AVI-SAT general-purpose smart RF admittance level system

The SAT general-purpose smart RF admittance level system, used for most continuous level measurements. It is a general-purpose continuous system comprised of one electronic unit, one connecting coaxial cable and one rigid or cable sensing element (or probe).  These components can be connected in an integral configuration or a remote configuration where all electrical components are mounted away from the measuring point The sensing element can be manufactured of many different metals and insulators depending on the specific application.   

Operation principle

Radio Frequency admittance level technology utilizes basic “capacitance” principles to determine level, special admittance technology to ignore conductive coatings (build-up) that stick to the probe and driven shield technology to compensate for capacitance changes in the coax cable due to temperature.

“Admittance” in Radio Frequency Admittance means the reciprocal of impedance In basic electrical theory it is comprised of the value of resistance, capacitance & inductance. “Radio Frequency ” refers to frequency of the signal induced on the sensing element. According to basic AC electric theory, capacitance can be measured by inducing an ac waveform across a capacitor according to the formula C = K×A / d， where K is the dielectric of the capacitor,  A is the area of the capacitor plates, and d is the distance between the plates.  

RF admittance employ a capacitance bridge and its main difference from simple capacitance technology is the addition of two important circuits: the oscillator buffer and alternating transition chopper driver.

The oscillator buffer is a special amplifier between the frequency generating oscillator and the bridge circuit to maintain a constant voltage on the sensing element regardless of level or the amount of coating left on the probe. The alternating transition chopper driver is another special circuit used to compensate for the capacitance and resistance created by coatings (build-up) left on  the probe as the level moves up and down. Conductive measurements pose two problems to basic RF technology.

The first problem is that conductive material increases the capacitance seen by the measuring circuit as the level rises, and the probe conducts more current as the level rises. This increase  in current starts to  load the oscillator circuit. This loading pulls the oscillator voltage down, causing changes in the oscillator signal or even stopping the oscillator. The FIT 8051 transmitter overcomes this problem by integrating a special buffer amplifier between the oscillator and bridge to compensate for current flow in the probe circuit. This assures that the oscillator voltage is stable and the signal on the probe is stable.

The second problem for conductive material measurements is the build-up left on the sensing element when the Level goes down. The build-up contains both capacitance and resistance and  is connected to ground through the actual level on the probe. The build-up is seen by the measuring circuit as level. If the material is very conductive the whole coating will be read as level Sketches and equivalent circuit.

Since the build-up has an infinite number of small resistances and capacitances conducting current to ground, theamount of current conducted to ground will be equal in magnitude but 90 degrees out of phase if the build-up is long.(capacitive current leads resistive current in ac circuits) Sketch of phase result

Due to the fact that the coating impedance of capacitance & resistance is the same value, total measured capacitance

 is C_level + C_coating ，then minus the same resistance value as C_coating，we obtain the real level value with virtually no coating

error.  e.g.        

Cmeasure = C_level + C_coating

Clevel  =C measure - C_coating

=Cmeasure - R

The AVI-SMT also uses a driven shield circuit to stabilize the coaxial cable. The electronic unit’s measuring (probe) signal is also passed through a parallel in-phase amplifier internally. This additional signal is connected to the shield of the coaxial cable and then is passed to the sensing element’s shield connection if a driven shield sensing element is used. If a fully insulated probe is used, the coax cable shield is left un-terminated and insulated at the probe end. This driven shield amplifier is an in-phase amplifier with a gain of 1. The signal on the shield has the same potential, phase, and frequency as the measuring signal but is electrically isolated from the measuring signal and measuring circuit. Since the center wire & shield wire of the coaxial cable have the above relationship, there is

no potential difference between them. No potential difference means that no current flows between the conductors, and no current flow (leakage) from the center wire to the shield means there is no capacitance between them (capacitance equal zero.) As a result, neither cable length nor cable temperature will affect the measurement. A separate ground wire connects the probe ground to the electronic amplifier ground.

By using a driven shield technology for the cable and special buffer and chopper circuits RF Admittance eliminates all the weaknesses of capacitance technology for conductive level measurements.

AVI-SAT serial smart RF admittance level product is the intelligence style of our AVI-SMT serial, except keep SMT serial original functions, AVI-SAT serial also adds software processing and digital communication function which extremely increasing the instrument accuracy/self check and reliability.

Characters 

Intrinsic safe design: two wire intrinsic safe design both for electronic unit and sensing element 

Free maintenance: No period maintain or replacement needed as other paddle、vibration or mechanical switches, the parts will not occur block、broken or abrasion. 

Coating rejection: Driven-shield electronic design makes the instrument ignore the coating affection on the vessel wall or sensing element, no period clean or re-calibration needed.

Chemical compatible: Variable sensing element design could meets any process material requirements. 

Wide application: process temperature could run from -183℃~815℃, process pressure could be vacuum~100bar.

Zero drift: will not drift along with the process temperature or density change  

Reliable Service life: unique technology give the instrument over 15 years service period 

HART protocol communication 

Software processing makes accuracy upgrade a step compare to AVI-SMT

Built-in self-check function

Safety Protection: Built-in sensing element input protection avoid the instrument destroyed under static energy/surge or electronic-chemical condition.

Easy installation: instrument could be mounted in the field through the vessel screw or flange, could be selected as integral or remote configuration, easy for installation and calibration.

Typical application

Liquid: conductive and insulating liquid (include liquefied gas) 

Slurry: conductive and insulating slurry 

Granular: food、plastic chips、coal etc.

Interface: liquid interface between two different dielectric constants 

Powder: plastic powder、cement or fly ash etc.

Specification:

Output: 4 ~20mA, HART protocol 

Fail Safe: HLFS/LLFS could be adjusted through software

Power: 12~35VDC, 1W max. 

Max. Load resistance: 450ohm under 24VDC 

Ambient Temp.: -40 ~+70℃ (-40 ~158 ℉)

Storage Temp.: -40 ~+85℃(-40 ~185℉)

Ambient humidity: <100% RH (while 25℃)

Response time: 1~30 seconds software adjustable (to 90% final value)

Accuracy: +/-0.5 %( unit, under standard condition)

Temp. Affection:0.25%/30℃ (54℉)

Span: 20,000pF (Max.) (While set the resistance component is infinity, then use capacitance denotation), maximum

distance is about 1000m(39370”)(while use difference sensing element the range will be different)

Inactive length: normally the allowed sensing element max. inactive length is 50m (1968.5″), for 50m (1968.5″)~100m (3937″) consult factory 

Spark Protection (for probe): surge strike less than 1000V, static strike less than 4kV/8kV

RFI (Built-in filter): the whole system has passed spatial 10V/m electromagnetic field and 3V/m electromagnetic field1current infusing tests 

Electrical Connection: double M20*1.5 (cable diameter less than 12mm, 0.47″) ,  3/4″ NPT optional 

Remote Coax Cable length: 5m(197″) standard, 0.1(3.9″) ~50m(1968.5″) optional

Over 50m(1968.5”) to 100m(3937”) consult factory

Process Connection: Standard:  BSPT(R) Thread, Optional: NPT or flange

Mounting: Typically vertical (both up and down) or slope installation 

Ingress Protection:  IP67  

Explosion-proof rating: Ex iaIICT4 

Approvals: CE Mark, ATEX, FM (consult factory for status)

 Probe specification: