Fundamentals of Cathodic Protection Measurements: Virtual Brew 8/15/2024

Transcript

Introduction

Cole Finney
Thank you — and as she introduced me, my name is Cole Finney. I’m the product manager for the Bullhorn remote monitors, the Triton test stations, and the Micromax portable current interrupters.

Taking a look at our agenda: we’re going to be covering a whole bunch of different measurement factors that are tied into a measurement device you’ll be using when taking your cathodic protection measurements. I have a whole bunch on the screen to the right there — looking at our DVM, the Micromax, and a few other tools. We’ll be covering how all these terms work together to calculate what measurement you’re actually receiving on your DVM, and then finally a few best practices for actually taking those measurements and maintaining the device you use to capture them.

Range

The first one we’re going to talk about today is range. Basically, range is the highest and lowest measurement that any measurement device can capture. I have a few examples on the screen — on the left is a simple waveform, and that waveform has a signal with a plus or minus 200 milliamps as the highest and lowest measurement you could capture, so that would be your range if you’re looking at a spec sheet.

I have the specs on the screen for the RM510, our remote monitor for a rectifier. For a rectifier, you’ll see that range can vary for DC and AC current, and oftentimes you’ll encounter various different types of ranges for whatever measurement tool you’re using. On a DVM, you’ll probably encounter a plus or minus 250 volts or greater oftentimes, and on a rectifier monitor, all the way up to almost 300 volts — it’s very common to encounter.

Resolution

Resolution is basically the smallest measurement that you can take on any measurement device — usually the smallest one it can represent. As you can see on the screen, I’ve got two Mesa tablets — or if you use an Allegro field computer, you’ll see the same type of screen — but basically these two devices are capturing the same measurement, and the resolution is smaller on the one on the right, so you can capture a more granular measurement on what you’re measuring. You have one volt on the left and 1,000 millivolts on the right, but these are the same measurement.

You want to make sure that whenever you’re out in the field capturing your compliance measurements, you have the resolution on your DVM set to the right resolution for what you’re capturing. Oftentimes a smaller measurement is better, and here’s a real-world example that shows why.

Real-World Example: Resolution and Compliance

Say I’m out in the field, I walk up to a test post, and my DVM is set up with a resolution of about 50 millivolts — so I can only take a measurement in increments of 50 millivolts. I hook my DVM up with my connectors, take my pipe-to-soil reading, and I see a reading of -850 volts. Looks good to me, I mark it as a pass, and I move on.

However, the actual measured potential on that structure was -833 volts. So in reality, I gave myself a false pass, because I thought I was capturing the right measurement based on the settings of my DVM, when in reality I needed a smaller resolution to capture a correct measurement.

If I had set my resolution to 1 millivolt instead of 50 millivolts, I’d come back to that exact same structure, take my pipe-to-soil reading, and actually see a -834 volt measurement. Then I would know that test post failed the compliance check, and I’d have to mitigate my CP system in some way. So this is really important — make sure that whatever tool you’re using, whether it’s the 1100 or the 2130 DVM, the resolution is set up correctly on your field computer.

Sensitivity and Noise

Sensitivity is the smallest signal that your measurement device can detect across its most sensitive range. I have a picture on the screen — a bug — because I use this as an example: the human hand has so many sensors and so much ability to capture varying degrees of signal on our fingertips, from something as large as a cell phone all the way down to a sticky note or a piece of paper. In reality, we can perceive a signal as small as around 40 micrometers, similar to the tip of a mosquito’s needle. That’s an example of showing that we have a wide range of sensitivity — as small as micrometers, but capturable across a wide range of settings.

Here’s how you might actually encounter this in the field. When you’re working with technology, sensitivity is often used interchangeably with the word noise — you probably hear those two terms come up quite a bit — but the main difference is that noise is any random signal from another source that could affect the value of your measurement. If you have a spectrum analyzer, you can see this really well. I don’t have one in front of me, but on the top example I have a source measure unit, which is basically a power supply — it generates a signal, and the same spec is called noise for a power supply because it’s generating power, but when it’s a measurement term, it’s called sensitivity.

