Updated for 2026: Gauss is often quoted when comparing magnetic separators, but it does not always give a practical indication of separation performance. A pull test provides a simple, repeatable way to measure the force needed to remove a defined steel test piece from the separator surface, helping users compare magnetic strength, monitor separator condition, and support routine inspection programs.
A magnetic separator is designed to attract, capture, and hold ferrous metal contamination. The magnetic strength needed to achieve that result is often stated in a quotation or specification, usually in terms of gauss.
However, gauss can be difficult to measure consistently, and a gauss reading alone does not always show how well a magnetic separator will capture and hold metal in a real process.
A more practical method is to measure the force needed to remove a defined steel test piece from the surface of the magnetic separator. This is known as a pull test. It provides a repeatable measurement that can be used to compare similar magnetic separators, check supplied equipment against a specification, and monitor changes in magnetic strength over time.
This guide explains why gauss can be misleading, how pull testing works, what the original pull test experiments showed, and how regular magnetic separator checks can support product quality, equipment protection, and audit requirements.
Contents
- Why Magnetic Separator Strength Needs to Be Measured
- Why Gauss Alone Does Not Tell the Full Story
- What Is a Magnetic Pull Test?
- How to Carry Out a Pull Test
- Where Pull Testing Is Most Useful
- Pull Test Experiment Results
- What the Results Show
- What Magnetic Separator Audits Can Reveal
- How Often Should Magnetic Separators Be Tested?
- What to Include in a Magnetic Separator Specification
- Frequently Asked Questions
- Getting the Right Magnetic Separator Support
Why Magnetic Separator Strength Needs to Be Measured
Magnetic separators are often installed to protect products, processing equipment, and customers from ferrous metal contamination. If the magnetic separator is not strong enough, damaged, poorly specified, or incorrectly positioned, metal contamination may pass through the process.
This can lead to:
- Reduced product purity
- Customer complaints or rejected product
- Damage to downstream equipment
- Unplanned maintenance and downtime
- Increased contamination risk
- Audit or quality concerns
Measuring magnetic separator strength gives operators a practical way to check whether the separator is still performing as expected. This is especially useful for equipment such as Tube Magnets, Grate Magnets, Plate Magnets, Drawer Filters, and Liquid Magnetic Separators.
In many cases, the most useful measurement is not the highest gauss reading. It is the amount of force needed to remove a defined test piece from the magnet surface.
Why Gauss Alone Does Not Tell the Full Story
Gauss is a unit used to measure magnetic flux density. It is commonly quoted by magnetic separator suppliers and is often used in purchase specifications. However, gauss is not always a reliable way to compare magnetic separators in practice.
There are several reasons for this:
- A gauss reading can vary depending on probe position and orientation.
- The highest gauss value may only occur at one point on the magnetic surface.
- Gauss does not directly show how strongly a magnet will hold a piece of ferrous metal.
- Two separators with similar gauss readings may perform differently depending on magnet design.
- A user may not be able to easily verify a quoted gauss value after installation.
Gauss reading
Measures magnetic flux density at a specific point. Useful, but sensitive to test method, probe position, and interpretation.
Pull test reading
Measures the force needed to remove a defined steel test piece from the magnet surface. Practical, repeatable, and useful for ongoing comparison.
For users, the practical question is simple: does the magnetic separator attract and hold the ferrous contamination it is intended to capture? A pull test provides a more direct way to assess that holding force.
What Is a Magnetic Pull Test?
A magnetic pull test measures the effort needed to remove a steel item from the face or surface of a magnetic separator. It is commonly carried out using a spring balance or pull test kit with a defined steel test piece attached.
The test piece is placed on the magnetic separator surface. Force is then applied using the spring balance until the test piece breaks away from the magnet. The force required to remove the test piece is recorded.
This does not provide a gauss reading. Instead, it provides a physical measurement of pull force. This can be used to:
- Compare similar magnetic separators
- Check the condition of a separator over time
- Create a baseline reading when equipment is new
- Identify changes in magnetic performance
- Support maintenance and audit records
- Help verify supplied equipment against a purchase specification
Simple pull test principle
1. Place test piece
A defined steel ball or test piece is placed on the magnet surface.
2. Apply force
A spring balance is pulled away from the magnetic surface.
3. Record breakaway force
The force needed to remove the test piece is recorded.
4. Repeat and compare
Readings are repeated and compared with previous results or specifications.
