String Wound Filter Cartridge

Filson string wound filter cartridge also called string wound filter is designed for efficiently removing dirt, rust and sediment from water. It is a low-cost liquid filter cartridge offering excellent depth filtration and strong chemical resistance.

Filson string wound filter cartridge is made with five kinds of material media, include cotton string wound filters, PP string wound filter cartridges, nylon string water filters, polyester string wound water filters, and glass string wound sediment filters. All above string filters can be with polypropylene, stainless steel or tin steel inner core.


Filson String Wound Filter Cartridges

Filson string wound filter cartridges are available in several particle retentions, filter ratings and lengths to meet your requirements.

Multiple string wound cartridge lengths, including 5,10,20,30,40 inch, are offered to fit existing water filter vessels and minimize change-out time when replacing most water filter cartridge brands.

Compared to other string wound cartridge filter manufacturers, Filson string wound sediment filters adopt single-strand continuous winding process for consistent quality, high particulate retention and reduced bypass.

During the production process, the wrap is usually brushed, thus providing increased working area whilst maintaining the rigid structure.

All Filson string wound filter cartridges get through integrity test in our lab to deliver the best filtration performance.

Micron ratings from 0.5 to 150 microns to meet required cleanliness levels.

Filson string wound 5 micron filter and 20 micron are the most popular sizes for drinking water applications.

As a string wound filter manufacturer with more than 15 years of experience, Filson can keep your water system contamination-free by being your OEM partner in string wound filter cartridges.

If you need string wound filter replacement cartridge, like Pall, Domnick Hunter, Eaton, Pentek, GE Osmonics…, Filson string wound filter is a better choice for you.

Materials of Filson String Filter

Filtration Media:

  • Polyester
  • Polypropylene (PP)
  • Bleached or Natural Cotton
  • Glass Fiber
  • Nylon

Inner Support Core:

  • Polypropylene (PP)
  • Stainless Steel
  • Tinned Steel

Features of Filson String Wound Filter

  • Depth and absolute filtration
  • Custom constructions and particle retentions
  • O-ring/end cap options available
  • FDA standard PP cartridges are available for food&beverage applications
  • High temperature and chemical compatibilityto meet a wide variety of applications
  • Multiple length cartridges to fit existing housings and replace most brands
  • Large dirt holding capacity with long life

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String wound filter cartridges VS melt blown filter cartridges

  1. Filson wound filter cartridges has a significant larger effective surface area than melt blown filter cartridges, due to the overlapping characteristics of the wound wires.
  2. A core is required for string wound filter cartridge structural firmness. Filson melt blown filter cartridges do not require a core because the fiber is very firm itself. Certainly, a core can be added for additional structural strength or higher temperature resistance.
  3. Filson string wound water filter cartridges are exceptionally useful for surfacewater filtration, such as river water, wastewater and process  Filson melt blown filter cartridges are especially effective in spring water and city water, RO filtration and pharmaceutical processes

Applications of Filson String Wound Filter

  • Process water
  • Sea water desalination
  • RO/DI pre-filtration
  • Beverage processes
  • Liquid clarification filtration
  • Plating baths filtration
  • Chemicals Filtration
  • Acids & Alkali Filtration
  • Filter Rating: 0.5 -150 µm, others on request
  • Length: 5″, 10″, 20″, 30″, 40″
  • Inner Diameter: 28 mm
  • Outer Diameter: 62 mm/114 mm
  • Max. Differential Pressure: 2.5 bar
  • Max. Working Temperature: 180°C–PP, 160°C –Cotton, 400°C-Glass fiber) 121°C- Polyester

String Filter: The Ultimate FAQ Guide

I know you’re looking for an efficient and reliable string filter.

That’s why this guide will answer all questions you have been asking about string filters.

So, before importing string filters, read this guide.

What is a String Filter?

This is a filter that is capable of eliminating rust, dirt, and other sediments from water by using fibers.

It is considerably cheap, offers good filtration, and has a strong resistance to chemicals.

The string material can be from nylon, polyester, cotton, etc.

It wraps on a center core like stainless steel or polypropylene.

 string filter

 string filter

What is the History of String Wound Cartridge Filter?

String wound cartridge filters date back several years.

The first one was sold in the United States in the mid-1930s.

The construction was a woven wire mesh bound in cotton wool, that was in a diamond pattern.

The edges of the diamond forms were the key points at which the contaminants would stick.

This was rather than filtering the sand through the entire formation.

Regular cartridges are built from ‘roving’ or ‘friction spun’ fiber.

Also, considerations like media displacement and chemical leaching have been among the main drawbacks to the efficacy of cartridges.

With recent innovation in filter media for the production of string wound cartridges, issues such as media migration and chemical leaching are on the minimum.

Filter cartridges, consisting of cotton yarn materials and metal frames, came about in the mid-1930s.

By the late 1970s, polypropylene (PP) core cartridges and yarn were becoming popular.

This is since they had a wide range of chemical resistance and one could use it in many applications.

Also, they could capture and resist the growth of microorganisms.

This era of string wound cartridge filters usually uses roving rather than conventional yarns.

These were rolls that twist or fiber strands that were more budget-friendly.

Also, the lifespan of the filter cartridge went up.

This is because more water was flowing through it easily.

In subsequent years, ‘friction spun’ wool took over.

The friction spun wool is only similar in appearance to roving but is relatively bigger and heavier.

The result of this is an increase in dirt-holding capability and lower resistance to fluid movement.

Where Are String Wound Filter Cartridges Used?

Several industries use this filter cartridge in cleaning water.

They include:

  • Food and beverage industries

Examples include:

  1. Water filtration
  2. Beverage processing filtration
  3. Syrup
  4. Alcohol
  5. Cola
  6. Drinks
  7. Edible oil
  • Electronic industries

Uses include:

  1. Films
  2. Resins
  3. Electroplating
  4. Pure water
  5. Electroplating
  6. Microelectronics
  7. Fibers
  8. Organic solvents
  9. Printed circuit board
  • Chemical and petroleum industries

Applications include:

  1. Pre-filtration of RO/DI
  2. Cosmetics
  3. Paints
  4. Photographic washing liquids
  5. Filtration of alkali and acids
  6. Coatings
  7. Petrochemicals
  8. Raw acrylic materials
  9. Chemical filtration
  10. Inks
  • Industrial oil

The applications include:

  1. Desalination of seawater
  2. Filtration of power plant condensation water
  3. Plating bath filtration
  4. Filtration of hydraulic, lubricating, cutting, and other industrial oil
  • Pharmaceutical industries

Uses include:

  1. Filtration of syrup, alcohol, injections, etc.
  2. Preparation water filtration
  3. VC raw materials
  • Other uses

These include:

  1. Laser cooling water filtration
  2. Filtration of electroplating liquids
  3. Ultrasonic cleaning liquids.

How Does a String Wound Cartridge Filter Work?

The string that goes around the core forms a gradient density in a string would cartridge filter.

Also, the core of the string wound filter cartridge is tighter and the exterior is lighter.

The light outer weaving makes it possible to filter out large contaminants first.

Then as the water continues to pass through the string, fine sediments get caught in each layer.

The winding technique and the specific thickness of the string wound cartridge filter will enhance the dust holding capability.

Also, it will lengthen the service life and the efficiency of the string wound cartridge filter.

The accuracy of the string would cartridge filter depends on the tightness of the string.

One can control the machine to ensure the inside of the string is tight and the outside is loose.

Water with contaminants will pass via the string wound and as it passes, contaminants remain behind.

The clean water will then pass through and enter the core of the string wound cartridge filter.

It then exits the filter as more water gets in to undergo filtration.

What are the Advantages of Using String Wound Cartridge Filter?

String-wound cartridge filters originate from the new media that provides an efficient and cost-effective solution.

The advantages include:

  • The spinning method ensures that no chemicals leach in during the filtration process.
  • Also, the string wound will allow no media to migrate since it has continuous filaments.
  • The filter has high structural integrity thus the media will not move therefore offering an impressive knife-edge sealing.
  • Besides, it has an impressive solid to void ratio which enhances the dirt holding capability.
  • The dirty liquid will flow via the whole string wound cartridge filter as there is a low-pressure drop.
  • Also, the sturdy and firm media configuration will enhance the resistance to the dirt unloading.
  • A string would cartridge filter will offer more consistent performance.
  • Also, it improves the filtration process as the inner layers of the strings are dense and the outer layers are loose.

What are the Technical Features and Specifications of String Wound Cartridge Filter?

These features and specifications may vary from one string wound cartridge filter to the other.

The technical features include:

  • A maximum differential pressure of 2.5 bar.
  • An inner diameter of 28 mm and an outer diameter of between 62 mm and 114 mm.
  • The length of the string wound cartridge filter can be 5, 10, 20, 30, or 40 inches.
  • Also, it has a filter rating of between 0.5 μm and 150 μm with the option of customizing on request.
  • The maximum working temperature of cotton is 160 °C, polypropylene is 180 °C, polyester is 121 °C, and fiberglass is 400 °C.
  • A maximum flow rate of 5 to 28 GPM and 10 to 30 GPM for a diameter of 2.5 and 4.5 inches respectively.
  • Besides, it comes in a wide array of micron ratings.

On the other hand, the specifications of a string wound cartridge filter are:

  • It has a large dirt holding capability and is also durable.
  • Besides, it offers an absolute and depth filtration process.
  • Also, one can customize the configuration and the particle retention.
  • The end caps or O-rings are some available options you can include in the filter.
  • Also, they conform to Food and Drug Administration [FDA] standard that is suitable for food and beverage usages.
  • It can meet a wide array of applications as they have high-temperature resistance and chemical compatibility.
  • They also come in varying lengths to fit onto existing filter housings.

What are the Main Components of a String Filter?

The components are the:

Inner Support Core

This is the core that supports the filtration material.

It has pores on it to allow clean water to pass through after undergoing the filtration process.

inner support core

 inner support core

This part can be from one of the following substances:

  1. Tinned steel
  2. Stainless steel
  3. Glass-filled polypropylene
  4. Polypropylene [PP]

Filtration Material

It operates by filtering out contaminants from the water.

The outer side is loose hence it allows the capture of large contaminants.

On the contrary, the inner side is tightly wound to capture the small contaminants.

 filtration material

filtration material

This material can be one of the following substances:

  1. Nylon
  2. Polyester
  3. Glass fiber
  4. Polypropylene [PP]
  5. Natural cotton
  6. Bleached cotton
  7. Rayon

What are the Key Differences Between String Filter and Melt Blown Filter?

Here is how these two compare:

melt brown filter

melt brown filter

Surface Area

String filters have a large and effective surface area than melt blown filters.

This is because the wound yarn will overlap with each other.

Structure of the Filter Cartridge

String filters have a yarn of a specific diameter that you will wind on a support core to create diamond-shaped patterns.

For melt blown filters, you will blow specific fibers from nozzles and then bond them thermally on rotating shafts.

This creates a dense and porous form that will offer the same porosity level where the fluid passes.

Also, it has no supporting core as the string filter as it is firm independently.

However, you can decide to add a core to provide extra structural support.

Path of the Process Fluid

The fluid in string filters passes via diamond-shaped winding while the path of melt blown filters is almost straight.

Number of Filtration Points

String filters have 16 or more paths while melt blown filters have an average of 8 paths.

Single or Multi Matrices Suspended Solids

It is possible to produce string filters in multi-gradient structures.

This makes it possible for suspended contaminants to penetrate deep within the stricture thus achieving true depth filtration.

One can produce melt blown filters in multi or dual gradient structures.

However, it mainly traps contaminants on the outer surface thus does not use the inner part for filtration.

This makes it have less dirt-holding capability than string filters.

Internal Core

String filters can have an inner core from stainless steel, polypropylene, or tinned steel.

Melt blown filters have an inner core from polypropylene [PP].

Dirt Holding Capability

String filters have a high dirt holding capability.

This makes it suitable for pre-filtration.

On the contrary, melt blown filters have a low dirt-holding capability.

Nominal or Absolute Cartridge Filter

The nominal rating for string filters is at an efficiency of around 95%.

Melt blown filters have a nominal rating with an efficiency of around 97%.

Micron Rating

The string filter has a micron rating that ranges from 0.5 to 150.

Melt blown filter has a micron rating that goes from 1 to 20.

Available Filter Media

String filters have filter media from sources like glass fiber, cotton, and polypropylene.

On the other end, melt blown filters use filtration media from nylon and polypropylene.


The string filter can operate on applications that need a temperature of 350 °C.

Also, is it suitable for applications that have a high dust load and requires depth filtration.

On the other hand, melt blown filters need a temperature rating of up to 120 °C.

Melt blown filters are suitable for applications that need an end or final filtration point.

Also, they need applications where the liquid has undergone pre-filtration.

Besides, in melt blown filters, you can use them in applications that do not have pre-filtration but minimal life cycle.

It is possible to filter river water and wastewater with string filters.

However, you can filter spring water, city council water, and pharmaceutical water with melt blown filters.

What are the Different Yarn Types used in a String Wound Cartridge Filter?

It is possible to use one of the following yarns when making a string wound cartridge filter:

  • Glass fiber

glass fiber

glass fiber

  • Polypropylene [PP]
  • Nylon
  • Natural or bleached cotton
  • Polyester

How Long Does a String Wound Filter Cartridge Last?

It is worth noting that a string wound filter cartridge has a high dirt holding capacity.

This characteristic makes it last for long periods.

Besides, the durability will also depend on the water you are filtering.

If the water always has lots of contaminants, the life span will be lower than when the water is relatively clean.

What is a Micron Rating for String Filter?

A micron is a unit of measurement which is similar to one one-thousand of a millimeter.

This is the size of the opening in a string filter that allows it to filter out dirt or sediments.

Large micron size allows large sediments to pass through than a small micron size filter.

Any size below 40 microns is not visible to the naked human eye.

A string filter has a micron rating that can be anywhere between 0.5 and 150.

What Micron Size Does my String Filter Need to Be?

The micron rating depends on the type of water you are filtering.

This is because dirty water has contaminants of varying sizes.

Always have a micron rating that allows the removal of microorganisms that you cannot see with your naked eye.

An absolute string filter contains pore sizes that are exact to its micron rating.

Nominal string filters contain pores that are an approximation to their micron rating.

For instance, a nominal 20-micron string filter can allow a small fraction of debris than have a 20-micron size to pass through.

Absolute string filters have an efficiency of over 95% thus they will not permit particles with their micron rating to pass through.

For an accurate filtration process, it is suitable to acquire an absolute string filter and not nominal string filters.

Nominal string filters are the most common but you can find absolute string filters.

Here is a guide that can assist you to consider the micron rating depending on the contaminants you are eliminating:

  • Contaminants with a thickness like human hair require a micron rating of 50.
  • When the contaminant is not visible to the human eye, you require a micron rating of 40.
  • If you have contaminants the size similar to white blood cells need a micron rating of 25.
  • A micron rating of 8 will eliminate contaminants the size like that of red blood cells.
  • One can eliminate contaminants the size of bacteria with a micron rating of 2.

What is the Ideal Length and Dimension for String Wound Cartridge Filter?

The length and dimension will depend on the filter housing you plan to place the string filter.

If you need more filtration, you can use long string wound cartridge filters as this increases the surface area.

What Are the Different Types of End Caps Used in A String Wound Cartridge Filter?

The end caps are several and provide the necessary filtration need for every application.

Examples of end caps you can find are:

  • Custom end cap
  • Fin end cap
  • Poly 22 end cap
  • Stainless steel core extender end cap
  • Flat or closed-end cap
  • Poly 222 end cap
  • Plastisol molded end cap
  • Flat open gasket end cap
  • Closed-end stainless steel 222 end cap
  • Open-end stainless steel 222 end cap
  • Polypro 226 end cap
  • Polypro core extender end cap

What are the Recommended End Cap Specifications for String Filters?

These include:

  • Ease of disposal
  • It should form a complete seal
  • Also, it should comply with FDA standards for the food and beverage industries.
  • Chemical compatibility.

What are Typical Format Combinations for String Wound Cartridge Filters?

These include:

  • Melt blown over carbon
  • String wound over melt blown

What is the Difference between Pleated Surface Filters and Would Cartridge filter as a Depth Filter Cartridge?

Pleated surface filters have thin filters that capture contaminants from the surface.

pleated surface filter

 pleated surface filter

Also, they have a large surface area which makes them have a high dirt holding capacity.

When the external surface is full of contaminants and particles, you can dispose of this filter.

Its wide surface area allows it to capture particles that have a uniform size.

Besides, the filtration happens on the surface and you can remove and wash it once the pressure drops.

The main features of pleated surface filters are:

  • It will provide an efficient and dependable filtration process.
  • One can use it in the food and beverage industries.
  • A high maintenance efficiency will offer great protection to downstream filters.
  • For broad applications, you can use O-rings.
  • It has a high surface area than other cartridges.

On the other hand, depth filter cartridges will also eliminate contaminants from water.

They are also known as string spun and string wound cartridge since it has 2 layers of filtration.

The first layer will capture large particles and the inner layer captures the tiny particles.

Also, you need to clean and change depth filters more often than pleated filters.

depth filter cartridge

depth cartridge filter

Some traits of depth cartridge filters are:

  • Large particle containment increases pressure.
  • It is durable and a cost-effective filtration method.
  • The inner attachments will eliminate contaminants.
  • Also, it can work at high speeds since it has a great diameter.
  • When manufacturing, you will not use adhesives or other external materials.

What are FDA String Filters?

This is a string filter that you can use in the food and beverage industries and pharmaceuticals.

What are the Recommended Standards for String Filter Media?

These include:

  • High dirt holding capability
  • True depth filtration
  • Free from chemical substances.

What are the Various Types of Cores for String Wound Cartridge?

The core can be of the following materials:

  • Gas-filled polypropylene
  • Stainless steel
  • Polypropylene [PP]
  • Tinned steel

What are the Considerations for Choosing String Wound Cartridge Filter Core?

These include:

Polypropylene String Wound Cartridge Filer Core

This is a cost-effective option for water, aqueous solutions with wide pH ranges, and a wide array of organic chemicals.

The appropriate maximum temperature is  82 °C [180 °F].

Glass-filled Polypropylene String Wound Cartridge Filer Core

This is a great substitute for applications that need more core stability for high temperatures.

Also, it replaces chemically compatible polypropylene.

Besides, it has a compatible fluid maximum temperature of 93 °C [200 °F].

Tin Plated Steel String Wound Cartridge Filer Core

It is a core great for solvents, paints, oils, and high-temperature applications.

The suitable maximum temperature for compatible fluids is 204 °C [400 °F].

304 Stainless Steel String Wound Cartridge Filer Core

This material is good for corrosive fluids or those that have chemicals that make polypropylene expand or soften.

The suitable maximum temperature for compatible fluids is 399 °C [750 °F].

316 Stainless Steel String Wound Cartridge Filer Core

It is suitable for some acids and corrosive agents that are not good for 304 stainless steel.

The suitable maximum temperature for compatible fluids is 399 °C [750 °F].

Nylon String Wound Cartridge Filer Core

This is good for several organic chemicals and fuels.

It has a suitable fluid maximum temperature of 135 °C [275 °F].

Polyester String Wound Cartridge Filer Core

It is a suitable core substance for several oils, fuels, and caustics.

Besides, it has a suitable fluid maximum temperature of 149 °C [300 °F].

PVDF String Wound Cartridge Filer Core

It is a premium-priced fluoropolymer substance.

Also, it offers compatibility with acids, fuels, oils, and oxidizing chemicals.

It has a suitable fluid maximum temperature of 121°C [250 °F].

PTFE String Wound Cartridge Filer Core

This is also a  premium-priced fluoropolymer substance.

It offers a broad array of chemical compatibility.

The suitable maximum temperature for compatible fluids is 149 °C [300 °F].

What are the Consequences of not Using Good Quality Wound Cartridge Filter?

These include:


If you use a yarn that is round and smooth to make a wound cartridge filter, it can easily roll and move in that position.

The rolling and displacement effect of the yarn can occur during these two processes:

  1. Knife-edge sealing.
  2. Pressure and fluctuations of the flow.

When this happens, it will dismantle the pattern and form gaps between the filtration medium.

This will compromise the efficiency of the filtration.

Also, achieving an accurate micron rating will be a challenging task.

Chemical Leaching

When chemicals leach into the liquid, it will cause a major concern as it will compromise the quality of the filtrate.

When you start using the string wound filter, chemicals that begin to leach include:

  1. Antistatic
  2. Resin binders
  3. Release agents
  4. Lubricants

These chemicals will begin to mix with the filtrate hence lowering the efficiency and quality of the filtration process.

There are two solutions to combat this effect:

  1. Prewashing the wound cartridge filter before using it.
  2. Allow the liquid to flow via the material for some time before you start to officially use the wound cartridge filter.

One of the best ways of ensuring minimal leaching is to ensure filters that are 100% free of these chemical compounds.

Media Migration

Using good quality yarn is one of the determinants to achieve the best filtration with a wound cartridge filter.

When you use short and chopped fibers, they will loosen and move with the filtration liquid.

Some extra deterioration may happen due to:

  1. Rough handling of the wound cartridge filter.
  2. Poor and bad packaging.
  3. Wear and tear can occur during knife-edge sealing.
  4. Inconsistent flows.

How do you Install a Cartridge Filter?

Here is how you can install a cartridge filter and ensure it operates properly:

  • Locate the inlet and outlet ports.
  • Properly align the piping system while avoiding it from pressing the cartridge filter.
  • It is possible to include shutoff valves that enable draining contents while replacing the cartridges.
  • Select stable support and volt down the system.
  • Install the right size of cartridge filter on inlet and outlet valves, then connect the piping.
  • Use the cartridge filter manual to connect all components and cartridges.
  • Make sure you align the seal and gaskets, then close them.
  • Allow fluid to enter the cartridge filter by opening the shut-off valves.

Cross-check the manual to ensure differential pressure reading is correct

What are the Recommended Operating Conditions for String Wound Cartridge Filter?

These include:

  • Maximum operating temperature
  • Maximum Differential Pressure
  • Change-out Differential Pressure

How easy are String Filters to Look After?

Maintenance ensures it operates optimally and offers quality filtration processes.

Do not backwash the string filter but rather replace it once it is full of dirt and contaminants.

Clean the filter regularly to eliminate any dirt and impurities.

If possible, ensure the yarn is always wet to prolong its use time.

What are the Major Drawbacks of Wound Cartridge Filters?

These limitations include:

Media Migration

This happens when you use short and chopped fibers, as they loosen and move with the filtration liquid.

Chemical Leaching

This happens when chemicals will leak into the fluid and cause health hazards.

Roving and Friction on Spun Yarns

This happens when the yarn starts to shift and form tunnels.

It may tend to roll on one side thus give an inconsistent outcome as the fluid passes via a less resistant path.

Which Tests Do String Wound Cartridge Filters undergo?

Some of the tests it undergoes include:

  • Destructive test
  • Non-destructive test
  • Bubble point test
  • Pressure hold test
  • Diffusion test

What are the Recent Developments in String Wound Cartridge Filter?

These developments will combat any drawbacks of these filters.

New media has a continuous filament and not short chopped fibers.

This means each filament will span the whole length of the yarn this combating the media migration effect.

These new filaments have no spin-finish chemicals.

Also, the yarns form a stable and bulky yarn as the filaments are random, intermixes, loops, and intertwines over each other.

With this new development, every yarn will trap the neighboring yarn thus achieving a stable configuration.

These yarns lock in place and do not shift side to side.

This effect enables it to overcome particle unloading and offers great knife-edge sealing.

Winding technology also assists in controlling the start and finish of the yarns.

It allows one to form tight inner weaves with no change in tension thus enhance a consistent performance.

Also, the new winding technology will form stable cartridges with no diamond patterns.

How Do you Choose the Best String Wound Cartridge Filter for Your Application?

Selecting the best filter ensures that the filtration process is accurate and of good quality.

Some of the components of a string wound cartridge filter are the core, yarn, gasket, and endcap.

The best string wound cartridge filter applies the best materials and the weaving is of good quality.

This is to ensure you achieve the best filtration process.

Some suitable qualities to look out for in a string wound cartridge filter are:

  • High precision manufacturing
  • A durable and sturdy core
  • The best quality yarn

When you use good quality materials, you will enhance the performance of the filter.

Also, it will achieve a high filtration capability and be durable.

Always select a string wound cartridge filter from a reputable manufacturer who uses the latest technology.

How Often Should You Change Your String Filter?

Changing a string filter depends on the quality of water that it is filtering and this is dependent on each supply.

When it is full, simply change the string filter.

To change it, start by turning off the supply and then unscrew the housing and empty the fluid from inside.

Take the filter out and replace it carefully until you achieve a suitable seal.

As you can see, there are many factors you should consider when choosing string filters.

However, if you have any question regarding string filters, FilSon team is here to help – contact us now.

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