Nace CP 1, Course Manual

Page: 176 / 535

Field Measurements 5:29
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Using the Wenner method, the soil resistivity,
ρ
(Greek letter
rho
), in ohm-
centimeters is determined by:
ρ
= 2
π
a R = 6.28 x a x R
For “a” in centimeters and “R” in ohms
To obtain “
ρ
” in ohm-cm, if “a” is in feet and “R” is in ohms, the formula
becomes:
ρ
= 191.5 x a x R
You might be asked to collect data so that the resistivities of various layers
of soil can be calculated. Figure 5.13 shows a typical layer situation.
ρ
1
avg
ρ
2
avg
ρ
3
avg
S
1
S
2
ρ
1
layer
ρ
2
layer
ρ
3
layer
S
3
ρ
1
avg
ρ
2
avg
ρ
3
avg
S
1
S
2
ρ
1
layer
ρ
2
layer
ρ
3
layer
S
3
Figure 5.13 Average and Layer Resistivity
This measurement is done by taking a series of measurements at ever-
widening pin spacings. It is important that the pin spacing be centered on a
fixed point between the center two pins. This means moving all four pins
out to wider spacings so that the center point of your measurement location
is always centered between the two inside pins.
To ensure accuracy, it is good practice to take two sets of data,
perpendicular to each other. This will help expose any anomalies in the soil
layers.