How to Carry Out a Pull Test
A pull test should be carried out in a consistent way so the results can be compared over time. The same test piece, test location, test direction, and recording method should be used each time.
A typical pull test process is:
- Select the correct magnetic test piece, such as a defined steel ball attached to a non-magnetic ring.
- Attach the test piece to a spring balance or pull test kit.
- Place the test piece on the magnetic separator surface at the required test point.
- Pull the spring balance steadily away from the magnet until the test piece detaches.
- Record the force required to remove the test piece.
- Repeat the test at the same point and calculate the average.
- Repeat the process at other defined test points.
- Compare the results with previous readings or the original supplied specification.
For a Tube Magnet, readings are commonly taken at the center of the tube and at the end poles. The test should be repeated several times and the average recorded.
Pull test record should include
Where Pull Testing Is Most Useful
Magnetic separator designs vary considerably depending on the application. Pull testing is most useful for smaller magnetic separators where a defined steel test piece can be applied directly to the magnet surface.
Typical examples include:
Used in hoppers, chutes, gravity systems, and as part of more complex magnetic separator assemblies.
Used to remove fine ferrous particles from dry, free-flowing products passing through hoppers and bins.
Plate Magnets
Used in chutes and above burden flows to attract and hold ferrous contamination from dry materials.
Use rows of magnetic tubes to capture ferrous contamination from powders, pellets, granules, and other gravity-fed products.
Use magnetic tubes to capture ferrous contamination from liquids, pastes, and slurries.
Pull testing is not the only way to evaluate magnetic separator performance, and it is not suitable for every separator design. For larger or more complex systems, the correct assessment may include process testing, material trials, inspection of captured contamination, or review of the full installation.
Pull Test Experiment Results
In the original pull test experiments, Tube Magnets were tested using Ceramic Ferrite, Standard Neodymium Rare Earth, and High Strength Neodymium Rare Earth Magnets.
Three steel ball sizes were used as magnetic test pieces: 6 mm, 12 mm, and 25 mm. Non-magnetic spacers were introduced to assess magnetic strength at different distances from the tube surface: no gap, 3 mm, and 6 mm. All recorded measurements are in kilograms. “NR” means no noticeable force was recorded.
Ceramic Ferrite Tube Magnet
| Gap | 6 mm Ball | 12 mm Ball | 25 mm Ball |
|---|---|---|---|
| None | 1.25 kg | 1.75 kg | 2.5 kg |
| 3 mm | NR | NR | 1.2 kg |
| 6 mm | NR | NR | NR |
Standard Neodymium Rare Earth Tube Magnet
| Gap | 6 mm Ball | 12 mm Ball | 25 mm Ball |
|---|---|---|---|
| None | 2 kg | 4.25 kg | 9 kg |
| 3 mm | 1.2 kg | 1.6 kg | 2.45 kg |
| 6 mm | NR | 1.25 kg | 1.5 kg |
High Strength Neodymium Rare Earth Tube Magnet
| Gap | 6 mm Ball | 12 mm Ball | 25 mm Ball |
|---|---|---|---|
| None | 3.75 kg | 9 kg | 14.5 kg |
| 3 mm | 1.5 kg | 2.5 kg | 4.6 kg |
| 6 mm | 1.1 kg | 1.5 kg | 2.25 kg |
What the Results Show
The pull test results show two important points. First, magnetic force drops significantly as the test piece moves away from the magnet surface. Second, the magnet type and test piece size both affect the recorded pull force.
Distance matters
The force drops quickly when the test piece is moved away from the tube surface. This highlights why product presentation and contact are important.
Magnet type matters
High strength rare earth magnets produced a much higher pull force than ceramic magnets in the original tests.
Test piece matters
Larger steel balls recorded higher pull forces, which is why the test piece size must be defined and kept consistent.
For Tube Magnets arranged as a grate, this also explains why product flow matters. If material passes through the gaps between tubes without being directed into the strongest magnetic zone, separation efficiency may be reduced. Deflectors or staggered tube arrangements can help direct product toward the areas of highest magnetic strength.
What Magnetic Separator Audits Can Reveal
Magnetic separator audits often identify problems that have gone unnoticed during normal operation. A good inspection should not only measure pull force. It should also check the physical condition and suitability of the separator.
Common findings include:
Weld failure
Failed welds can allow liquid or product ingress into a magnetic tube. This may damage the magnet assembly and reduce magnetic performance.
