Metal Samples MS2500L, Operator'S Manual

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V. Circuit Description
A. Introduction
The MS2500L is a Linear Polarization Resistance (LPR) type corrosion monitoring instrument that
translates corrosion rates obtained from polarization techniques into a 4-20ma current which is then
transmitted through a two-wire twisted cable.
This instrument also incorporates the ability to monitor and transmit the Corrosion Potential, obtained
from LPR probes using a reference electrode. The power for the instrument is supplied on the same two
wires that return the corrosion rate signal. Therefore, to power its internal circuitry, the instrument can
use no more than the four milliamp, live zero, current supplied on the loop. This 4-20mA type of moni-
toring instrumentation has become a standard in the process monitoring industry. The 4 and 20 milliamp
values are used for several reasons:
1. A power supply voltage of 25 volts or more can be supplied to overcome long cable drops or to
fulfill power requirements for the instrumentation.
2. The 4 milliamp live zero not only provides current to power the instrument, but also provides a
method to determine loop failure. If the zero output point of the instrument was at zero milliamps,
there would be no way to differentiate between a zero output or a failure in the loop.
3. The transmission of a low impedance signal, (such as a current), rather than a high impedance signal,
(such as a voltage), sets up a transmission technique that is much more impervious. Generally, noises
induced onto signal cables are of very high impedances. It would take a very large external force to
induce a noise signal onto the loop at a current of 4mA or more. Again, if a true zero point were
used, it might be possible to induce a noise error around the zero current level. However, the use of
the 4mA live zero precludes this possibility.