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Several point-contact level sensors are available. Most of these types of level sensors include a switch that is activated when the level reaches a specific point. Each of these types of level sensors will be discussed in this section.
Float-Level Sensor
The float-level sensor is the simplest level sensor to understand. From the diagram in ill. 1 notice that a float is connected, to an arm, and the arm will activate a limit switch when the arm is raised. The float will be lifted when the level of the liquid is high enough. The limit switch can be adjusted so that the exact level where the switch is activated can be set.
Above: ill. 1: A float-level sensor that uses a float and arm to activate a limit switch when the liquid level is high enough.
Another type of float switch uses a magnetic-activated switch. From the two diagrams in ill. 2 notice the float in this type of switch has a rod connected to it just like the previous type float-level sensor. This type of sensor has a permanent magnet connected to the end of the rod. ill. 2b shows that when the float raises with the liquid level, the magnet is moved into position so that it's near the magnetic-activated switch that is mounted in the head of the sensor. When the permanent magnet on the rod is in the correct position, it will pull the movable magnet that activates the switch. When the movable magnet is pulled to the magnet on the rod, the switch contacts close. Notice in ill. 2a that when the liquid level drops and the float allows the rod to be lowered, the magnet will no longer have any attraction to the switch. Small springs will then cause the contacts to move to their normally open position. (Notice that the switch has a single-pole. double-throw configuration so that the common terminal can be connected for normally open or normally closed operation.) The switch is typically connected to a pump motor a solenoid valve that is used to change the amount of material entering or leaving the tank.
Above: ill. 2 (a) A float-level sensor that uses a magnetic-actuated switch.
This figure shows the sensor at low level. (b) The float-level sensor with
the sensor at high level.
Multiple Float-Level Sensor
The multiple float-level sensor is similar to the single float-level sensor except the multiple float-level sensor provides the ability to have switches activated at more than one point. From ill. 3 Notice that the liquid level moves the float up and down on a guide that keeps it aligned with the switches. The float has a magnet mounted inside it and the switches are magnetic-activated reed switches. When the magnet in the float comes near each switch, it turns it on, and when the magnet is not near the switch, the switch s turned off. Any number of switches can be added to the system so that the level at numerous points can be detected.
Above: ill. 3 (a) Multiple reed switches are mounted so that a magnet
in the float will activate each switch when the magnet is near the switch.
The float moves up and down a guide with the level of the liquid in the
tank. (b) Close-up view of the float with the magnet encased in it, and the reed switch that is activated when the float moves past it.
Displacer-Level Sensor
The displacer-level sensor is very similar to the boat-level sensor in that it has a displacer element located directly in the liquid. The displacer element has a rod that connects it to a switch, and when the level increases, the displacer element rises with the level and moves the rod so that it activates the switch. The switch in this sensor is exactly like the magnetic switch explained in the float-level sensor. ill. 4 shows an image and ill. 5 shows a diagram of this type of sensor. Notice that when the rod moves up, the magnet on the rod moves to where it's located close to the magnet that activates the switch. The magnet in the switch activator is movable, and when it's pulled toward the magnet on the end of the rod, it will activate its contacts. When the level drops and the displacer drops and the magnet on the rod drops, the magnet on the rod will no longer have any effect on the switch magnet. Small springs will cause the contacts to move back into their original position.
The major difference between the float and the displacer element is that the float is totally supported by the surface of the liquid, and the displacer is partially submerged. This is known as the buoyancy principle. The displacer element must be slightly more dense than the liquid that it's used in. Since the displacer element is partially submerged, it's not subjected to the action on the surface of the liquid. Since pumps and agitators tend to make the surface very rough, a float sensor may be subjected to false actuation, or it may wear prematurely. If the surface is subject to floating debris or suspended solids, an additional displacer may be used that is submerged further below the surface so that it can activate safely.
Above: ill. 4. A displacer-level sensor from Tempsonics.
Above: ill. 5 (a) An example of a displacer-level sensor. The displacer
element uses the buoyancy principle to allow the sensing element to be
partially submerged so that action on the liquid surface does not interfere
with the sensor's action. (b) The liquid-level sensor is measuring liquid
level at an intermediate level. (c) The liquid-level sensor is measuring
liquid level at a low level.
