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

HVAC Control in the New MillenniumDirect wiring transmits the low-level sensor output over sensorextension wires. Sensor extension wires are fragile and cost about threetimes more than common shielded copper wire. Transmitters amplifyand condition the signal, and transmit it over a twisted wire pair.Temperature Measurement AccuracyAccuracy is a gauge of how much a measurement may vary fromthe true theoretical temperature value. Generally speaking, a high-accuracytemperature measurement is greater than ±1°C for a span of 200degrees or less.Temperature measurement uncertainties may be introduced by thetransmitter and the sensor. Temperature transmitter inaccuracies includeinput/output accuracy, linearity, load and line voltage effects. The inputaccuracy is the uncertainty of the measurement after conversion throughthe transmitter’s analog to digital converter. Output accuracy is theuncertainty of the output after the signal is converted from digital toanalog, it includes the input accuracy.Resolution is the smallest change that can be detected in a measurement.It is listed as a percent of span or as a number of bits. Linearityis the amount of deviation from a straight line between zero and fullscale input. Deviations may be caused by electrical noise or interferenceand the resistance of the leads depending on the sensor type and leadlength. Repeatability is the ability of the unit to generate the same outputvalue for the same input for consecutive measurements under thesame operating conditions.In some sensor specifications linearity is used in place of accuracy.Others use an accuracy specification that includes linearity and repeatability,and assumes specified ambient temperature conditions.RTD output can change due to temperature cycling, corrosion inlead wires, moisture, and contamination. Two and 3-wire RTDs are subjectto lead wire imbalances. A 4-wire RTD can provide true lead wirecompensation when it is used with a transmitter that has a 4-wire input.Each ohm of imbalance in an RTD’s lead wires results in as muchas 2.5°C measurement error. Imbalances are due to terminal block corrosion,connector corrosion, extension wire splices, loose connections,lead length differences and work hardening from bending.A 2-wire RTD does not compensate for lead wire length or resis-©2001 by The Fairmont Press, Inc. All rights reserved.