HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New Millenniumthe additional heating power can reduce the element life. The sensingelement should be located in a relatively quiescent zone or must beprotected from direct impingement of the flow. In ducts, a sheet metalhood can be installed over the element. It is open on the downstreamside. In piping, the element can be installed in the side outlet of a tee ormounted separately in a sampling chamber. This sensor requires periodicmaintenance and must be removable.Surface Conductivity HygrometersEvery object in a moist atmosphere has water molecules on itssurface. The concentration on these molecules is related to the temperatureof the object and the dew point of the atmosphere. If the temperatureof the surface is above the dew point of the atmosphere, the thinlayer of molecules is not visible. As the surface cools to the dew point,the density of water molecules at the surface becomes so great thatwater condenses on the surface and dew can be seen.At surface temperatures above the dew point, the moisture densityat the surface can be detected electrically although the water vapor isnot visible to the eye. This water vapor will permit a current to flow onthe surface of an insulator. This current flow depends on the moisturedensity at the surface.The measuring element consists of a polished inert surface with ametal grid and a thermocouple imbedded in the surface. A fixed potentialis maintained across the grid and the current flow is compared to thereference current flow at dew point.This signal is amplified and used to modulate a cooler so that thesurface is maintained at the dew point of the sample. The cooler is acrystal that pumps heat away from the sensor when electric power issupplied to it. From the thermocouple imbedded in the surface, theoutput can be presented as dew point temperature or in terms of relativeor specific humidity.Chilled Mirror HygrometersThe optical, chilled mirror dew point technique is a fundamentalmeasurement of the humidity of a gas since the saturation temperature©2001 by The Fairmont Press, Inc. All rights reserved.