You might notice I have resolution and sensitivity highlighted — they’re directly connected to each other. The sensitivity of your device is often measured in the smallest resolution that your device can capture. You’ll see on the RM510 below, you have a sensitivity of about 5 microvolts, but the resolution is 1 microvolt — so you’ll be able to capture measurements in increments of microvolts, but the lowest one you can actually capture will be about 5 microvolts. They work together in your technology to help capture the right measurement and determine how sensitive of a measurement you can capture.

Accuracy

Accuracy is basically the expected difference between a traceable standard and what your device’s measurement is. Most of the time these standards are set by a certain body — I have NIST on the screen as an example — and you can measure against those with a calibration device or validation device. This is really crucial for when you calibrate your DVMs.

Here’s another way to show it — this is a bow-tie plot, which shows the actual calculation for accuracy, which is really your percentage of offset plus your percentage of gain. The x-axis is your traceable standard, the y-axis is your margin of error, and the red arrows on the top and bottom are the accuracy spec bounds. If you have a device that’s within the accuracy spec bounds, you’ll see a measurement pattern like this across the entire range of that device’s measurement range — that device is within calibration and within the acceptable standard for the measurements you’re trying to take.

It’s important to know about this because every year — we recommend calibrating your device at least once a year — anytime you’re in the field taking measurements and something like an audit comes up, oftentimes validation paperwork is going to be asked for the device that was used to capture a DVM reading. I’ve seen it happen with digital voltmeters, and I’ve heard about it potentially happening with an RMU as well, so it’s very important to be aware of this and how it relates to making sure you’re taking an accurate measurement.

Precision and Repeatability

This comes up a lot as well with accuracy — accuracy versus repeatability. Repeatability is another word for it — you’ll probably hear it referred to more as precision. Basically, precision or repeatability is the variation of one successive measurement to the next.

I have a few examples on the screen of how this is usually encountered in the real world. On the left, you have a precise measurement, but it’s not accurate — why wouldn’t it be accurate in this case? Because that red diamond is considered our accuracy spec, if you refer back to the bow-tie plot we looked at on the last slide, and this person was able to repeat that measurement three times within the same specs — this reading is repeatable, but it’s not within the accuracy specs that were set for what they wanted to capture. This is a really good example of what a non-calibrated DVM that’s out of calibration would look like — you’re probably going to get the same readings over and over again, but they’re not within the accuracy specs of the device, so you’re going to have to get it calibrated.

In the middle, you have an accurate reading — it’s within the accuracy spec bounds, so you know what you’re looking for, but you can’t repeat the measurement. Then you’re aware that maybe there’s interference, some noise coming out of that rectifier that you need to mitigate, or stray current, and you’re wondering what’s going on that you can’t repeat your measurement. The end goal is that you want to have an accurate and a repeatable measurement — the last picture is all of those measurements repeated over and over again within a certain margin of error, and all within the accuracy spec bounds of your device and what you’re looking for. That’s the end goal — an accurate and repeatable measurement.

Input Impedance and Input Resistance

This next one is very important — input impedance and input resistance. These two words are different in the scientific world, but for us today, they’re going to be used pretty interchangeably, especially in the CP world. Basically, input impedance and input resistance are each a measurement of opposition to current flow through a conductor. It’s really important to have an input impedance that’s higher than any source impedance you may encounter, such as soil resistivity, to make sure that whatever load you’re measuring in your circuit, you minimize the margin of error you’re going to see.

Here’s a better way to show this than just a definition — this is a formula you can use to capture your meter voltage against the true potential. A lot of the factors that go into this are the actual potential on your structure, and then input impedance and circuit resistance. When you put these all together, you’ll see that depending on what your input impedance is, it’ll affect the margin of error you’re going to see on your digital voltmeter.

