EXPERT TIP #25: PIPE-TO-SOIL POTENTIALS: ENSURING ACCURATE MEASUREMENT IN CATHODIC PROTECTION

Measuring Pipe-to-Soil Potentials Most cathodic protection technicians will measure pipe-to-soil potentials (PSP) using a Digital Multimeter (DMM) and a Cathodic Protection Reference Electrode (CPRE).

The following scenario may be familiar: Your task is to conduct a PSP survey on a cathodically protected pipeline. You have history with this system and expect good levels of protection over the entire pipeline. You set up your DMM and portable CPRE to measure a PSP at a known test point and find that the reading is 50% lower than expected. As you have done many times before, you add water to the soil around the CPRE (which you probably should have done in the first place) and find that the PSP is now reading as expected.

Question: Why was the potential reading higher after adding water? Voltage is voltage. Certainly, the water does not create voltage. So, what is going on?

All DMMs will impose a load on all voltage circuits measured. Most professional-grade DMMs have an input resistance of 10 MΩ. (million ohms). When the DMM is connected to a voltage source, it imposes a load of 10 MΩ. This may seem high, but when used to measure very sensitive (high resistance) circuits, the meter’s load on the circuit can result in a significant error, as illustrated in the above example. This condition is referred to as "Meter Loading." In the example above, adding water to the soil around the CPRE reduces its contact resistance to earth and voltage drops across the CPRE-to-soil interface. This allows the CPRE to produce adequate current to properly operate the DMM resulting in a more accurate PSP measurement.

Meter Loading Explained

Once connected to the circuit under test (CPRE-to-Structure), the current required to operate the DMM changes the conditions in the circuit compared to what it was before the meter was connected. If the CPRE has a high contact resistance to the soil, the ability of this compromised or weak galvanic cell cannot provide adequate energy to operate the DMM properly. As illustrated in the above example, the meter is imposing a load on the CPRE circuit that can result in a significant (lower than actual) error.

An analogy is when the batteries in a flashlight become weak, and the light output reduces. That is because the light bulb is not getting the required voltage to operate it properly. Consider the light bulb as the DMM, and the weak batteries as the galvanic cell created between the CPRE and the structure in this example.

The ideal DMM would have infinite input resistance, creating zero-meter loading and no reduction of the measured voltage. Meter loading will affect the ability to obtain an accurate reading from any CPRE, even under the best conditions. Fortunately, for most instances, the effect from meter loading is minimal, and the resultant error is considered acceptable.

In the case of stationary CPREs and/or coupons, adding water is not typically possible or practical. Therefore, is the low PSP reading you see truly low, or is it low due to meter loading? Without specialized metering equipment, there is no way for the technician to know if the reading is accurate or compromised.

One area where meter loading is common is when measuring PSP readings from a stationary CPRE. When a lower-than-expected reading is indicated, the question is: Now what? Once a stationary CPRE is tested and confirmed to be compromised, the technician has five choices: 1. Accept the lower reading, understanding that it may be inaccurate or compromised due to meter loading. 2. Purchase a specialty DMM with a higher input resistance to obtain a more accurate reading. 3. Retire the CPRE and consider it unserviceable. 4. Install a new stationary CPRE to replace the compromised CPRE. 5. Purchase a specialty instrument to eliminate the effects of meter loading. Visit: https://www.farwestcorrosion.com/ref-check-vpr.html

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Why Do Copper Sulfate Reference Electrodes Become Unserviceable?

The most frequent problem with many permanent, soil-type CPREs is that they simply dry out over time. Once the CPRE is dehydrated, the resistance-to-earth will increase, and the CPRE will eventually be affected by meter loading. Even if the CPRE is installed in permanent moisture, they still have a finite life. For a copper sulfate CPRE to function, an adequate amount of copper sulfate solution must permeate the porous end plug and migrate into the soil or water. Eventually, the volume of copper sulfate will be depleted, and once that occurs, the CPRE will become unreliable even in a damp environment. How long it takes for the copper sulfate to be depleted is a question that has a complex answer. The physical size of the CPRE, the volume of copper sulfate reserves, the soil composition and relative moisture all affect the service life of the CPRE.

About Farwest:

Farwest Corrosion Control Company is an industry pioneer and leader in comprehensive cathodic protection and corrosion control services and related products. Named a Top Ten Corrosion Solution Provider from Manufacturing Technology Insights magazine in 2020, Farwest is known for finding solutions to difficult problems through quality products, sound engineering solutions and onsite installation services. Farwest was founded in 1956, remains privately held and is a certified, family- and woman-owned business. The firm is headquartered in Downey, CA, has eight regional operations and over 175 employees nationwide. Farwest: The Right Product. The Right Solution. The Right People. For more information about Farwest products and services, FarwestCorrosion.com or (310) 532.9524.