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

Advanced HVAC Control, Information Technology and Open Systemscal linkage is used to amplify the element movement for readout.Good circulation is required and in ducts there is generally anadequate gas velocity to insure a dependable measurement. However, ifthe instrument is to be mounted in a room, the location should be carefullychosen. The sensing portion should be in the room or compartmentwhere the relative humidity is to be measured. The sensor should not bemounted near doors or other openings where it will be exposed to spuriousdrafts. Flush mounting on a panel should be avoided because theatmosphere in the back of the panel is stagnant.Dew Point HygrometersThe solution conductivity type of hygrometer depends on solidscondensing and absorbing minute quantities of water on their surfaces.The solution vapor pressure depends on the moisture content of theatmosphere.The sensing element is a thin-walled, hollow-metal socketwrapped with tape impregnated with lithium chloride salt crystals. Twowires are wrapped over the tape and connected to a regulated, alternatingcurrent voltage source. The electric circuit between the wires is completedby the salt crystals.When the sensing element is exposed to the sample atmosphere,water condensing on the crystals forms an ionic solution which permitsan electric current to flow between the wires. This current in turn heatsthe solution and raises its vapor pressure.As the water condenses, more current flows, which results in afurther increase of the solution vapor pressure until equilibrium isreached. A decreasing moisture content will cause water to evaporatefrom the element, decreasing the current flow and resulting in a newequilibrium at a lower vapor pressure. A temperature sensor inside thehollow socket is used to detect the temperature.The output can be calibrated in terms of dew point temperaturewater vapor pressure or specific humidity. Relative humidity is obtainedby converting sample and dew point temperature readings to percentrelative humidity.A sample velocity in excess of 1 foot (0.3 m) per second can resultin a poor measurement and shortened element life. At high samplevelocities, convective heat losses swamp the measurement signal and©2001 by The Fairmont Press, Inc. All rights reserved.