Understanding Soil Resistivity and pH Measurement: Virtual Brew 7/20/2023

Measuring and cataloging the pH and resistivity of soil is a vital part of designing and implementing a cathodic protection system for corrosion prevention. Whether it’s ground bed installations or general cathodic protection system maintenance, understanding the pH and resistivity of the soil surrounding your pipeline will help optimize your CP system, saving you time and headaches.

Join Andrew Wooster, Bullhorn Remote Monitoring and PCS Field Integration Product Manager, for an informative session highlighting the importance of pH and resistivity, various methods and tools to obtain measurements, and how American Innovations can help your organization use soil pH and resistivity to your advantage with the SRM-100A.

Transcript

Introduction

Andrew
Thanks, everyone, for joining this morning — hope you have your coffee, I’ve already finished mine for the day. As I mentioned earlier, one of our recent product releases this year was the SRM 100A, a new and improved revision of the SRM 100. Since we have this new revision, I wanted to take the opportunity to talk about some of the theory behind it — why it exists, what it does — so you can make a more informed buying decision if this is a product you need for what you’re doing in the field. I’m going to talk about soil resistivity and pH measurement, specifically how they relate to corrosion.

Resistance vs. Resistivity

Let’s start with the difference between resistance and resistivity — they’re very similar, but resistance is the more common property for materials: the opposition to the flow of electricity through something, a resistor, a wire, or some material, and the magnitude in ohms dictates how easily or difficultly electricity flows through it.

Resistivity is similar, but you could almost think of it as an average — the average resistance across or through the body of a material. This is the more appropriate measurement for soil, since soil conditions around a pipeline or other oil and gas asset can change dramatically depending on location, and soil isn’t a homogeneous material, so it’s good to get an average resistance through it.
The SI unit for resistivity is the ohm-meter, indicating resistance across a length of material, and the symbol you typically see for resistivity is the Greek letter rho, which looks like a lowercase, soft “p.”

Why Soil Resistivity Matters

Soil resistivity is very influential on the corrosivity of an environment where a pipeline might be buried — in general, low resistivity corresponds to high corrosivity, and high resistivity corresponds to low corrosivity. This is an essential measurement when a CP system is being designed, or when evaluating long-term changes in a CP system or its environment.

In a high-resistivity environment, it takes more power and energy to pump current through, so typically a more expensive CP source or ground bed is used. Low resistivity means electrons flow much more readily, making you more susceptible to faster corrosion. This table on soil resistivity and corrosivity rating is pulled from some of the AMPP standards and documentation — generally speaking, this is what one could expect in terms of soil resistivity in the world of CP or corrosion. You typically see ohm-centimeters used, rather than ohm-meters (the SI standard unit), since our probes are generally sized in centimeters rather than meters.

Factors That Affect Soil Resistivity

Soil resistivity can vary pretty widely depending on location — even from one side of a room to where I’m standing, let alone more dramatically different parts of the country, based on the characteristics of the environment and weather. It also changes seasonally — in rainier seasons, soil resistivity is going to be lower, since added moisture conducts electricity better. It’s also affected by electrolytes and other minerals and how they interact with moisture in the soil, temperature (frozen soil is going to be in more opposition to the flow of electrons), and soil type and compaction — more rocks, as opposed to sand or more compact soil, will affect resistivity. More compaction generally means lower resistivity, since everything is in closer contact, allowing electricity to flow more easily.

Methods for Measuring Soil Resistivity

There’s a whole variety of methods for measuring resistivity. The ones typically used in the corrosion industry are the three I’ll cover here — no method is inherently better than another, it depends on your use case and what you’re trying to get out of the reading: the four-pin Wenner method, the soil box method, and the single-probe method.

The Four-Pin Wenner Method

In the four-pin method, you place four probe pins in the ground — typically a measurement taken over a large surface area, often prior to a dig or installing a structure or CP system, since it gives a general idea of soil resistivity conditions in an area for designing the CP system, or for choosing materials to put around a pipeline, like gravel or sand.

