LPR Meter Handbook Page 6
4. Corrosion Rate
Measurement Operation
Introduction
Overview
Current Measurement
Autoranging
The LPR Handheld meter provides a complete corrosion rate
measurement system on a single Eurocard PCB.
Corrosion rate measurement is carried out potentiostatically,
using the three electrode linear polarisation resistance
measurement method.
The circuit samples the free corrosion potential of the test
electrode, applies a 20mV anodic polarisation and measures
the resulting polarisation current. This is directly
proportional to the corrosion rate of the test electrode.
The current measurement circuitry is fully autoranging and
the board is simply configured to take the measurement on
an instruction from the Main Control Unit.
Two outputs proportional to the polarisation current are
provided. The first is simply the output of the current
measuring amplifier. This varies during the time the
polarisation is applied to the test electrode, usually
approaching a steady state value after some 100 seconds or
more. At times when polarisation is not being applied to the
test electrode this output will be zero. The magnitude of
this output is simply equal to the polarisation current times
the range resistance.
The second output is proportional to the absolute value of
the polarisation current at the end of the polarisation period.
It is generated by a digital to analog converter and retains
its level during the ‘off’ (no polarisation applied) periods.
The value of this output represents the last polarisation
current measurement before the polarisation was removed
from the test electrode. The voltage of this output is equal
to the absolute value of the polarisation current times the
range resistance divided by two.
The polarisation current is measured as the voltage drop
across a resistance in the auxiliary electrode path. Some
+/- 12V are available at the output of the inbuilt potentiostat
for the test electrode polarisation, any voltage drop across
the current sensing resistor will reduce this amount
proportionally, in the extreme leading to a loss of
potentiostatic control of the test cell and erroneous
readings.
The autoranging circuitry is programmed to switch to the
next less sensitive range (lower measuring resistance) as
this voltage drop approaches some 3V and to switch to a
more sensitive range (larger resistance) as it decreases
below some 0.26V. In the autoranging mode the output of
the current measuring amplifier will therefore normally be
somewhere between these two limits. In certain situations
