Exercise 4-1 Float Switches EXERCISE OBJECTIVE In this exercise, you will study the construction and operation of a magnetic reed float switch. You will use a float switch to limit the amount of liquid pumped into a column. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Introduction Reed switch float switches Industrial applications Advantages and limitations Installing the float switch DISCUSSION Introduction A float switch is a point-level detection device used to actuate another device when the level in a vessel rises above, or drops below, a set point. Like all pointlevel sensors, float switches allow level detection of a single discrete level. A float switch that stops a pump when a tank is full is an example of a typical float switch application. Figure 4-3 shows a point-level detection mechanism that uses a simple float switch. Figure 4-3. Float switch application. In this example, the float of a float switch is displaced when its buoyancy makes it move upward when it comes in contact with the liquid. The float can be linked mechanically to an actuator, such as in the example of Figure 4-3, or via another mechanism, such as a reed switch and magnet mechanism. The working principle of float switches using this type of mechanism is described in the section below. Festo Didactic 87996-00 161
Ex. 4-1 Float Switches Discussion Reed switch float switches Reed switch float switches have two main components: a reed switch and a float with an encased magnet. Depending on its position along a stem, the float actuates (or does not actuate) the reed switch. The reed switches used in float switches usually have a single-pole, single-throw (SPST) or a single-pole, double-throw (SPDT) contact configuration. The reed switch in the float switch of the training system is of the SPST type. How a reed switch works The reed switch was invented in 1936 by Walter B. Ellwood from the Bell Telephone Laboratories. As stated in the original patent application, this device eliminates the complicated mechanical and magnetic structures of the usual electromagnetic relays. Figure 4-4 shows a typical reed switch diagram. This type of switch is named after the thin metal reeds enclosed in its glass capsule. The capsule is filled with an inert gas to prevent the reeds from rusting. The portions of the reeds that come in contact when the switch closes are plated with precious metal to ensure good conductivity. Glass capsule Reed Inert gas Terminal Gap Contact plating Figure 4-4. Reed switch. The working principle of a reed switch is simple. In the absence of an outside force, the reed switch is open. The gap between the two reeds isolates them and no current can flow through the switch. However, if a permanent magnet is placed close to the switch, the magnetic flux generated by the magnet draws the two reeds together. As shown in Figure 4-5, the plated tip of one of the reeds becomes a magnetic north pole, while the tip of the other reed becomes a magnetic south pole 1. Since the tips of the reeds have opposite magnetic poles, they attract each other and the two reeds snap together to close the electric circuit. When the magnet is removed from the proximity of the switch, the stiffness of the reeds brings them apart and the electrical circuit is open again. 1 Note that each reed also has an opposite pole at its other extremity. Magnetic poles always come in pairs; a magnetic monopole has never been observed. 162 Festo Didactic 87996-00
Ex. 4-1 Float Switches Discussion Permanent magnet Magnetic flux S N N S N S Contact closed Figure 4-5. Actuated reed switch. The float and the stem In a float switch, the reed switch described above is located in the stem, a nonmagnetic tube. The stem is waterproof and protects the reed switch from water. The float has a cylindrical shape and the stem passes through a hole along its axis. The float is made of a material of a lesser density than water so that it floats. A permanent magnet is embedded in the float and, when the float moves along the stem, the magnet changes the state of the reed contact. Construction and operation The working principle of the float switch can be summarized as follows. The float is made of a material that is lighter than water and incorporates a permanent magnet. As the float rises and falls with the changes in the water level, it moves the magnet along the stem, thereby opening and closing the reed switch contact, which consists of two thin moveable blades of ferromagnetic metal. The free ends of the reeds are the contact points and form the actual reed contact. Float switches can be normally open or normally closed switches. For example, the factory default configuration for the float switch, Model 6570, is with the reed contact acting as a normally closed contact. With this configuration, the magnet in the float closes the reed contact when the level is below the float. When the level rises above the float, the magnet rises with the float and, because it is too far from the reed switch, its magnetic field is too weak to keep the reed switch contact closed. Thus, the two reeds separate from each other and the circuit opens. Figure 4-6 shows how a float switch operates in NC configuration. Festo Didactic 87996-00 163
