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SECTION 1 INTRODUCTION
1.1 System Description
This instruction manual is for the AVI-PDU Series smart two wire RF admittance level limit switch, which is used for
most level measurements and control and is suitable for almost all industrial or civil applications where measuring level of
liquids, powders, slurries and interface. The equipment can be either installed indoor or outdoor, no special mounting
requirements needed.
The equipment is a level switch comprised of one electronic unit, one connecting coaxial cable and one rigid or cable
sensing element (and also it could be called probe). These components can be connected in an integral configuration or a
remote configuration which all electrical components are mounted away from the measuring point and the sensing element. The
sensing element can be manufactured of many different metals and insulators depending on the specific application. See figure
1.1 as AVI-PDU series integral system scheme.
Figure 1.1 AVI-PDU series RF level switch
The AVI-PDU series RF level switch is strictly followed the International/Chinese standards which list below.
1.2 Operation Principle
Radio Frequency admittance level technology uses a basic capacitance technology to detect the difference in materials, i.e.
air/powder, air/liquid, oil/water. But it has advanced features for rejecting coatings, more reliable and stable operation, higher
accuracy and remote electronics.
Radio Frequency admittance technology employs a capacitance bridge and differs from capacitance technology in the
electronics; cable and sensing element 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. 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, build-up on the sensor, cable length and cable temperature have no affect on the
measurement. A separate ground wire connects the probe ground to the electronic amplifier ground.
