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