I have three examples on the screen — one with 1 megaohm input impedance, one with 10 megaohms, and one with 100 megaohms — and we have one circuit across the board with a -900 millivolt true potential. If you take that same reading with a device that has 1 megaohm of input impedance, assuming everything else is constant, you’ll get upwards of 9% margin of error on that reading — a pretty large spec difference than you might be anticipating when you go out and take a measurement. If you go to 10 megaohms, it’s about 1% margin of error, and up to 100 megaohms, you get down to 0.1% margin of error.

So if you know you have a really high input impedance on your device — which the DVM 2130 does, at 100 megaohms — you know the percentage of error you’re going to see is going to be very small, unless you’re in an area with a really high source impedance. There are standards out there as well, from AMPP and other bodies, that recommend something higher than 10 megaohms, but that’s why we shoot for 100 megaohms or greater as a good recommendation.

Q&A

[Moderator]
Hey Cole, we got a question in the chat from Casey Wheeler — he says: when taking waveforms with the FDC device, and it’s set to Auto Range, why does it change from 250 volts to 5 volts, and then it ends up giving sections of invalid data within the waveform — zero reads for a short section?

Cole Finney
That’s a really good question, Casey — I unfortunately am not as familiar with the FDC side, so I may have to get back to you on that, but I can talk about it real quick.

On the original version, on the QX, we did have auto-ranging — when you were doing interrupted surveys, it would auto-range down from the 250 to the 5-volt range in the field data collector application. Now we don’t do the auto-ranging, so you shouldn’t see that anymore once you’re using FDC on the Mesa, the Allegro AX, or iOS.

[Moderator]
Thanks, Lon — did that answer your question, Casey?

Casey Wheeler
Awesome, no — thanks, Lon, greatly appreciate you hopping in there. And feel free to throw your questions in the chat while we’re going through the presentation — I’d like to take them ad hoc, but I know we’re pressed for time, so we’ll have more dedicated time for questions at the end of the presentation for sure.

Best Practices: Calibration

Cole Finney
We’ve talked through a whole lot of measurement terms and how they all work together to help build what your device is capturing and displaying. Now we’re going to get into some best practices for actually using your measurement tool, and the first one is calibration.

As I highlighted earlier when we were talking about accuracy, making sure your device is calibrated regularly is probably one of the most important pieces of maintenance you can do for a measurement device — in this case, a DVM. We at AI recommend getting your DVM calibrated at least once a year, but really what matters is that you can validate that the device is within calibration.

As you can see on the screen — before I took my DVM to get calibrated, I had this same bow-tie chart from earlier: my measurements across the entire range, and I was only within the accuracy spec bounds for a certain section of my measurement range. I had a huge margin of error. If I had done a whole line of surveys with this device and gone under an audit, there’s a pretty good chance that PHMSA, or whoever was conducting the audit, would ask for validation that the DVM was calibrated — and that’s not going to look very good for me if I hadn’t gotten this device validated and calibrated beforehand and I’d gone and done that survey.

So I learned my lesson — I got my device calibrated, and now that green line is what it looks like after calibration. You can see the margin of error is very small, and we’re basically right along that traceable standard. Also, just a quick call-out — we can do calibrations for your DVM at AI World, so if you come by, make sure to drop it off and we’ll get you squared away.

Best Practices: Know What to Expect

The next thing that’s really important when you’re taking measurements is knowing what you’re going out there to capture — having an idea of what your structure is supposed to read like. So if you have PCS, make sure you use that database of record to check, before you go take your survey, that this structure has been reading within 50 millivolts of -800, or -900, for however long — so that you know ahead of time what you’re expecting to see when you go out in the field.

As an example, I have two field computers up here. If you walk up to your rectifier to take a shunt reading, hook up your leads, take a measurement, and see 237 volts, you might think that’s definitely not what you’re supposed to be seeing on a shunt — so you’d know something’s wrong with your asset. But if you didn’t know you were supposed to be looking for something like 25 millivolts on that shunt, you might take the reading and move on. So it’s really important, as a tech and as a person taking these measurements, to understand what your legacy structure looks like, what your readings are supposed to be, and at least have a general idea of what to look for going forward.

