Psychometric – Instrument used for measuring air temperature and humidity:
Psych meter, Kata thermometer, Globe Thermometer : The psycho-meter basically consist of two thermometers -a ‘dry bulb’ and a wet bulb’ – over which air passes at a certain speed. Dry bulb thermometer just a liquid (most frequently used liquid mercury) in glass thermometer. Wet bold thermometer a similar one, but having its bulb covered by a cotton wick. The wick which covers the bulb of one of the thermometers fully wetted with distilled water.
The psycho meter placed at the point of measurement and air at a certain speed passed over the bulbs. When air passed over the thermometer bulbs, the reading in the dry bulb remains unchanged. While reading in the wet bulb decreases until equilibrium attained. The air movement which directly influences the evaporation of water from the wick will have a cooling effect, thus decreasing the temperature in the wet bulb, referred to as the depression of the wet bulb the air velocity over the bulbs must sufficient to ensure that equilibrium reached rapidly.
The temperature values in both thermometers are read and recorded as I dry bulb’ and wet bulb temperature which are plotted on to a psychometric chart, and the values for relative humidity, few point and absolute humidity can be obtained.
Sling Psych meter (Whirling Hygrometer):
Psych meter is a very simple instrument; in which the thermometers are mounted on a sling. The air movement over the bulbs made by whirling the whole assembly. It is recommended to whirl at about 60 revolutions per minute. Usually one minute is enough to get the wet bulb thermometer to its lowest reading. After’ checking the reading, it is advisable to whirl a few. More times,’ and check if the wet bulb temperature remains the same. If it continues to fall, the process should go on until the reading is stabilized.
Aspirating psych meter:
This instrument is basically the same as the sling psycho-meter. However, The air circulated over the bulbs by means of an aspirator bulb to eliminate’ error from manual rotation. The same precautions as for the sling psycho-meter should be applied.
Instruments to measure air velocity are generally anemometers. The air velocity measured from the cooling power of the moving air. These are very useful for evaluation of heat stress because they also measure non-directional, turbulent air movement which is important for heat transfer. Example of instruments in this category is
- Kat Air movement Kata thermometers thermometer,
- Anemotherm air me.
Kata thermometers mostly used for measuring movement of air:
This instrument designed by Dr. William Hill Kata to measure the cooling capacity of the surrounding air, which depends on it movement whether linear or not. The Kata thermometer is glass thermometer which has alcohol (liquid) with a large bulb and an upper reservoir with two marks on the stem. These instruments made for different ranges (1050 – 1000F, 1300 – 1250F, 1300 – 1400 F), and hence the selection should made bearing in mind the thermal conditions before its use. The kata thermometer can used to evaluate the cooling per of the air movement even for very low air velocities – round or below 0.25m/ sec. (50 fpm).
- The bulb warmed (usually by placing it in warm water) until the alcohol dilates and fills up the whole thermometer (until about 400C).
- The thermometer then carefully tried and place at the sampling site,
- Due to the cooling effect of the air movement, the alcohol will contact; in a time interval inversely proportional to ‘cooling capacity’ of the air (air movement.)
- The mean radiant temperature of the surrounding calculated – riot measured.
- The values The time elapsed from the moment the alcohol passes by the upper mark to the moment
- The values of the ‘cooling time’ air temperature arid instrument factor (Kata Factor marked on reaches the lower cooling time determined by means of a stopwatch; each Kata thermometer) combined through equations or monograms to calculated on movement
for this calculation are the air temperature (dry bulb), the globe temperature (Temperature in a globe thermometer) and the air velocity.
Globe Thermometer (Vernon Globe):
This instrument consists of a hollow sphere made of metal “copper (globe – with a diameter of 15 cm (or 6 inches) painted in a matte black to absorb as much as possible of the incident infra-red radiation. A thermometer inserted in the globe so that the bulb(or sensing element located at its center).
The globe suspended, at the point of measurement, about 1.20 m (4 ft.) above the ground, with as little contact as possible with any other solids. Thus the globe gains heat by radiation and loses by convection (not conduction).
When thermal equilibrium, reached; which usually take about 25 – 30 minutes, the reading in. the thermometer will give the globe temperature (t8). The mean radiant temperature (tw) can then calculated, if necessary, from globe and ‘air temperatures, and air velocity. For most practical application, the globe temperature (t8) required.
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