Measuring Thermal Comfort.
Radiant-field asymmetries result from environmental conditions other than variations in air temperature. A glass pane or an exterior wall that receives direct sunlight will radiate heat much like a radiator. Conversely, when a wall is colder than the ambient indoor air, heat is readily lost from the portion of the body facing that wall. The result is a localized chilling effect. Quite simply, the north-facing side of the building will generally be a bit too cold in the winter, and the south-facing side will be too warm in the summer.
For all intents and purposes, thermal comfort is a moving target and can be described as "any condition of the mind that expresses satisfaction with the thermal environment." Dissatisfaction may be due to general bodily discomfort, to warm or cool conditions, or to unwanted heating or cooling of a particular part of the body. In other words, thermal comfort is more qualitative than quantitative. The thermal balance of the body is affected by a combination of air temperature, mean radiant temperature, air movement and velocity, and relative humidity The most sophisticated climate control systems recognize only air temperature, air velocity, and relative humidity Radiant temperature is a variable that we must sometimes define separately-- and one that we can't necessarily control by adjusting the ventilation system.
To measure the effects of radiant-field asymmetries, we recommend a few basic techniques and two devices. One of the devices is a bit more sophisticated than the other.
The simplest technique is to measure the difference between the temperature of an outer wall and that of the air. This technique constitutes another use for the versatile infrared thermometer: Simply point the thermometer at the outer wall and compare the wall temperature with the air temperature within the enclosed area. This measurement is particularly helpful in quantifying the effect of a cold surface on an individual and will provide some measure of the relative discomfort experienced by the occupant.
At the other end of the temperature spectrum, radiant heat from sun shining through a window or onto an outer wall can easily be measured with a device known as a globe thermometer. The globe thermometer consists of a thermometer suspended in a blackened metal globe housing. Unfortunately, these devices are quite expensive when purchased through a scientific catalogue--about S400. The good news, however, is that for about $15, you can easily fashion your own globe thermometer from readily available materials. This homemade device will provide a reasonable estimate of radiant heating. To construct a globe thermometer, you will need a 5-inch copper-ball toilet float (the newer plastic floats sold in megamalls will not work) and an inexpensive bimetal dial thermometer. Clean the copper float with a solvent to remove any residual oil or grease and spray-paint the surface with a matte-black finish. Next, drill a slightly oversized one-eighthinch hole through the tapped female fitting on the end; insert the stem o f the dial thermometer through the hole; and seal the top with a bit of silicone caulk, It is important to prevent the sensor portion of the thermometer stem from being in contact with the globe. If you follow this process correctly, the bimetal thermometer should fit perfectly. To use the new globe thermometer, simply place it directly in the rays of the sun and let it stabilize for a few minutes. The reading on the dial will approximate the radiant-heat effect.
Like all living things, we interact with our surroundings in a multidimensional way. A true measure of our thermal environment addresses air movement, thermal radiation, and evaporative forces on our bodies. Thus, to assess our environment accurately, we must consider factors that run the gamut from humidity and air velocity to ambient-air, dry-bulb, wet-bulb, radiant-heat, and convective temperatures. We have to measure each parameter individually and integrate the results into a single "index" number that accurately reflects the true physiological effect on the body. Absolutely the best and easiest-to-use instrument for this purpose is the "Botsball." This rather simple device, developed by J.H. Botsford of Akron, Ohio, consists of a globe thermometer covered with a black cotton wick that serves as a water reservoir. In this case, the globe thermometer is a black copper ball, 6 centimeters in diameter, into which is inserted a temperature-standard thermometer with a 30[degrees]-to-120[degrees] F range. To use the Botsball, wet the cotton wick and place the instrument at the site of discomfort (desk, bed, workstation, etc.). Let it stabilize, and read the temperature. This figure will reflect fairly accurately what the body feels. The simplicity of the instrument belies its value in measuring and integrating the four critical environmental factors needed to determine comfort. Unfortunately, we do not know if the Botsball is still being manufactured. If it is, we would appreciate any information readers can send us on availability and price.
Interpreting the globe thermometer or Botsball readings is still somewhat subjective--and more of an art than a science. Although criteria exist for determining comfort requirements for light, mainly sedentary activity, we use a practical rule-of-thumb to interpret radiant temperatures and put a number on comfort levels. In the winter, for instance, if a wall, floor, or ceiling is 20[degrees] F colder than the air temperature (at 68[degrees] to 75[degrees] F), occupants will feel chilled. Likewise, in the summer, if the globe thermometer registers 10[degrees] F warmer than the air temperature (at 73[degrees] to 79[degrees] F), the occupants will feel thermal discomfort.
The solution to radiant-field asymmetries is not absolute. For the most part, however, adverse effects can be mitigated with air barriers, and controlling radiant-field asymmetries makes good economic sense. Consider the savings, not only from direct energy costs, but also in time spent on grievances and on repairs to makeshift controls.
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|Author:||Balsamo, James J.|
|Publication:||Journal of Environmental Health|
|Date:||Dec 1, 1999|
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