Best Practices: Maintaining Your Equipment

The last thing I’ve got here is maintaining your equipment. Something as simple as making sure that if you’re going out and doing CI or DCVG surveys, you clean your reference cell — something really simple like that can save you a lot of headache down the line. Go through that checklist before you go out in the field: when was the last time I cleaned my reference cell, have I charged it recently, is it contaminated, do I need to calibrate it? That will save you a whole lot of time and make sure that whatever readings you’re capturing are the most accurate readings you can capture.

Another one that comes up pretty often — I hear about it a lot on the support side too — is just making sure your cables are connected right. Something as simple as: did I plug my leads in correctly, are they hooked up on my structure right, is there any interference I might be able to see based on my connections? General maintenance on these items can really help make sure that whenever you go out in the field, the readings you’re capturing are working for you, your assets are in compliance, and you know what to look for in the field.

All right — that’s all the content I had for today. We still finished with some time, so if anyone has any questions, I’ll go ahead and open the floor. If you prefer, you can also drop your question in the chat, or you can unmute yourself since we have a couple minutes left.

Closing Q&A

[Unclear name]
My question is around the DVM meters — after you’ve left them running for a while and you toggle to see the waveform, sometimes the waveform freezes up, and what I’ve found helpful is to shut down the system and reload it. Is there any fix for that?

Cole Finney
That’s a good question — I may have to pass that info along to our FDC product manager, but I’m happy to talk to you after the meeting as well and get some more info on what you’re encountering. Unfortunately, at this time I probably wouldn’t have an answer for you on that.

[Moderator]
Can you put in the chat what version you’re running, just so we know — once we talk to the product manager about that, if there were any issues at that time?

[Unclear name]
Okay, I’ll pull it up. Thanks.

[Moderator]
Perfect, thank you — awesome, thank you. Okay, Eric, I see your question there: is there a big change in the setup and off-delay adjustment between the QX and the DVM 2130? I may pass that one over to Lon, if you have any insight on that.

Lon
No — as far as the QX and the DVM 2130, the DVMs are very similar. In the 2130, what was added was the battery and GPS, but internally, as far as their measurements, they’re all the same — they’re both 100 megaohm input impedance, for the QX, the AX, and the DVM 2130. So all the settings should be exactly the same.

[Moderator]
Awesome, thank you, Lon. I think we missed one other question: if you have past PCS data for a location, does the surveying program on the Mesa automatically flag a point during a survey that has a significant change from the last reading?

Lon
Not currently. It sounds like you’d want to set a value — say, this has changed by 50 or 100 millivolts since the last read — not currently, but that’s something you could put into the ideas portal, and it’s definitely something we could look at.

[Moderator]
Mhm, actually not a bad idea — yeah, that’s pretty good, I’m glad I thought of it. I’ll drop a quick link to the ideas portal as well if you have any other suggestions.

I also see Joel Fellers asked: any recommended leads for your DVMs?

Lon
You can use your Fluke leads — the same ones, even if they’re the safety-shielded leads. There’s a grommet on the DVM — if you pull that off, you can use those Fluke-style leads on there as well. We’re currently looking at a lead set to provide as well, so we’re looking into leads.

[Moderator]
Nice — William, good to see you. Yeah, that’s a good question — I know we’re running out of time here, but I see you’ve got a question on AC and DC interference, and setting up an ideal setup of equipment to reach good and reliable measurements and readings. I know we’re low on time, so we might have to take that one off the webinar, but I’d love to talk to you about this directly.

Lon
I will add that the filtering on the DVM 2130 for AC is extremely good — they put a lot of really good filtering in there, so if you have some AC on there, it shouldn’t affect your DC readings.

Closing

[Moderator]
Sounds good — if you have any other questions, feel free to reply to our emails, we’ll send a follow-up as well. If you have any additional questions, you can always reach us, or reach marketing staff at AI Worldwide, and we’ll make sure these get to Cole and our team. We’ll see you for the next one — thank you so much for joining us this morning. Be on the lookout for the recorded version of the video soon, so you can watch it on demand. Thank you all, have a good day.