movement in the given area is very important at location of various objects.

Devices for Estimation of Air Movement Speed

 

Fig. 13. Anemometer

 

A large pointer goes around a dial having 100 divisions, indicating meters; small pointers move around a dial having 10 divisions and showing hundreds, thousands, tens of thousands and more meters. Each small pointer at a complete revolution shows 10 times larger values than the previous pointer; for example, the transition of the first small pointer to one division (100 m) is equal to a complete revolution of a large pointer; the transition of the second small pointer to one division is equal to a complete revolution of the first small pointer, etc.

On observation a person faces a wind and establishes the device in such a way, that the dial should face the observer. The indications of pointers are written down (a large pointer is established on zero), then the scales should rotate idle for 1-2 minutes, and after that the counter of anemometer is switched on, simultaneously with a stop-watch. In 10 minutes the counter is switched off and new indications of pointers are written down. The difference in indications between the number of meters passed by air flow for the period of observation is found out. The obtained number of meters is divided into amount of seconds of the anemometer work.

 

Fig. 14. Katathermometer

 

Very weak air flows are determined with the help of katathermometer (cylindrical or spherical), representing a special spirit thermometer with a scale of 35-38º or 33-40º.

At first the cooling ability of air is determined. For this purpose a katathermometer is put into hot water (about 80ºC) and heated up until the spirit rises up to half of top expansion of a capillary. After that the device is wiped and hung up on a support or held on the outstretched arm in place of investigation, protected at that from effect of sun energy by means of a small screen (cardboard, plywood). Then, keeping an eye on the stop watch, it should be found out, how much time it takes the spirit to go down from 38 to 35º; the experiment is repeated twice and the average value is calculated.

CAA (cooling ability of air) is estimated by the formula:

Н = F/t (mCal per second/cm2),

where F — a katathermometer factor (it is indicated on the back side of each device),

t — time of cooling a katathermometer.

 

 

Knowing CAA, the speed of air movement is calculated according to the Hill’s formula:

 

 

where Н — the cooling ability of air;

Q — difference between a body temperature of the man (36.5º С) and temperature of a premise; 0.2 and 0.4 — empiric factors, Х — speed of air movement in m/sec (for small speeds up to 1 m/sec).

Atmospheric Pressure. Devices for Estimation of Atmospheric Pressure

 

Fig. 15. Barometer

 

A barometer consists of a vacuum metal box with elastic wavy walls. The fluctuations of atmospheric pressure have an effect on capacity and form of the box, which walls are flexed inside at increase of pressure, and are straightened at decrease of pressure. With the aid of a spring and system of small levers these changes are transmitted to a pointer moving around a dial, on which the divisions corresponding to a scale of a mercury barometer are marked. The figures of the scale designate hundreds and tens millimeters mercury; units are counted by intermediate divisions of the scale. Before readout it is necessary to knock cautiously on the device glass to prevent friction of the metal transmission parts. On the dial there is also a thermometer, which indications should be written down.

Metal barometers are less precise than mercury ones, but they are portable and convenient in application. Some of them have the second pointer for indicating pressure at a given moment.

 

Fig. 16. Barograph

 

For continuous observation of fluctuations of atmospheric pressure a self-recording device — barograph is used, which sensor part is composed of a number of aneroid boxes connected with each other. At change of pressure these boxes move, that is transferred by the system of small levers to the pointer with a writing point rotating with the speed of one complete revolution per week. All components of the device are placed in a case, which opens only at change of tapes.