What’s neat about this method is that the spacing of the pins directly correlates with the depth of the soil being measured — space the pins six feet apart, and your resistivity measurement correlates to conditions about six feet below the surface. You can space the pins according to how deep you’ll be burying the pipeline.

There are two variations — one uses a galvanometer, one uses a separate voltmeter and ammeter — but a galvanometer is really doing the job of both in one box. A stimulus current is generated and flows through the earth, and the voltage drop from that current is measured, then converted, along with the pin spacing, into resistance and then resistivity.

The Soil Box Method

Another method — more of a lab-setting method — is the soil box method, which is identical in theory to the four-pin Wenner method, except you obtain a soil sample and measure resistivity in a controlled box. This is useful if you want to evaluate a corrosion incident, or if you’ve done a dig and want to take a sample back to a lab, either sending it out or measuring it yourself if you have the equipment.

One disadvantage of this method is that you lose, or influence, the soil compaction variable in the resistivity measurement — the moment you dig up soil and put it in a box, the compaction differs from what’s in the ground, so that variable comes into play, and you need to keep that in mind when using the data to make decisions about your CP system or the conditions around a structure.

The Single-Probe Method

The third method is the single-probe method, using the same methodology as the previous two — a stimulus current is generated, and the voltage drop is measured across the path that current flows. The difference is you don’t have pins spaced apart or spaced in a box — instead, you have probe terminals located at the end of a metal rod, and current flows between them while voltage is simultaneously measured. This gives a very localized reading of resistivity.

This method is particularly useful if a dig has already been conducted and you want to get into the hole and measure soil resistivity conditions right around the pipeline. We generally recommend, and hear from technicians and engineers in the field who do this, taking multiple samples around a structure and averaging them, since resistivity can vary even around a single section of pipe — top, bottom, left, right — so it’s a good idea to get multiple measurements and average them for a general picture.

Q&A: Pin Depth and Measurement Localization

[Moderator]
On the previous slide, Gord asked: can you address how the depth of the pins affects the measurements?

Andrew
Like I mentioned, the single-probe reading is very localized — it’s just going to give you the soil resistivity right around the probe itself.

[Moderator]
What about on the four-pin method — since you’re taking a sample of soil away from the environment for the soil box method?

Andrew
For the four-pin method, the spacing of the pins is fixed and is used for the resistivity calculation — the depth the pins themselves are pushed into the ground doesn’t really have a bearing on the depth of your structure. If you took your sample at the depth of the pipeline, you’d get a general idea of resistivity at that depth. In my experience, the depth the pins themselves go into the ground is fairly negligible, as long as they’re deep enough to stay in place — the depth you’re really trying to get an average resistivity sense of is much larger than the pin’s own depth. You just want to get through the upper layers — mulch, grass, topsoil — so you’re not just measuring the surface, and get down to where the soil is more representative of what surrounds the structure.

[Moderator]
Does that answer the question? I believe he was asking about the Wenner method specifically, right?

Andrew
Yeah, I think that covers it — we’re on the right spot.

I’d summarize the soil box method as more of a lab-accurate method, whereas the single-probe method for resistivity is more of a field-accurate method.

Introduction to pH

Now let’s shift gears and talk about pH — generally, a measure of the acidity or alkalinity of a solution, and its relationship to the corrosiveness of a material depends on what that material is. For steel and iron, which we commonly deal with in oil and gas pipeline structures, more acidic conditions are more corrosive, and more basic conditions are less corrosive — based on the chart we’ve shown, you’re not really experiencing corrosive conditions once you get above a pH of about 10.

This is another important variable to measure, whether preparing to bury a structure, evaluating conditions during a dig, or at some point during the structure’s life cycle. Much like resistivity, location plays a huge part in the pH you’ll generally experience — these maps show general transmission pipelines across the U.S. on the left, and general soil acidity across the U.S. on the right, showing how pH plays different levels of influence on corrosiveness for buried pipelines in different parts of the country.
Methods for Measuring pH

Like resistivity, pH has a number of methodologies, and again none is necessarily better than another — it depends on your goals.
The visual method is one of the most familiar, since we’ve all done science experiments with litmus paper since elementary school — it’s inexpensive, but best suited for fluids rather than soil.

