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

HVAC Control in the New Millenniumparticular sensor. Most RTD temperature sensors can be obtained withconstants that are then programmed into a transmitter that uses theequation to correct for divergence.Using a 1000 ohm RTD rather than a 100 ohm unit will give bettermeasurement resolution, since each degree change in temperature willresult in a resistance change ten times greater.Using the digital output from a smart transmitter instead of ananalog output will provide higher accuracy. This eliminates the errorassociated with the D/A converter.StabilityAccuracy is the level of uncertainty of a transmitter’s or sensor’soutput at a given time while stability is the uncertainty of a transmitter’sor sensor’s output over a period of time. Stability is usually specified asa percent of temperature span per year. It indicates how often the systemwill need calibration.Total accuracy also depends on the equipment used to calibrate themeasurement loop. Test equipment should provide a 3:1 accuracy ratio,where the test device is rated at least three times more accurate than theunit under calibration. Equipment should be traceable to an accreditedmetrology institute such as NIST (National Institute of Standards andTechnology).Humidity ControlThe relative humidity sensors are installed in the return air stream.The relative humidity in the clean room should be within RH +3% so thehumidity sensor must have an error of less than ±3% RH. The repeatabilityof human hair element sensors is about ±1% RH. These units can beused for clean rooms provided that they are individually calibrated foroperation at or around 35% RH. Without individual calibration, theywill not perform satisfactorily in this application, since their off-the-shelferror, is about +5% RH.Hair HygrometersThese devices operate by a change of length of organic and syntheticfibers when these are exposed to a moist atmosphere. A mechani-©2001 by The Fairmont Press, Inc. All rights reserved.