Ex. 4-1 Float Switches Discussion Stem Contact open Contact closed Float Lower stop Figure 4-6. Float switch (NC configuration). Industrial applications Float switches are mainly used to perform on/off control and to start sequential operations in batch processes. They are also used to trigger alarms when high or low liquid levels are detected in vessels. High levels can indicate unusual change in normal load demand due to clogged pipes or valves accidentally closing, while low levels can indicate pump or liquid supply failure. Advantages and limitations Float switches are relatively inexpensive, reliable, and require minimum maintenance. However, they are limited in their use as on/off control devices or alarm switches. They do not provide continuous measurement of the liquid level. Moreover, some types of float switches are not recommended for processes where dirty liquids are handled. For example, liquids that contain particles of ferrous metal can cause malfunction of magnetically operated float switches. Installing the float switch The float switch is not designed to operate in a pressurized column. To avoid damage to the equipment, be sure the column is open to atmosphere when using the float switch. 164 Festo Didactic 87996-00
Ex. 4-1 Float Switches Discussion The float switch is designed to be installed in the column, through the opening at the top of the column. Use Figure 4-7 and the instructions below to install the float switch on your system. a. Loosen the positioning lock of the mounting screw on the float switch. b. Insert the float switch all the way into the column. Secure the float switch to the column by tightening its mounting screw fully. c. By making the float switch slide within the column, adjust its position so that the bottom of its float is aligned with the desired mark on the graduated ruler of the column. Lock the float switch into position by tightening its positioning lock fully. Positioning lock Mounting screw (a) (b) (c) Figure 4-7. Installing the float switch. Festo Didactic 87996-00 165
Ex. 4-1 Float Switches Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Level regulation End of the exercise PROCEDURE Set up and connections 1. Set up the system shown in Figure 4-8. In this circuit, the float switch is used to limit the amount of water that can be pumped into the column. Adjust the position of the float switch so that the bottom of the float is aligned with the 52 cm (20.5 in) mark on the graduated ruler of the column. Overflow hose Plug Figure 4-8. Using a float switch to limit the amount of water pumped into a column. a The overflow hose causes the column to be open to atmosphere through the reservoir of the pumping unit. 2. Make sure the reservoir of the pumping unit is filled with about 12 liters (3.2 gallons) of water. Make sure the baffle plate is properly installed at the bottom of the reservoir. 3. On the pumping unit, adjust pump valves HV1 to HV3 as follows: Open HV1 completely. Close HV2 completely. Set HV3 for directing the full reservoir flow to the pump inlet. 4. Turn on the pumping unit. 166 Festo Didactic 87996-00
Ex. 4-1 Float Switches Procedure Level regulation 5. Make the pump rotate at maximum speed. 6. Observe the water level as it rises within the column. What happens when the level reaches the activation level of the float switch? Explain. 7. What causes the pump to alternately start and stop? Explain. 8. Note and record the level of the water in the column when the float switch activates and when it deactivates. 9. Stop the pump. 10. Reposition the float switch tube so that the bottom of the float is aligned with the 47 cm (18.5 in) mark on the graduated ruler of the column. 11. Restart the pump at maximum speed. Does the float switch activate and deactivate at the levels recorded in step 8? Explain. 12. Set pump speed to 50%. Do you observe a difference in the levels at which the float switch activates and deactivates? Explain. Festo Didactic 87996-00 167
Ex. 4-1 Float Switches Conclusion End of the exercise 13. Stop the pump and turn off the pumping unit. 14. Disconnect the circuit. Return the components and hoses to their storage location. 15. Wipe off any water from the floor and the training system. CONCLUSION In this exercise, you used a float switch to limit the amount of water pumped into a column. You saw that the float switch automatically turned off the pump when the water level approached the top of the column. It automatically restarted the pump after the water level had returned below the activation point of the switch. REVIEW QUESTIONS 1. What are the most common functions for the float switches? 2. Name one advantage of using a reed switch. 3. Explain how a reed switch works. 4. Why is the reed switch glass capsule filled with an inert gas? 168 Festo Didactic 87996-00
Ex. 4-1 Float Switches Review Questions 5. Name one advantage and one limitation of float switches. Festo Didactic 87996-00 169