Tube magnet wear
Abrasive product flow can wear the stainless-steel tube surface, especially around magnetic poles where ferrous particles collect.
Weak magnets
Older separators may have lost their original specification details. Pull testing helps identify magnets that no longer provide suitable protection.
Where magnetic separators protect critical processing equipment, these issues can create a serious risk. A weak or damaged magnetic separator may allow tramp metal or fine ferrous contamination to pass downstream, increasing the risk of equipment damage, downtime, and product contamination.
How Often Should Magnetic Separators Be Tested?
The correct test frequency depends on the application, contamination risk, process conditions, product type, audit requirements, and the consequence of a failure. A low-risk application may only need periodic checks, while a high-risk food, pharmaceutical, powder, or equipment protection application may need more frequent testing.
As a practical starting point, magnetic separator testing should be considered:
- When new equipment is installed
- After maintenance, repair, or cleaning changes
- After impact, dropping, overheating, or suspected damage
- During routine maintenance inspections
- As part of internal quality audits
- Before critical customer, safety, or compliance audits
- Whenever contamination performance is questioned
The most important point is consistency. Testing should be carried out using the same method, the same test piece, and the same test points so that changes in performance can be identified over time.
What to Include in a Magnetic Separator Specification
If magnetic strength is important when purchasing a magnetic separator, the specification should not rely only on a quoted gauss value. A more useful specification includes both the physical separator details and the pull force requirement.
Specification checklist
Including a defined pull force requirement helps the buyer verify that the supplied magnetic separator matches the agreed specification. It also creates a useful baseline for future testing.
Frequently Asked Questions
1. What is the best way to measure magnetic separator strength?
For smaller magnetic separators such as tube, grate, cartridge, drawer, and plate magnets, a pull test is often the most practical method. It measures the force needed to remove a defined steel test piece from the magnet surface.
2. Is gauss a reliable way to compare magnetic separators?
Gauss can be useful, but it does not tell the full story. A gauss reading depends on how and where it is measured, and it does not directly show how strongly the separator will hold ferrous contamination in a real process.
3. What is a magnetic pull test?
A magnetic pull test measures the force required to pull a defined steel test piece away from the surface of a magnetic separator. The result can be used to compare similar separators and monitor magnetic strength over time.
4. What equipment is needed for a magnetic pull test?
A typical pull test uses a spring balance or pull test kit with a defined magnetic test piece, such as a steel ball attached to a non-magnetic ring. The same test piece should be used each time for consistent results.
5. Can a pull test measure gauss?
No. A pull test does not measure gauss. It measures breakaway force, which is the force needed to remove the test piece from the magnet surface.
6. Which magnetic separators can be pull tested?
Pull testing is commonly used on smaller magnetic separators where the test piece can be placed directly on the magnetic surface. This includes tube magnets, cartridge magnets, grate magnets, plate magnets, drawer filters, and some liquid magnetic separators.
7. Why does magnetic strength drop away from the surface?
Magnetic force reduces rapidly as distance from the magnet surface increases. This is why product presentation, contact with the magnetic surface, burden depth, and separator design are important for effective metal capture.
8. How often should magnetic separators be pull tested?
The frequency depends on the risk level, process conditions, product type, audit requirements, and consequence of failure. Many plants include pull testing within routine maintenance or quality audit programs.
9. What does a low pull test reading mean?
A low reading may indicate that the separator is weak, damaged, incorrectly specified, worn, overheated, or no longer suitable for the application. It should be investigated against the original baseline or required specification.
10. Should pull force be included in a magnetic separator specification?
Yes, where magnetic strength is important. Including a defined pull force requirement, test piece size, and test location helps the buyer verify the supplied separator and creates a baseline for future inspections.
Getting the Right Magnetic Separator Support
Measuring magnetic separator strength is important because it helps users move beyond quoted gauss values and assess the practical holding force of the separator.
A pull test provides a simple, repeatable way to compare similar magnetic separators, create a baseline reading, and identify changes in performance over time. When combined with visual inspection, it can also help identify wear, weld failure, damage, or magnets that are no longer providing suitable protection.
Bunting designs and supplies magnetic separation equipment for food, plastics, recycling, ceramics, powder and bulk handling, minerals, liquids, and other industrial applications. Our team can help review your process, assess the correct separator type, and advise on magnetic strength, pull testing, and ongoing inspection.
For support with magnetic separator strength, pull testing, or equipment selection, contact Bunting’s technical team.
