Measuring levels of liquid and bulk solids is commonplace and can be done for control or inventory purposes. Numerous technologies are available for measuring liquid levels including magnetic float, magneto strictive, hydrostatic, radar, TDR and electromechanical devices. However, an overlooked technology that can be used for measuring both liquid and bulk solids level exists and is often the least expensive approach, for bulk solids, and that is the RF admittance level transmitter.
The RF admittance level transmitter continuously measures the changing level of liquids and bulk solids by measuring the effect of the target materials dielectric properties on the total capacitance of the mass of material in the vessel at any given moment. Every material has a dielectric property or electric Permittivity. From Wikipedia: “In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how much electric field (more correctly, flux) is ‘generated’ per unit charge in that medium. More electric flux exists in a medium with a low permittivity (per unit charge) because of polarization effects. Permittivity is directly related to electric susceptibility, which is a measure of how easily a dielectric polarizes in response to an electric field. Thus, permittivity relates to a material’s ability to resist an electric field (while unfortunately the word stem “permit” suggests the inverse quantity).”
So the RF admittance level transmitter simply measures the change of electrical capacitance. As the level of the target material in the vessel goes up or down, the capacitance increases and decreases respectively. The use of RF admittance level transmitters is typically limited to materials with a dielectric constant or relative permittivity of 1.8 and higher, though exceptions can exist.
To apply to many industries and materials the RF admittance level transmitter is typically available with a wide number of probe styles and types including rod and cable probes.
4-20mA 2-wire loop powered design
Linearity ± 1% FS or ± 0.5pF
Low temperature effect (±0.2% FS/⁰C or 0.1pF/⁰C)
Easy 2 point calibration
Probes for general purpose, high temperature,
high pressure, corrosive and explosionproof
Measuring range of 20~2000pF
Feed & Grain Processing Mining & Cement
Setup of the RF admittance level transmitter is very easy. Most are 2-wire powered, meaning the low voltage DC power is supplied of=ver the same pair of wires as the transmitter output, usually 4-20mA. And programming is also simple with setting a high and low point of capacitance by taking the material to as low a point as possible (preferably 0%) and a high point that is as point as possible (preferably 100%). Programming is usually accomplished by keystroke and not potentiometers or switches. Cost effective? You bet. Pricing can be from under $1,000USD. Most long range applications, as example a 40ft unit, will be around $1,400USD! That means many applications are less than even the maintenance heavy electromechanical units.