Constant Meter Capacity (Cmc) Pc Assembly - ABB COPA-XM 3000 Series Instruction Manual

Series 10DX3000 Magnetic Flowmeter Instruction Manual
Models 10DX3111/3311E flanged flowmeter primaries contain two flux producing coils wired in series
and a pair of diametrically opposed electrodes mounted at 90 degrees to the coil flux plane (refer to
FIGURE 4-2 below). Meter coils are excited with approximately +10 volts of pulsed DC. A precision
current sensing network is connected in series with the coils. The current sense network produces
what Fischer & Porter refers to as a "Reference Voltage", which is typically +70 millivolts. The
reference voltage is directly proportional to the strength of the magnetic field in the measuring tube
and is measured by the signal converter. The reference voltage must be measured, since any
variation in reference voltage will also produce a proportional change in electrode signal voltage,
assuming an unchanged flow velocity. The current sense (reference network) may be in the Primary
or the Secondary, depending on the model number and/or the design level.
FIGURE 4-2. SIMPLIFIED MAGMETER SYSTEM BLOCK DIAGRAM

4.3.2 Constant Meter Capacity (CMC) PC Assembly

The CMC PC Assembly provides several functions. These include:
1. Establishing interconnections between the Flowmeter internal wiring and the sig-
nal converter.
2. Permitting factory adjustment of meter capacity values to a fixed value for each
nominal size Flowmeter.
3. Establishing proper wiring connections for remotely mounted signal converters.
Meter coil current and, consequently, calibration factor are established by adjusting a precision
current-sense network which is in series with the meter coils. The current-sense network consists of
a low-resistance current-sensing resistor along with an adjustable resistive divider network placed
across the current-sense resistor. A potentiometer is also used to provide fine-tuning of the meter
capacity. Should this resistive network be damaged, repair and recalibration are only possible by
using precision electronic calibration instruments.
4-6