Chapter 4 - Sensors and Auxiliary Devices

Abstract

This chapter discusses the functions and importance of sensors and auxiliary devices in heating, ventilation, and air conditioning (HVAC) systems. Sensors are used to measure the controlled variable. Sensors are also used for monitoring purposes to keep the operator informed about elements in the system that indicate proper operation. Sensors commonly used in HVAC applications include temperature, Carbon Dioxide (CO2), Carbon Monoxide (CO), relative humidity, dewpoint, differential pressure, sensors used to estimate indoor air quality (IAQ), and velocity/flow sensors. Sensors can be categorized into temperature sensors, moisture sensors, pressure sensors, flow sensors and meters. In order to specify and design sensor systems, evaluation of “first cost” effectiveness such as qualitative items like installation time, accuracy, precision, reliability, repeatability, durability, maintenance, repair/replacement costs, compatibility, and various other factors are considered. In addition to controllers and sensors, most control systems require additional devices to completely implement the desired control sequence, called auxiliary devices. This is true whether the control system is electric, analog electronic, or digital. These devices are absolutely needed for every control system in order to make it work per its sequence.