AMAZON multi-meters discounts AMAZON oscilloscope discounts
The RF admittance-level sensor uses pulsed radio frequency waves to determine when material or liquid is touching the end of its probe. Since this type of sensor uses the change in dielectric ro determine the level of a liquid or granular solid material, it's also called a capacitance-level sensor. ill. 1 (right) shows an example of this type of level sensor and ill. 2 (below) shows the electronic circuit that produces the radio waves. From the first diagram, notice that this type of level sensor has a long probe. The circuitry in the top of the sensor produces pulsed radio frequency waves that flow through the probe to the tank wall or the bottom of the tank, which provides a ground for the circuit. This means that the probe must be strategically located in close proximity to the outside wall or to the bottom of the tank so that the amount of dielectric in the air between the probe and the tank wall is known. If the bottom of the tank is used as the ground plane, the amount of capacitance between the probe and the level of the liquid or solid granular material will change as it rises and moves closer to the probe. The change of capacitance can continue to be measured as the material or liquid begins to cover the probe. If the side of the tank is used, the probe will use the side as the ground plane reference and it can also continue to determine the change of capacitance as the liquid moves up the probe. In both applications the dielectric of each type of liquid or material can be measured, and the probe can be calibrated to show the precise level of liquid or granular material as it gets closer to the probe or moves up on the probe.
Above: ill. 1 Example of an RF admittance
(capacitance) level sensor. The probe can be mounted so that the side
of the tank or the bottom of the tank can be used as the ground reference
for the probe.
Above: ill. 2: Electrical diagram of an RF admittance-level
sensor.
ill. 2 shows an ac power source that is used to provide the radio frequency for the probe. An op amp is used in conjunction with a transformer to provide a series of pulses whose high frequency is in the radio wave range. The secondary of the transformer is center tapped and connected to an op amp that measures the amount of power used by the probe to transmit the energy wave. When the probe is in free air and no material or liquid is close to it, the amount of energy will be minimal. When the material or liquid begins to rise and changes the dielectric between the probe and the tank, the amount of energy to transmit the energy wave will change. The amount of change will be proportional to the level of the material or liquid. The output of this op amp will provide a 4-20 mA signal that is proportional to the level of the liquid. The sensor has a zero and span circuit to provide accurate calibration.
A two-probe version of this type of probe is also available for use in tanks that are nonmetallic. Some chemicals and other liquids must be stored in plastic tanks. In these types of applications, the tank wall or bottom can't be used to create the ground plane. A two-probe capacitance-level sensor uses two probes that are located in close proximity to each other. If the entire level of the tank is to be measured, the two probes must be long enough to reach to the bottom of the tank. When the level in the tank is near empty, the probes are exposed to air which allows the circuit to use the minimal amount of energy. When liquid level begins to rise and cover the probes, the capacitance will change because the dielectric between the probes is changing. The electronic circuitry for this type of sensor is very similar to the RF admittance-level sensor.
Related Articles:
