Static Electricity Professionals

Static Dissipative Interlocking Flooring: 4 Things You Must Know

Hi Dave,

We are considering an antistatic interlocking vinyl floor for our new call center. Should we use a static dissipative or static conductive one?

Over the past couple of years, interlocking floors have become a very popular choice. The idea of buying a floor that can be installed with no adhesive and then moved and reinstalled in a new location is very enticing - and quite legitimate. When it comes to static control, there are a few things you need to know before choosing an interlocking version. 

1. Static control vinyl floors do not reduce static on people walking on them wearing standard everyday footwear. Static control vinyl (PVC) is actually a static generator when it contacts most shoe sole materials. Research shows that static control vinyl will only reduce static if a person is wearing special static control shoes or special heel straps and shoe covers. If people will be wearing ordinary shoes, you might as well save your money and skip the purchase of a vinyl interlocking floor.

2. Choose a fooring material that prevents static regardless of footwear. Inerlocking flooring isn't just available with PVC - vinyl. You can also get an interlocking floor with EC rubber or carpet tile attached to the top. Both EC rubber and static control carpet will provide excellent static controls regardless of what composition of shoes people wear when they walk on the floor.

3. Interlocking floors are expensive. They are not the only solution for a buyer looking for flexibility and mobility. Carefully compare the real cost of a glue down solution before assuming you will save money by taking the floor with you. In many cases, you can buy "dry adhesive" systems that will provide the stability of a hard set adhesive but you will also have the ability to remove and reinstall the tile in a new location. Some floors like EC rubber are available in rolls making them the ideal solution for long-term but temporaty installation.

4. Static dissipative floors do not perform as well as static conductive floors. Recently there has been a very strong push to eliminate the use of static dissipative flooring when conductive versions are also available. If you are getting steered in the direction of static dissipative, ask if there is a cost difference for the same floor in the static conductive range. There should not be any cost difference.

Don't Let Moisture Ruin Your ESD Flooring Installation

August 13, 2010

Yesterday I was contacted by a client who wanted my opinion about a small company's new inexpensive system that would eliminate moisture vapor problems in concrete. The client was excited because his facility has a major vapor emission problem in the concrete floor. Measurements using Test Method ASTM F2170 had determined concrete humidity levels in excess of 90% all over the facility. These measurements are quite high; Essentially these numbers equate to a near 100% probability that any static dissipative or static conductive floor installed with low VOC conductive adhesives will fail without some form of moisture mitigation system. The supplier of the new moisture prevention system told the client that their system will prevent this problem from happening and they will guarantee it for the life of the floor. Their warranty would cover up to "8 pounds" of moisture per the ASTM F1869 calcium chloride test method. Their guarantee states "in the unlikely event of any failure resulting from moisture vapor below 8 pounds, XYZ Inc will either replace the material or refund the cost of the floor.

Let's talk about what's wrong with this scenario

1. The system is brand new and it is from a "small company." How can the warranty claim "the life of the floor?" the system hasn't been around long enough to be vetted.  How will a small new company handle a warranty claim that could run into 100s of thousands of dollars?

2. The moisture mitigation manufacturer is citing a different moisture test method than the method the client used for the first set of tests. Why isn't the warranty tied to the currently favored method of measuring concrete humidity? Instead, the warranty is rated to the less accepted and outdated calcium chloride test method.

3. The guarantee only covers the replacement costs of material. It does not cover labor or the costs associated with lost business due to shutdowns and inconvenience. Labor will cost as much as material in many cases. Depending on the industry, lost business could cost 5, 10 maybe 50 times more than the cost of the ESD floor.

 

The bottom line: Moisture problems are expensive to solve. However, these problems are even more expensive if they are not handled with lock tight solutions up front. if moisture testing determines you have a bad problem - you have a bad problem Don't go looking for contrary test results so you can pretend there isn't a problem. Someone will always be willing to take on a risk in order to gain your confidence. But, will they pay for your down time if the floor needs to be replaced?

Here's a link to some pictures of a failing floors failing from moisture. http://www.staticworx.com/articles/articles-moisture-barrier.php  These floors were installed by individuals who either didn't test for moisture or didn't properly address moisture because they didn't have adequate money n their budget.

If it sounds too good to be true - it probably is too good to be true. You generally get what you pay for...

Please note: Staticworx is not in the moisture mitigation business. This information is merely provided as a service to the many clients and contractors who use Staticworx ESD flooring.

Why do you “precision mill” solid vinyl ESD tiles?

Understanding what solid vinyl tile (SVT) is and how it is made will help you understand the advantages of precision milling.

 

 

SVT has the same color pattern and material composition on both sides of the tile. This is

because the color pattern goes through the whole tile to the back. Solid vinyl tiles are just what the term implies, vinyl without any fillers. ESD solid vinyl tile also has the same conductive elements through the full thickness of the tile. You can read more about SVT here at this link: extension.usu.edu/files/publications/factsheet/hi_03.pdf

How solid vinyl tile is made?

 

·      Step One: Hot Press

Solid vinyl tile is made in large batches starting with a process known as hot pressing. Large pans 37 inches by 37 inches by approximately 2 inches deep are filled with a combination of plastic resins, conductive plastocol and pigments. Under heat and pressure the resins and pigments mix with the conductive materials. The result is what we call a rough slab.

 

·      Step 2: Skiving

The slabs are automatically removed from the pans and placed between a series of pinch rollers. The rollers transport the warm slabs through a horizontal cutter called a skiving machine. The skiving machine cuts the slab’s thickness in half, yielding 2 rough slabs  - one half the thickness of the original slab. This process is repeated until we have eight 37-inch square slabs approximately 5/32” thick.

 

·      Rough sanding:

The 5/32” slabs move from the skiving operation on to a conveyor. The tiles are rough sanded on one side. The sanded side will become the bottom of the tile. The sanding allows the tile to hold adhesive and provide a good bond during installation.

 

·      Polishing:

After sanding the rough slabs move through a wet polishing operation. This operation seals the top surface of the tile. Polishing produces a factory finish on the tile. This finish can be further enhanced after installation using nothing more than a low speed buffer with a polishing pad.

 

·      Die cutting:

The 37” by 37” slabs are die cut in-line using a long centre bevel die. These types of dies cut the tile edges on a slight angle.  Cutting on an angle eliminates material deformation during the cutting process. However, angle cutting leaves the tile with a reverse bevel that needs to be removed. High quality ESD vinyl tiles should be cut approximately 1/32” larger than their final dimension.

 

 

·      Precision milling:

Precision milling is the most critical step in the manufacturing of perfectly square solid vinyl tiles. European and North American SVT manufacturers use this process. In this process we create a perfect 90-degree angle by removing approximately 1/32” of material along both edges.  Most imported tiles are not precision milled due to the added time and expense.

Precision Milling accomplishes 3 goals:

 

1.     Milling removes the weak point on the tile edges – the beveled edge from die cutting and the void created by the reverse bevel when 2 tiles are placed next to each other. Precision milling or micro-polishing, removes angles, enabling adjacent tiles to fit flush against one another.

2.     Eliminates Shingling and Creeping: Since any sub floor will have slight deviations in flatness, a reverse bevel allows lower tiles to climb over higher adjacent tiles. This is called the “shingle effect.” Shingling causes lines of tile with the installation to go out of square. Periodic cutting of rows of tiles is the only way to regain a straight line. . The best way to prevent shingling and resultant seam gaps is to produce tiles with pure 90-degree cuts.

3.     It polishes the edges of the tile. Edge polishing eliminates the unevenness and micro fractures created by die cutting - enabling a tighter fit between tiles.

4.     It precision sizes the tiles after resin curing and chemical cross-linking have reached completion. Solid vinyl tile cures for several hours after it has been produced. The best assurance against size deviation is to wait at least 24 hours before final sizing of the tile.

 learn more about milling at this link: http://www.staticworx.com/esd-flooring/anti-static-vinyl-bevel.php

Ask your installer: Most experienced installers can identify a precision milled floor tile after installing just a few rows of tiles. A precision milled tile installs tight without gaps, shingling or row creeping, enabling a faster cleaner installation. A gap free installation stays clean for the life of the floor because there is now where for dirt to collect in the first place.

How to discuss and recommend the right static control flooring with a client

A guide for architects working within client environments and applications that require conductive flooring

I recently visited with an architect who asked me how to best discuss and guide a client interested in using static control carpet in their application and environment.

There are two main question areas to begin the client discussion and help determine the right static control flooring recommendation:

1. What is the electronic industry supplier environment? Are they actively handling static sensitive electronic parts and sub-assemblies (as opposed to using them)?
   
2. What is the end user environment? Is the organization reliant on electronic systems that may be affected by static discharge events?

Specifying for electronic industry supplier environments handling static sensitive electronic parts and sub-assemblies

Electronic industry supplier environment organizations make, handle, test, store and repair electronic parts. A static event in these types of spaces would damage or destroy the part, assembly or final system. Grounding is achieved by wearing conductive heel straps or special shoes in conjunction with a grounded static control floor. In other words, personnel need to be grounded in any operation in this environment.

Generally, organizations handling and producing static sensitive parts designate certain areas as “ESD safe areas” or static free zones; an ESD safe area requires vigilant compliance with personnel grounding procedures as well as numerous other protocols for preventing the generation and accumulation of static electricity. The processes in these environments are usually governed by international standards like ANSI/ESD S20.20-2007.

Best flooring solutions for electronic industry supplier environments

Staticworx works with the architect to help narrow down which flooring to specify for the client for this type of application:

• EC ESD Rubber
• ESD Carpet Tile
  (not the same as low kV or computer grade carpet)
• ESD Vinyl Tile
  (Also called static conductive and static dissipative tile)
• Conductive Epoxy Coatings

For more information on conductive flooring solutions for electronic industry supplier environments, read the article, “Taking the Mystery Out of Selecting ESD Flooring” which is part of the Staticworx Knowledge Center.

Specifying for electronic industry end user environments: operations reliant upon electronic systems

The term “reliant upon electronic systems” means that the operation does not involve active handling, testing or assembly of electronic sub assemblies. These operations are electronic systems end user environments requiring sophisticated electronic equipment to accomplish their mission.

These spaces are often called Mission Critical Environments: examples include data centers, R & D labs, 9-1-1 call centers, hospital ERs, MRI suites, communication centers, flight towers, control rooms. These spaces utilize operational electronic systems which are manned by personnel who will not use special footwear. A static problem in these spaces would lead to an event that would impact either the equipment, operational mission or both. Examples of these types of problems include: data corruption, system lock-up, dropped phone call, unintended signals in a system.

In these applications, the static control floor must be fault tolerant. (See my recent blog entry on fault tolerant design.) The flooring must prevent the generation of static without the added benefit of special footwear; conversely the anti static floor must prevent static despite the use of static generating footwear.

Best flooring solutions for electronic industry end user environments

Most static control flooring will not inhibit static without the use of special footwear. That said, there are two flooring materials that perform very well despite this obvious deficiency and that Staticworx recommends for electronic industry user environment applications:

• EC ESD Rubber
  (with carbon chip conductors)
• ESD Carpet Tile
  (with conductive fibers wrapped around nylon tuftsand conductive backing.
  Not the same as low kV or computer grade carpet)

For more information on conductive flooring solutions for electronic industry end user environments, read these articles, part of the Staticworx Knowledge Center:

• Static Control in a 9-1-1 Call Center: City of Lowell Case History
• Static Control Flooring for Data Centers (featured in The Data Center Journal)

Emergency Call Center Personnel and Anti Static Flooring

Removing Static for a Call Center Environment

It's no secret that emergency personnel need to be “well-grounded” and focused on day-to-day and minute-to-minute aspects of their job. I recently wrote an article for the January/February 2010 issue of Emergency Numbers Professionals Magazine which featured the importance of removing static from a call center environment. Read the full Emergency Call Center article online now.

More Free ESD Resources

Other articles I've authored on the topic of anti static flooring in 9-1-1 Call Centers are available in the Staticworx Knowledge Center, including:

Managing Static, The Invisible Threat to Call Centers

and Case History: Emergency 911 Call Center Static Discharge Problem Solved in LaVergne, TN Police Department

How to Ground SD Monolithic Seamless Epoxy Coating Floor

Client Question
We are considering applying Staticworx Groundworx ESD epoxy paint over two surfaces in our building. The areas are either sealed concrete or covered with regular VCT. Will the Groundworx floor meet ESD standards even though it will be installed on top of non-ESD flooring? - Keith, Texas

Answer
My company's Staticworx Groundworx Epoxy is produced using graphite particles. These particles create a conductive matrix within the liquid. When applied over any surface, insulative or otherwise, the Groundworx cures into a homogeneous ground plane in all directions. Continuity is achieved across all points of Groundworx regardless of the electrical properties of the surface underneath it.

However, the “Groundworx ground plane” still needs to be grounded once every 1000 square feet using our copper ground strips. The strips are 2 inches by 24 inches in length. They can be attached to the floor using glue or double-sided tape. Part of the strip touches the floor and the other end of the strip should attach to an electrical box, a piece of conduit, building steel or a metal stud that is in continuity with the AC or building ground. (See article: Best Way to Ground ESD or Static Dissipative Flooring) When applying the Groundworx, make sure you coat over the portion of the copper strip touching the floor. This is the only necessary step for grounding a Groundworx floor.

Learn more about Staticworx Groundworx Epoxy
Read this article on the Staticworx website, in the Knowledge Center Area

Are You Grounded? Evaluating Waxed, Solid Dissipative Vinyl Tile vs. Wax-Free, ESD, Conductive Vinyl Tile

Recently we decided to do some spot testing of various “conductive” flooring products in our office in Massachusetts as an exercise to further develop our library of resources available to those who struggle to understand ESD flooring and the underlying principles of static electricity. It is important to note that the day we tested it was very cold outside (and in our office) and that the low relative humidity is the main reason for choosing this particular day to conduct these tests. Static problems are more prevalent when the air is dry. It is well known that many static control products lose effectiveness when it is dry – just when you need them the most.

Hypothesis
The hypothesis that we are looking to test with these evaluations is that static is dissipated least effectively by humidity dependent products like SDT and more effectively by carbon loaded conductive ESD tile. Testing day's super-dry winter conditions eliminate any chance that relative humidity will enhance the discharge properties of any tiles. It is a known phenomenon in the industry that elevated relative humidity can increase the effectiveness of anti-static flooring, one very important reason floors should be tested at low relative humidity and without the aid of waxes or antistatic polishes.

Samples and Testing Methodology

Remembering that the ideal range for ESD flooring systems is 10⁵ to 10⁷, we decided to compare the results of the following flooring samples:

· SDT VINYL SAMPLE
Armstrong SDT Static Dissipative Tile (figure 1)
Samples that re provided by the manufacturer ‘s distributor

· Carbon loaded VINYL SAMPLE
Staticworx Foundation Series ESD Vinyl Tile (figure 2)
This sample was pulled from the Staticworx warehouse of samples

 

· EC RUBBER SAMPLE
Staticworx EC ESD Rubber (figure 3)
This sample was pulled from the Staticworx warehouse of samples

 

   (figure 1)  

   (figure 2)    

  (figure 3)

 

The size of the sample does not influence the effectiveness of the results of this test. These sizes appear as provided by the manufacturer. All samples are shown here, prior to testing, on a standard 8.5” x 11” piece of paper.

 

To conduct the test, we used:

·  Prostat, PRS-812 Resistance Meter (figure 4)

· Copper stripping that Staticworx supplies with all ESD flooring orders

· The three flooring samples listed above.

 

 

(figure 4)

 

Any easy way to determine if a flooring material can be grounded effectively is to measure its resistance with a megohm meter (see figure 4.) A good reading is usually below 1,000,000 ohms, that’s 10⁶ in ESD speak and on the meter it’s represented as “1.0 E 06”. An unacceptable reading is any reading above 3.5 E 07-0—it’s critical to understand that the meter’s highest measurement is 10 E 10 and lowest reading is 0 E 00. According to ANSI/ESD S20.20-2007 the system ground should always measure below 3.5 E 07.

 

Sample Testing

After connecting the meter to the two-weights/conductive cords, we placed each sample with its midline on the copper-grounding strip. Then, we connected the meter—one weight on the copper strip and one on the flooring sample. At this point, we turned on our meter and measured electrical resistance. Here are the results.

 

  (Figure 5)     

 

 (Figure 6)         

 

 (Figure 7)

 

In figure 5, the Armstrong SDT Vinyl Tile sample, reads 2.8 E 09, or 2.8 10⁹. In figure 6, the Staticworx Carbon loaded Vinyl Tile sample, reads 8.7 E 05, or 8.7 10⁵. In figure 7, the Staticworx EC Rubber sample, reads 9.8 E 05, or 9.8 10⁵.

 

Test Results

As hypothesized, the tests confirm that SDT is the least effective conductor of static; Carbon loaded ESD tile and EC rubber flooring conduct static most effectively - regardless of the dry conditions.

 

Conclusion: What does this mean?

The main purpose of static control flooring is to prevent discharges between people and static sensitive electronic assemblies. A good static control floor must safely ground static charges away from people regardless of environmental factors. A floor with poor or no conductivity, like Armstrong SDT, cannot effectively ground static charges because the resistance is too high. A high resistance (ohms rating) prevents charges from reaching ground. At low humidity, static charges will actually accumulate on a people as they walk on SDT. Carbon loaded flooring like Staticworx vinyl and EC rubber will perform equally well regardless of relative humidity.

 

Read more about how Staticworx ESD, Conductive Solid Vinyl Tile (wax-free) compares to Armstrong Static Dissipative Vinyl Tile (requires wax) on the Staticworx website >

Polyurethane is an insulator and a static generator

QUESTION:We are considering installing ESD tile in our facility. We have heard that tiles with polyurethane coatings (PU) might be easier to keep clean. Is this true? Can you recommend a floor that requires low maintenance?
Josh, Androgon Networks

 

ANSWER:
Polyurethane (PU) coatings are used by luxury vinyl tile (LVT) manufacturers. A coating of PU helps reduce the use of waxes and sealers on flooring installed in retail spaces, hospital corridors, shopping malls etc. Unfortunately PU is a non-conductive coating so defeats the purpose of the conductivity of the tile. That’s why they shouldn’t be used on floors that are intended for low static generating applications. This coating would insulate access to the conductive, carbon matrix sealing it inside the PU, keeping the tile from eliminating static effectively.  (See the Staticworx illustration on how conductive tile works > )

 

 

The PU coating would increase the electrical resistance of the tile leaving the floor unable to meet ANSI/ESD S20.20-2007 ESD standards. PU is an extreme static antagonist making it a major static generator; a floor coated with PU would allow for the accumulation of high static charges on people when they walk on the floor with any shoes.

 

If you think maintenance will be a major problem in your facility, you should at least consider a precision milled ESD tile with high vinyl content. These tiles are commonly called solid vinyl tile (SVT.)  The surface of SVT can be buffed and returned to its original appearance regardless of age, traffic or soiling. The properties run all the way through the thickness of an SVT tile so you never need to worry about abrasion affecting performance. The precision milling ensures a tight gap-free installation. Tight seams prevent that ugly dark line that often appears in between VCT tiles.

 

You may also want to explore EC rubber flooring. Unlike vinyl, rubber doesn’t have any pores that could trap dirt in the first place. And, conductive rubber is the only floor that will reduce static on people wearing standard street shoes. All other resilient ESD flooring requires the use of static protective footwear. EC Rubber is the most fault-tolerant ESD flooring available.

Fault-Tolerant Anti Static Flooring

Wikipedia defines “fault tolerant design” or “fail safe design” as “design that enables a system to continue operation, possibly at a reduced level…rather than failing completely”.¹

Over the years, I have found that this is a very successful way for customers to think about their ESD flooring needs. For most environments, the hazard of static to equipment and/or operations is a risk that cannot be taken. Fault-tolerant ESD flooring is intrinsically anti-static: the end user won’t need to do anything after it is installed except walk on it while it allows for the most user errors. Fault-tolerant ESD flooring allows for the most flexibility in an environment while keeping that environment safe:

• People can wear the wrong shoes but still remain protected
• Waxes, polishes, and sprays need not be applied or monitored
• Ambient humidity, temperature, traffic, and the age of the tile are taken out of the equation

In mission critical environments, like labs, call centers, healthcare and cleanrooms, there is no room for static or the human error that can bring static if high maintenance ESD flooring is used. Based on the application where the ESD flooring will be installed consider the following elements when evaluating the selection of anti-static flooring:

1. Footwear: Will the shoes worn in the environment be standard “street shoes” (insulative) or will they be specialty footwear (conductive)?
2. Volts (kv): What is the acceptable amount of human body charge allowable in the environment?
3. Static decay: Upon contact with the floor, how quickly does the charge leave the body?
4. OHMs: What is the floor’s measurable ability to conduct electricity?

(See image, below)

¹ “Fault-tolerant_design,” Wikipedia, http://en.wikipedia.org/wiki/Fault-tolerant_design, Dec. 1, 2009

Confusing Terminology

Have you been asked to select a floor that is supposed to eliminate or minimize static charges accumulating on people when they walk on the floor? What do you call such a floor? If you have done any research in this area you’ve probably discovered something rather unusual. There seems to be many different and seemingly contradictory descriptors for this type of floor. To make this point, here are just a few descriptors used by the construction industry to describe this floor (and a free resource to help you decode them).

 

Section 09 61 36 of The Construction Specifications Institute (www.csi.com) calls this kind of floor a  Static-Resistant Flooring Treatment. That makes sense; the purpose of the floor is to resist static. But wait; would it also make sense to control static? Control as in eliminate, mitigate, inhibit. There appears to be another CSI category (09660) called Static Control Flooring. What’s the difference between static control and static resistant? Before you answer that question here’s another CSI category: 09 65 33. This category is called Conductive Resilient Flooring. (Info courtesy of www.aecinfo.com) That’s three descriptions for what seems to be the same thing.

 

The construction Specification Institute isn’t the only source of confusion in this dilemma of identifying technically sound walking surfaces that can eliminate static on human beings. Try using the www.google.com search engine to find your solution. Start out with words that make logical sense in a search for something that should prevent static. Try the words “antistatic flooring.” The first thing you will notice is that Google will ask you, “did you mean anti static flooring?” So try searching anti static flooring instead of searching the way you originally spelled it. Now you get different search results than you did using the first spelling. Ditto if you hyphenate anti-static instead of spacing the two words. Now things really get really blurry. You’re looking for “something” that the construction industry can’t describe consistently and the worlds of Google, Bing and Yahoo can’t seem to spell right. And worst of all, you are only looking at antistatic. You haven’t even searched other descriptive synonyms like conductive, static dissipative, grounded, static-free, ESD safe, ESD flooring, shock resistant, static mitigating or non-static and low static generating. Some Internet searches even resort to application-oriented strategies in the hopes that they can whittle down the possible options based on suitability. They’ll plug in words like like: data center flooring, hospital operating room flooring, cleanroom tile, MRI suite flooring or flooring for handling explosives.

You could have a field day thinking up all the possible word combinations for this kind of walking surface. Several years ago we actually created an Excel spread sheet that contained over 2000 possible word combinations for something that is supposed to do the following:

· Permanently prevent static on people who are standing or moving

· Be certifiable to some standard that verifies safety and performance

· Meet the requirements of the space, environment and application

 

I hear questions and comments about this dilemma ever day. This confusion is the number one reason we get calls from end users asking us to test their static flooring solution because something doesn’t seem to be right. Recently, we were asked to test some anti static high pressure laminate (AKA HPL) installed on raised access flooring panels in a semiconductor fabrication facility. The client believed their maintenance department must have harmed the HPL floor because the floor was no longer antistatic. And as a result, extremely low levels of static — lower voltage discharges were zapping static sensitive parts than can be felt by a person. It turned out that the HPL floor was never a good investment because it was incapable of discharging static on people walking on it. The floor lacked the most basic ingredients required to transport electricity from one place to another; it was not made with electrical conducting materials like carbon, graphite, silver or gold. Without the presence of electrical conducting materials, it is not possible for electricity to move through a material. Instead, electricity was getting stuck. The static charges were staying on the people and on the floor. We call that kind of electricity – static electricity. If you don’t understand this concept think of an electrical plug. There’s a reason you don’t get a shock when you plug your hair dryer into an AC electrical outlet. It’s because the plug is made of a non-conducting plastic material. If the plug were made with conductive materials electricity would flow through it. You would also get a shock.

 

So, how do you get on track and find this elusive floor that has no universal name, can’t be spelled properly and apparently if you don’t get lucky and describe it right, you could end up with the opposite of what you wanted?

 

Oh, and one more thing, once you get the description down you will need to decide if the floor should be resilient, carpet, paint, wax, rubber, vinyl, epoxy, urethane, HPL, or maybe matting. Could this be confusing? We think so. That’s why we built www.staticworx.com. Every single article, specification, test result and comment has been vetted by independent standard organizations or technical editorial boards. And, if that is not enough, we update and refute any technical premises that have been superseded or proven to be incorrect.