Paddlewheel-Level Sensor
The paddlewheel-level sensor uses a paddlewheel that is similar to the paddlewheel flow sensor. In this application the paddlewheel is turned slowly by a small motor. When the level of the liquid or solid granules comes into contact with the paddlewheel, it will become stalled. When this occurs an amperage detector determines that the current has increased slightly, which indicates that the level has increased to a point where it stops the motor shaft from turning. The motor that is used for this type of sensor is a shaded pole motor. The operation of the shaded pole motor allows it to have its rotor - stalled and not damage the motor. When the rotor stalls, the motor windings act similarly to a primary winding of a transformer, and a slight increase in current will occur.
ill. 6 shows an example of the paddlewheel-level sensor. The paddlewheel can be mounted at any level on the side of the tank to indicate the level has been reached or exceeded. This type of sensor is used frequently to measure the level of granular plastic or other similar types of raw material that is stored in bins and hoppers.
Above: ill. 6 (a) An example of a paddlewheel-level sensor (from Omega)
that uses a shaded pole motor to turn the shaft of the paddlewheel-level
sensor very slowly. When the level of the material increases to a point
where it reaches the paddlewheel, it stalls the motor. A sensor detects
the slight increase of current when the motor stalls and activates a switch.
(b) Application where paddlewheel sensor is mounted through the top of
a tank to measure when the tank is full, and through the side of a tank
to measure when the tank is half full.
Vibrating-Tines Level Sensor
The vibrating-tines level sensor uses a set of tines that acts like a tuning fork to determine when the level of material or liquid has exceeded the level setpoint. An electronic circuit makes the tines oscillate at a specific frequency. When the level of material rises and covers the tines, it stops them from oscillating. An electronic circuit detects the change in the oscillating frequency and activates a switch. ill. 7 shows an example of this type of level sensor.
Above: ill. 7 Example of vibrating tines used to determine the level
of material. The tines are oscillated at a specific frequency. When the
level of material reaches a point where it covers the tines, they will
stop oscillating. The change of frequency is detected, which activates
a switch.
Beam-Breaker Level Sensor
A beam-breaker level sensor uses a light beam to detect the level of solids to determine when the setpoint level has been exceeded. From ill. 8 notice that two probes are mounted horizontally so that one can send a beam of light that is focused directly on the other. When the level of the material increases sufficiently, it will block the beam of light and the photoelectric detector will activate a switch. These types of sensors are useful to measure the level of granular solids. In some applications, multiple sensors are used to determine the level at several points.
Two-Wire, Conductance-Level Sensor
Another way to determine the level of certain liquids is to mount two wires at different heights in a tank where the level is measured. One wire is mounted near the bottom of the tank, and the second one is mounted at the level that is used as the setpoint. When the liquid level rises to a point where the second wire is covered, a small current is conducted through the liquid between the two wires. This small current is detected and the electronic circuit activates a switch. When the liquid level is below the second wire, the resistance of the air is too large and no current will flow (see ill. 9). Some applications of this type of sensor use additional sets is of wires that are mounted at several points along the side of the tank to detect the level of the liquid at more than one point. It's also important that the liquid has the properties that make it a conductor. Some liquids are dielectrics or insulators and this type of sensor would
Above: ill. 8. An example of a beam-breaker level
sensor, which focuses a beam of light on a receiver. When the level of
the material increases, it will block the beam of light, and a photoelectric
sensing circuit will detect the difference and activate a switch .
Above: ill. 9. Example of two conducting wires used to detect the level
of liquid. One wire is mounted near the bottom of a tank, and the other
is mounted near the top. When the liquid level covers the top wire, a
small current is conducted through the liquid. The small current is detected,
which activates a switch.
Another version of this type of conductance-level sensor uses one or two probes instead of the wires. If one probe is used, the side of a metal tank is used as the other probe. When the liquid level increases to a point that it touches the tip of the probe, a small current is conducted between the probe and the side of the tank. If the sensor has two probes, a small current will flow through the liquid as it rises and touches both probes. ill. 10 provides a diagram that shows examples of the probes being used as level sensors and for alarm points. Notice that one of the sensors has four separate probes of different lengths to indicate when the tank is one-quarter, one-half, three-quarters full, and completely full. Each probe is connected to an indicator lamp.
Above: ill. 10 Example of conductance probes being used to sense the
level of liquid in a tank. The probes are shown as alarms to indicate
high liquid levels and low liquid levels.
Thermistor-Level Sensor
The thermistor-level sensor operates from a principle that the temperature of the liquid being measured will be different from the temperature of the surrounding air. The thermistor is mounted in the tank at the specified level. Since it's not submerged in liquid, it will be measuring the temperature of the air. When the liquid level rises and covers the thermistor, it will be measuring the temperature of the liquid. The small change of temperature between the air and the liquid will cause the thermistor's resistance to change, which can be detected in the bridge circuit. The bridge circuit can be designed to activate a set of switch contacts.
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