The photometric method uses a spectrophotometer to measure wavelengths of light that translate to pH — typically not used for soil, but used in lab settings for other materials.

The method most commonly used for soil is the potentiometric method, which measures pH based on an electrochemical reaction between different materials — I’ll go through a couple of ways to do that.

Lab-Accurate pH Measurement

Much like resistivity has a lab-accurate method, so does soil pH — using a potassium chloride solution and a wire assembly. This is great for lab-accurate measurements, but not particularly useful in the field, because the whole assembly is contained in a glass container, and the moment you jab something like this into the ground, you’re going to shatter it. It’s good for bringing samples back to a lab, similar to a soil box resistivity measurement, but not for field measurements.

For this type of measurement, you take multiple samples for a given area and average them, and compare the readings to reference standards — solutions of a known pH, like 4 or 5 — to determine the pH of your material.

Field-Accurate pH Measurement

The more field-accurate measurement is what we most often use in the corrosion industry — using an antimony reference electrode along with a copper-copper sulfate reference electrode. These two materials are most commonly used to measure that delta potential — a voltage is generated when you place these two reference materials in the soil, and that voltage translates to a pH level.

Again, you want multiple samples for a given area to get a general idea, and you can average and track that over time. The SRM 100A is a bit different in that, much like with resistivity, it pairs these reference electrodes together at the bottom of the probe, so you can jab it straight into the ground. There’s a copper ring, serving the same purpose as the copper-copper sulfate reference electrode, and an antimony ring — a special metal that reacts readily to changes in the pH of a material.

The SRM 100A

To sum it up, this is the SRM 100A we’ve mentioned a few times. Two of the probe terminals facilitate the resistivity measurement, and two of the materials in the probe facilitate the pH measurement — each probe terminal is separated by an insulator, with wiring running up into the housing, which handles all the resistivity and pH calculations. The resistivity range is zero to 700 kilo-ohm-centimeters, covering the majority of soil conditions you’d experience across the U.S. and around the world for corrosion purposes. As a lab-accurate instrument, the resistivity accuracy is plus or minus 5%, and pH accuracy is plus or minus half a standard pH unit.

I’ve got a sample of the short probe here, but we also offer a much longer probe, for reaching into a dig site or hole to get a reading.

Q&A: Time to Get a Reading

[Moderator]
Question: how long does it take to get a pH measurement value in the field?

Andrew
Great question — with this device, it’s a matter of seconds to get a single reading, both for resistivity and pH. It’s got three buttons: a power button, an ohm-centimeter button for resistivity, and a pH button — press either one, and within a matter of seconds you have your reading.

SRM 100A Design Improvements

I think Ed mentioned it, but I’ll reiterate — we’ve had the SRM 100 for the longest time, and this is the SRM 100A. You’re probably hearing a lot right now about supply chain issues, and one of the reasons for this revision was to account for parts that had gone obsolete and could no longer be sourced. In the process, we also took the opportunity to improve the product — one of the biggest improvements was adding a rechargeable battery with a charging port. The previous design used four AA batteries that needed replacing once used up. Even so, both the old AA-battery version and this rechargeable version are very low-power-consumption devices, so the frequency you’ll need to recharge this isn’t very high, given the power draw.

Q&A: Calibration Requirements

[Moderator]
Do you need to do a calibration prior to a pH measurement?

Andrew
Not immediately prior to a measurement, but this device, much like our digital voltmeters, is something we offer a calibration service for — you’d send it back to us annually to get it calibrated, similar to our DVMs, to make sure it’s still operating within specification.

Closing

[Moderator]
There’s a bit of a longer question here, Andrew, but we’re about 20 seconds from being out of time, so I’d ask you to follow up directly with Brad on that one, to make sure it gets answered — it’s a really good question, we’re just out of time.
Thank you, Andrew, for presenting, and thank you, everybody, for participating. Sorry we didn’t get to that last question, but as I said, Andrew will follow up directly. Everybody have a great day — we’ll see you in a month.