What is it?
How do I collect the data?
How do I interpret it?
Why is my boss making me do this?
This article will briefly discuss the foundation and importance of soil resistivity, collecting data, and how to interpret data.
What is soil resistivity?
Soil resistivity is pretty much how much the soil resists current flowing through it. The lower the resistivity, typically less than 10,000 ohms, the higher the corrosion rate will be on buried metallic structures - vice versa to higher resistivity. This is because with lower resistances, the corrosion cell can work easier. Ideally, you’d always want to aim for lower resistances on buried structures with cathodic protection because CP works much more efficiently.
How do I collect the data?
There are different methods to collect soil resistivity data, but the most common method in this industry is the Wenner 4-Pin Method. This method is practical, reliable, and it’s easy to train someone in how to perform it. The Wenner 4-Pin uses four metallic rods (pins) equally spaced apart, and the resistance meter shoves a known current from the two outside pins towards the two inside pins. The two middle pins measure the voltage drop between them – thus calculating the soil resistivity.
Important Notes and Considerations:
The pin spacing needs to be reflective of the depth you’re trying to accurately calculate. For example, if you want to know the soil res of 10’ below, your pin spacing needs to be 10’ apart.
ALWAYS perform soil resistivity tests PERPENDICULAR to the existing structure if that’s the goal of the test. If your pipe runs north to south, complete your testing east or west of the structure.
Cover multiple depths by spacing pins at different intervals: 5ft, 10ft, 15ft, 25ft, 100ft, 300ft, or whatever your boss tells you to do. It’s always good to have too much information.
How to use the Wenner 4-Pin Method:
Product Recommendation:
In my time, I’ve always used MCMiller’s Soil Resistivity box.
The soil res meter comes with color coded leads corresponding to their connection port on the meter.
Very dummy-proof and straightforward to use.
Wide range of resistances on scale.
Possesses the ability to fine tune readings for complete accuracy.
Okay. I collected this data from my soil resistivity meter. Now what?
Once you’ve watched my educational video and collected different soil res readings, now the fun part begins. There’s a formula that the collected readings must be entered into for any of these to be useful.
Soil Resistivity Formula: 191.5 x D (distance in feet of pin spacing) x R (resistance value collected from meter)
Example: Let’s say you measured 4 ohms on a 1 scale (4 ohms x 1 scale = 4 ohms) off the soil res box at 5ft spacing.
191.5 x 5 (pin spacing) x 4 ohms = 3,830 ohm/cm2.
There’s another calculation that uses meters instead of feet, but I’m an American, so I do not know meters.
How is this useful?
If you’ve collected multiple pin spacing resistances, you can determine what kind of cathodic protection design will be best, anode choice and longevity, and determining current requirements for buried structures. Your engineer will thank you for collecting these.
Example: If resistances are VERY high near the surface or even 20ft deep but continue to lower the deeper you go, a deepwell would be a wise consideration.
Tips:
Be mindful of where the pins are being shoved in the ground. If possible, shove pins all the way in, avoiding rocks and debris.
Make sure all connections are tight and in their correct positions: C1, P1, P2, C2.
Be consistent in your step up. Don’t hop from a .001 scale to a 10 scale. Small incremental steps.
Bring a helper. This greatly improves everyone’s experience.
I am a young technician, and I’m always eager to hear different people’s ideas, tactics, and opinions in the industry. Feel free to let me know how you collect, interpret, and use soil resistivity readings in your career.