Metek RCT 10 series, Инструкция по установке и эксплуатации

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Introduction
1
SECTION 1: INTRODUCTION
1.1 System Description
The instructions in this manual are for the AMETEK Drexelbrook
RCT Series RCT Series for level measurement in liquids, slurries,
interfaces, and granulars.
Each AMETEK Drexelbrook RCT Series system consists of a RCT
Series two-wire, 4-20 mA electronic unit and a 700 series sensing
element. A 380 series connecting cable is also supplied for connection
of the sensing element to the electronic unit.
The RCT Series system is an admittance-to-current transducer. A
change in level produces a change in admittance which results in a
change of current. It is termed a two-wire transmitter because the
same two wires that are used to power the unit also indicate the
change in level (4-20 mA).
1.2 Technology
In a simple capacitance sensing element, when the level rises and
material covers the sensing element, the capacitance within the
circuit between the sensing element and the medium (conductive
applications) or the sensing element and the vessel wall (insulating
applications) increases. This is due to the dielectric constant (k)
of the material which causes a bridge misbalance. The signal is
demodulated (rectified), amplified, and the output is increased. There
are drawbacks, however, especially when there is coating of the
sensing element.
An RF Admittance level transmitter is the next generation. Although
similar to the capacitance concept, RCT Series employs a radio
frequency signal and adds the Cote-Shield™ circuitry within the
Electronics Unit.
Built-in oscillator buffer and chopper drive circuits permit separate
measurement of resistance and capacitance. Since the resistance
and the capacitance of any coating are of equal magnitude (by
physical laws), the error generated by a coating can be measured and
subtracted from the total output.
This patented Cote-Shield™ circuitry is designed into the RCT Series
and enables the instrument to ignore the effect of buildup or material
coating on the sensing element. The sensing element is mounted
in the vessel and provides a change in RF admittance indicating
presence of material.
The Cote-Shield™ circuitry prevents the transmission of RF current
through the coating on the sensing element. The only path to ground
available for the RF current is through the material being measured.
The result is an accurate measurement regardless of the amount of
coating on the sensing element. By far the most versatile technology
available it works with all types of materials in a vast array of
conditions, from cryogenics to high temperature, from vacuum to
10,000psi pressure.
Figure 1-1 Capacitance
Sensing Element
k
air
A
d
C
k
media
C =
k A
d
C =
k A
d
A
k
air
d
C
k
media
Tank Wall
Coating
High Resistance
Little to No Resistance
TFE Insulation
Sensing Element
(More Coating is
Easier to Ignore)
R
Xc
Xc
R
Oscillator circuitry through phase shift
cancels small amounts of RF current
flow (both Resistive and Capacitive)
caused by coating
Figure 1-2
RF Admittance Sensing
Element with Cote-Shield