Metal Samples MS2601E - Interpreting Data; Metal Loss

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c. Interpreting Data
i. Metal Loss
The MS2601E measures the Metal Loss of an ER probe and converts that value to a linearized 4-20mA
current loop output. The 4-20mA output is directly proportional to the cumulative Metal Loss of the
ER probe at any given time. The overall span of the 4-20mA output is proportional to the life of the
probe in use, so for DCS/SCADA programming the 4-20mA signal can be scaled as follows:
4mA = 0 mils (0% Metal Loss)
20mA = Probe Life in mils (100% Metal Loss)
The Probe Life can be found in Table 1 and is also listed on the Metal Samples probe tag (as the
“Multiplier”.)
Figure 6. Metal Samples Probe Tag
Figure 7 illustrates the relationship between Loop Current and Metal Loss. In this example a probe
with a 10-mil life is assumed. However, the X axis could be changed to represent any Probe Life.
Figure 7. Output Relationship
As seen here, the transmitter output begins at 4mA for a new probe (zero Metal Loss) and increases as
the probe element corrodes, eventually reaching a maximum of 20mA when the probe sensing
element has been completely consumed by corrosion (in this case, 10 mils of Metal Loss.) At this time
the probe has reached its end-of-life and must be replaced.