Chapter 2: Input Devices and Sensors
Part 1: Digital Devices

Types of Switches
The following sections outlines common switching devices currently used by the industry.

Hand Switches
Hand switches are used as digital input devices and in hardwired electrical control circuits associated with digital outputs. Hand switches come in numerous sizes, shapes, and configurations. Common switch types include rotary, selector type switches, toggle switches, and pushbuttons. Selector and toggle switches are almost always maintained contact type. Pushbuttons may be momentary or maintained contact type. Selector switches can have key operators to prevent tampering.

Figure 2.2- Pushbuttons and Selector Switches (courtesy IDEC)

Limit Switches
Limit switches convert mechanical motion or proximity into a switching action. Limit switches are most commonly used in DDC control systems for HVAC to provide position status feedback to the controller for valve and damper positions. A wide variety of configurations are available. Common types include industrial limit switches, mercury, and proximity switches.

Figure 2.3-Industrial Limit Switches

Figure 2.4-Mercury Limit Switches

Figure 2.5-Proximity Switches

Temperature Switches
Temperature switches (also called thermostats, aquastats or freezestats depending on application) are commonly used in DDC control systems to provide a digital input when a process medium temperature rises or falls to a set temperature. Switches with a number of different operating principles are manufactured. Some of the common types include bimetallic, fluid thermal expansion, freezestat and electronic.

Bimetallic temperature switches use a bonded "bimetal" strip consisting of two dissimilar metals with different thermal coefficients of expansion. When the temperature changes, the metals expand or contract at different rates causing the strip to bend. Various configurations such as coiled elements are used to increase the thermal movement to cause two contacts to come together or separate. Some configurations use the bimetallic principle to change the orientation of a bulb containing liquid mercury so that the mercury flows into contact with two electrodes, completing the circuit.

Fluid thermal expansion temperature switches use the principle of thermal expansion of a fluid to cause displacement of a bellows, diaphragm, bourdon tube, or piston to open or close a set of contacts. Fluid system based temperature switches can be connected to a remote fluid containing bulb by a capillary tube, allowing the switch element to be remote from the sensing bulb.

Figure 2.6- Remote Bulb Thermostat

The freezestat is commonly used to prevent water or steam coils in air handling units from freezing. Freezestats use a fluid that is a saturated vapor at the switch set point temperature. This fluid is confined within a long capillary tube. The tube is installed in a serpentine fashion over the area of the air stream to being monitored. If any point along the tube falls below the saturation temperature, the vapor begins to condense causing a rapid change in pressure in the system and actuating the switch mechanism.

Electronic temperature switches use the same sensing technologies used for analog temperature sensing to electronically operate a set of output contacts. Refer to the Temperature Measurement portion of the Analog Input Device Section for more details of sensing technology.

Figure 2.7-Freezestat

Humidity Switches
Humidity switches, or humidistats, are used in DDC control systems to provide a digital input when a process or space humidity level rises or falls to a set level. Common applications are high limit safety interlocks for humidifiers, space or process humidity alarms, and simple on-off humidity control.

Mechanical humidistats use a hygroscopic material such as animal hair, nylon or other plastic material that changes dimension with changes in relative humidity. The dimensional change is amplified via a mechanical link to causing a switch to operate.

Mechanical humidistats are rapidly being replaced by electronic humidistats that use thin film capacitance or bulk polymer resistance analog humidity sensing technologies combined with electronic switching circuitry to produce a switching action at an adjustable set point. These sensing technologies are described in the Humidity Measurement portion of the Analog Input Device Section.

Flow Switches
Flow switches are used to provide a digital input to DDC controls systems when a fluid flow rate has risen above or fallen below the set value. Common applications include safety air and water flow interlocks for electric heaters and humidifiers, chiller safety interlocks, and burner safety interlocks. Numerous technologies are available, but the most common types used in DDC systems for HVAC control are mechanical and differential pressure types.

Mechanical flow switches operate on the principle that the kinetic energy of a flowing fluid creates a force on an object suspended in the flow stream. The magnitude of the force varies with (the square of) the velocity of the fluid. Various configurations are used to transfer this force into operation of a switch. Common configurations include paddles or sails, pistons or discs.

Differential pressure type flow switches (Figure 2.8) operate on the principle that a difference in pressure is always associated with fluid flow, or the principle that the total pressure of a flowing fluid is always greater than the static pressure. These differences in pressure can be accurately predicted for a given situation and related to the fluid flow rate. For more information see the Flow Measurement portion of the Analog Input Section.

Level Switches
Level switches are used in DDC control systems (for HVAC) to provide a digital input when the fluid level in a tank, vessel or sump has reached a predetermined height. Common applications include cooling tower sump level control and monitoring, steam condensate tank level, storm water and sewage sump level monitoring and control and thermal storage tank level monitoring. Numerous mechanical and analog technologies are currently available. Some analog technologies include capacitance, ultrasonic, and magnetostrictive-based devices in combination with solid-state electronics to provide a switching action based on level. More commonly used technologies include devices that employ the use of a float (integral, rod and float, submersible), conductivity probe, or differential pressure mechanism.

Integral float type level switches typically combine an metal or plastic float attached to the arm of a submersible rotary switch mechanism, or a float that encloses a magnet which slides on a hollow rod enclosing one or more reed switches.

Submersible float switches utilize an encapsulated integral float type switch or mercury switch suspended on a fluid tight cord in the vessel being monitored. When the level is below the cord attachment, the float hangs down and the switch is in its normally open or closed position. When the fluid level rises, the float rises above the cord attachment point, changing the float orientation. When the float has position has inverted sufficiently, the internal switch changes position.

Conductivity probe-type level switches work for conductive liquids only and use the liquid itself to conduct low level electrical signals between two or more electrodes to operate higher level electronic switching devices such as transistors or triacs.

Pressure Switches
Pressure switches are used in DDC systems to provide status indication for fans, filters and pumps, and to provide flow and level status indication by virtue of the predicable relationships between pressure and these values. Pressure switches may be mechanical or electronic.

Mechanical pressure switches use a piston, bellows, bourdon tube or diaphragm and a magnetic or mechanical linkage to convert the forces resulting from the measured pressure into repeatable motions used to operate one or more switches (Figure 2.3). Low pressure switches commonly used to measure air pressures in the range of 0.05 inches water column to 1 psig typically use a flexible diaphragm. Piston, bourdon tube and bellow type switches are available for higher pressures ranging from 1 to over 100 psig.

Vibration Switches
Vibration switches are used to provide a signal when vibration levels in rotating machinery such as fans, reach unsafe levels. Vibration switches are commonly applied on large cooling tower and air handling unit fans.

Moisture Switches
Moisture detecting switches are commonly used to detect moisture under raised floors, in piping and tank containment areas and in the drain pans of air handling units to alert system operators before damage or flooding occurs. Most moisture detecting switches are instruments of the float type or conductivity type. Float types are adapted to actuate at very low levels. Conductivity types may consist of point sensitive probes located very close to the bottom of a low point or sump where water will collect, or they may be ribbons or strips with wires separated by a non-conductive material, such that when any portion of the ribbon is exposed to liquid moisture, the electrical circuit is completed and the switch mechanism activates.

Current Switches
Current sensing relays are used in DDC systems to monitor the status of electrical devices. The devices typically have one or more adjustable current set points. Common applications include fan and pump on/off status feedback. Current switches can detect broken fan belts if properly adjusted. Current relays can also be used for phase monitoring.

Continue on to Chapter 2: Part 2: Analog Devices