Principle and application of level switches (meters)

Thread Content

Excerpt from the China Machinery Community: A level switch, as the name implies, is a switch used to control the liquid level. Formally, they are mainly divided into contact and non-contact types. Non-contact types include capacitive level switches, while contact types include float-type level switches, electrode-type level switches, and electronic level switches. Capacitive level switches can also be implemented using a contact-based method. http://www.cmiw.cn/data/attachment/forum/201811/26/131008bbx5qpx6346mcwiu.gif Level control system. A level control system mainly consists of three parts: the sensor section, the control section, and the actuator section. Common level sensors can be classified based on whether they come into contact with the medium being measured: Contact types include float balls, pressure sensors, level switches, tuning forks, photoelectric, and resistive-capacitive sensors; non-contact types include radar and ultrasonic sensors. Cable float switch – Working principle: A cable float switch uses microswitches or mercury switches as the contact elements. When the switch is tilted at a certain angle relative to its pivot point (usually, the microswitch tilts by 28°±2°, while the mercury switch tilts by 10°±2°), it generates an ON or OFF signal. http://www.cmiw.cn/data/attachment/forum/201811/26/131009sz1c77ahg77h66cv.gif The cable float level controller is designed based on the principles of gravity and buoyancy. It mainly includes a float body, a high-capacity microswitch installed within the float body and a driving mechanism that can put the switch in either an on or off state, as well as a three-core cable connected to the switch. http://www.cmiw.cn/data/attachment/forum/201811/26/131010d0boecheqck5z5ib.gif Features of the application: The cable is made of special materials that are resistant to oil, acids, and alkalis, offering excellent corrosion resistance. Simple structure and reliable performance. It outputs stable and reliable on/off switch control signals, with no malfunctions and high reliability. It is easy to install and convenient to adjust; by moving the positioning weight up and down, the control range of the level surface can be adjusted as desired. Wiring and installation method for instruments: For single-phase pumps of 1KW and below, the float switch can be connected directly in series in the circuit (Figure 1). For pumps with a power rating of over 1KW, a float switch can be connected in series in the control circuit of the electrical control box (Figure 2). The control level of the liquid level is determined by the length of the cable in the liquid and the position of the weight on the cable. Use brown and black wires: When the float is at the lower liquid level, the connection is closed. When the float is at the upper liquid level, the contacts are open. Use brown and blue wires: when the float is at the upper liquid level, the connection is closed. When the float is at the lower liquid level, the contacts are open. Unused wires must be absolutely insulated. ) It is recommended to use a voltage of 24V for float level controllers. http://www.cmiw.cn/data/attachment/forum/201811/26/131011cff53zur0ovgovy0.jpg Figure 1 http://www.cmiw.cn/data/attachment/forum/201811/26/131011ierl69zvv0il24f6.jpg Figure 2 Method of installing the weight: 1. Pass the cable of the float switch through the recessed circular hole at the center of the weight, then gently push the weight so that the plastic ring located above the circular hole falls off due to the force exerted by the cable end. (If necessary, this plastic ring can also be removed using a screwdriver.) Then wrap this detached plastic ring around the cable at the location where you wish to fix the weight to set the level of the liquid. 2. Gently push the weight to pull out the cable until the center of the weight catches onto the plastic ring; the weight will not slip as long as it is lightly seated on the plastic ring. If this plastic ring is damaged or lost, a bare copper wire of the same diameter can be used to connect to the cable in order to replace the float level switch. Precautions: 1. The weight is an accessory provided only if required. 2. Please pull the cable directly to the control box, and try to avoid using intermediate connectors. If a connection must be made, the cable connector must under no circumstances be immersed in liquid. 3. Unused wires must be absolutely insulated. Rod float level switch: http://www.cmiw.cn/data/attachment/forum/201811/26/131011bpcaa1mcq1vmkbkc.jpg Operating principle: The rod float level switch operates on a principle similar to that of a small float level switch. Inside a sealed metal or plastic tube, one or more reed switches are installed; then, one or more hollow floats with ring-shaped magnets inside are placed within the tube. A fixing ring is used to keep the float in the appropriate position relative to the reed switches, allowing the float to move up and down within a certain range. http://www.cmiw.cn/data/attachment/forum/201811/26/131012u6bzqupieqbjebpe.jpg The magnet inside the float is used to attract the contacts of the reed switch, thereby enabling opening and closing actions for the control or indication of liquid level. The image below shows the relevant positions of the float switch in the normally closed (NC) and normally open (NO) states: http://www.cmiw.cn/data/attachment/forum/201811/26/131012s6rn2zbrjbq5blo5.jpg. Application characteristics: The normally open and normally closed states correspond to the conditions when no liquid is present. The linkage float level switch is a custom-made product; the appropriate type of float is selected based on the temperature, pressure, specific gravity, and acid-alkali resistance of the liquid being measured. When purchasing, it is also necessary to determine the connection specifications (flange mounting or threaded mounting, etc.), the positions of the various operating points, the operating mode (normally open or normally closed), and the total length. http://www.cmiw.cn/data/attachment/forum/201811/26/131012c2ddoa3yxx22coyc.jpg When installing, the following points should be taken into account: 1. The installation location should be far away from the water inlet; otherwise, the switch may malfunction due to fluctuations in water level at the inlet. (This also applies to other types of float level switches.) 2. If the switch is installed on the wall of the container, an L-shaped angle steel bracket can be used. (A relatively simple installation method; generally, it is a threaded float level switch. 3. If the switch is installed in the mixing area, a wave guard or wave-blocking baffle can be installed.) (In cases where the installation environment is like this, it is recommended to first consult the manufacturer to determine the size of the float and the installation threads before installing the wave barrier.) 4. Choose a flanged pipe with an inner diameter larger than the diameter of the float. 5. It is recommended to use multi-core cables for wiring. (This ensures its safety; it is generally chosen for outdoor installation. For example, cable-type float level switches use multi-core cables, as shown in the image: http://www.cmiw.cn/data/attachment/forum/201811/26/131013ogtxuvgt8tiuubgo.jpg 6. The load of the circuit being controlled must be compatible with the capacity of the contacts of the float switch.) (Involves voltage and current aspects). 7. The specific gravity of the liquid being tested must be greater than that of the floating ball. (If the buoyant density is greater than the liquid level density, the buoyant ball will not be able to sink.) 8. The operating point of the float has been adjusted at the time of manufacturing in accordance with the customer’s specifications; please do not adjust the position of the float arbitrarily. (If there is a slight difference in position, you can contact the manufacturer to determine whether it can be adjusted, or confirm this with them before placing the order.) Routine maintenance: 1. Regularly remove dirt from the linkage and float (the frequency depends on water quality). 2. Check whether the screws on the ring fastener are loose. Common faults and solutions: The float does not move: ① The density of the liquid is lower than that of the float; ② The float is leaking; ③ Foreign objects are blocking the float. Solutions: ① Recheck the density of the float; ② Contact the company to have a new float installed; ③ Remove any foreign objects. The float moves, but no signal is output: ① The position of the float is incorrect; ② The reed switch is damaged. Solutions: ① Adjust the position of the float; ② Replace the reed switch. Abnormal signal output due to magnetic field interference in the vicinity. Solution: Eliminate the magnetic field; the signal remains unchanged, it cannot be restored, the float ball cannot return to its original position, as there is a foreign object stuck there. Solution: Remove the foreign object; this will result in two signal outputs, and the position of the ring buckle will change.