Meteorological monitoring

Mercury and alcohol thermometers are used to measure air temperature. The mercury thermometers are more precise in temperatures -35...+357 deg C. Alcohol thermometers are less precise and applied in low temperatures (under -130 deg C). The alcohol boils at temperature over 78.3 deg C.

Mercury thermometer indicates actual temperature at position of the upper part of meniscus, the alcohol - at the lower one.

Measuring air temperature in the area with the source of heat radiation they take pair-thermometer what’s made up from two thermometers. The surface of one of the mercury bulb is covered by black paint, and the other one is silver. The actual temperature is calculated taking both indications by the formula

t_a = t_b - k(t_b-t_s), (8)

where t_b - indication of thermometer with black bulb, deg C; k - instrument constant; t_s - indication of thermometer with silvered bulb.

Time rating air temperature is read by thermograph.

Psychrometers are applied to measure air humidity. It consists from two thermometers - dry and wet one. The bulb of wet thermometer is covered with wet fabric; vaporizing from the bulb absorbs the heat, therefore the wet thermometer indication is lower than the dry one.

The most simply model of psychrometers is August psychrometer what’s made up from two alcohol thermometers - dry and wet one. The bulb of wet thermometer is covered by cambric the end of which is dipped into glass with distil water, which vaporizing from the bulb absorbs the heat, therefore the wet thermometer indication is lower than of the dry one.

More precise measurements use aspiration psychrometer (Asman psychrometer). It also consists from two thermometers the bulbs of which are in the metal sleeves, what prevents additional heating of thermometers by heat radiation. Protective sleeves connect to the pipe with aspiration ventilator set on its upper part.

Difference in thermometers’ indications is used to find humidity by psychrometeric table 1, or by formula.

, (9)

where f - partial pressure of saturated vapor at the temperature of wet thermometer, millimeters of mercury column (mm.mr.cl.) (table A.3); a - psychrometerical coefficient, grad^-1 (for psychrometers «M-34» and «MB-4M» a = 0.0007 grad^-1); t₁ and t₂ - indications of dry and wet thermometers accordingly, deg C; P - barometric pressure, mm.mr.cl.; M - partial pressure of saturated vapor at the temperature of dry thermometer, mm.mr.cl.

Time rating air humidity is read by hygrograph.

Table 1

Psychrometeric table

Wet thermometer indication, °C	Air humidity, %, at the difference of dry and wet thermometers indications, deg C
	3.0	3.5	4.0	4.5	5.0	5.5	6.0	6.5	7.0	7.5	8.0	8.5	9.0

For measuring air circulation in the frames of 0.3... 5 mps revolving-vane analyzer is applied. Its vane has light aluminium or micaceous vane placed into wide metal ring. The worm-gearing transfers the vane revolution to counter. The air circulation is defined by means of calibration graph taking the difference of counter indication before and after the measuring divided by time of measuring in seconds.

For measuring air circulation in the frames of 1 … 50 mps revolving-cup analyzer is applied.

To measure very slight air circulation electro-anemometers and catathermometer are used.

Catathermometer is alcohol thermometer with cylindrical or spherical bulb. Cylindrical catathermometer scale is calibrated in frames 35.. 38 deg, spherical one - 33.. 40 deg C.

The heat measured in milicalories, that 1 cm² of bulb surface radiates for the time the catathermometer cools down from 38 to 35 deg C is called device factor F. It’s signed on the catathermometer capillary bore.

If to divide factor F by time a (in seconds), during which alcohol column of heated catathermometer goes down from 38 to 35 deg C, we’ll get the total cooling ability of air H:

. (10)

If measuring is carried out in the temperature frames 40.. 33 deg C, or 39.. 34 deg C

, (11)

where t’ - temperature at the beginning of measuring, deg C; t’’ - temperature at the end of measuring, deg C.

, (12)

or

, (13)

where Dt - difference between average temperature of catathermometer (36.5 deg C) and air temperature in room.

Heat radiation is measured by actinometer.

Дата добавления: 2014-01-04