Flashcards in 10.1 c Temp. and Thermal Environment Deck (10):
A whirling hygrometer (or sling psychrometer) is commonly used for
The whirling hygrometer consists of
two thermometers, a wet bulb and dry bulb. The ‘wet’ bulb is covered with a ‘wick’ or ‘sock’ that has been thoroughly wetted using distilled (de-ionised) water. The hygrometer is swung (like an old football rattle) for about 30 seconds, this allows air movement to pass over the wet bulb thermometer and cause water from the wick to evaporate. After 20 – 30 seconds, the aspirated wet bulb temperature is read first, followed by the dry bulb temperature. These values are noted and the measurements repeated three times. From the dry bulb and aspirated wet bulb temperature it is possible to calculate the partial vapour pressure (P a ), relative humidity (RH) and dew point (t dp ) or determine the relative humidity via a psychrometric chart At 100% relative humidity (saturation) there will be no depression of the aspirated wet bulb temperature.
As air movement varies in time, space and direction it is the ‘mean’ air velocity over the body, integrated over all directions and over an exposure time, that is of interest. Air velocity can be measured by: 3
hot-wire anemometer vane anemometer kata thermometer.
The hot-wire anemometer works by
an electrical current heating the sensor to a temperature above ambient, and being cooled by air movement. The amount of cooling is dependent on the air velocity, the ambient air temperature and the characteristics of the heat element. These devices are directional and can be inaccurate in low air velocities due to natural convection of the hot wire.
The rotating vane anemometer consists of
a number of blades that are configured to allow the air movement to rotate them in one direction. The number of rotations are then counted over a period of time (usually 1 minute) and converted to air velocity. These devices are not accurate at low air velocities, are not omnidirectional and cannot be used where the direction of airflow is variable.
The kata thermometer consists of
a silvered bulb that has two levels marked on the thermometer, corresponding to a temperature drop of 3°C. The thermometer is heated to above the temperature of the upper graduation, wiped dry and allowed to cool while still clamped in place. The time taken to cool over the marked temperature interval is measured. Air velocity may then be derived by formula or nomogram (graphical representation of numerical relations) from three known quantities, i.e. the cooling time, the dry bulb temperature of the air, and a calibration factor for the particular kata thermometer, which represents the heat loss per unit surface area as the thermometer cools.
Heat stress indices typically fall into two types:
empirical indices and theoretical rational indices.
Empirical indices have been developed by assessing the physiological effects on a test group of people under varying environmental test conditions, and include: 4
Effective temperature (ET). Corrected effective temperature (CET). Predicted 4-hour sweat rate (P4SR). Wet bulb globe temperature (WBGT).
Theoretical or rational indices are derived by
consideration of the effects of the environment on the body’s heat balance. An example of a theoretical or rational indices is the Heat Stress Index (HSI), which was modelled on the heat balance equation, and is based on a comparison of evaporation required to maintain heat balance with the maximum evaporation that could be achieved in that environment.