Product purity – including the elimination of metal contamination – is essential for food safety.
Reliable and effective magnets are an important part of a facility’s quality assurance program as they help to ensure ferrous metal is removed from the product stream prior to packaging. Testing the strength of those magnets helps to identify threats to product purity before a breakdown in metal management occurs.
What Does Testing a Magnet’s Strength Reveal
A magnet’s strength can change over time. A reduction in strength can be caused by fluid penetration into the magnet housing, welding that damages the raw magnet, and, most commonly, operating temperatures that exceed what a specific magnetic material can withstand. While changes to an individual magnet are likely caused by localized issues, facility wide magnetic degradation can indicate a mismatch between the magnetic material chosen and the maximum operating temperature in the facility.
While magnet strength degradation will lead to ineffective metal capture if not identified and resolved, equally dangerous – but less obvious – is an increase in a magnet’s strength.
When a magnet’s values go up, it indicates that the walls, housing, or face of the magnetic tube are thinning. This is most likely due to abrasion from product flow, but it can also occur if the equipment is not the right fit for the application.
Testing a magnet’s strength allows an operator to monitor their performance over time – and should be completed at regular intervals. If the strength changes – whether by increase or decrease – the unit should be replaced before it becomes ineffective for product protection.
There are two methods to measure a magnet’s strength, Gauss and Pull Strength. Both methods can define a magnet’s circuit design and field gradient - what is the difference, and when should they be used.
Gauss Testing
Gauss measurement is one way to measure a magnet’s field strength. This is accomplished by using a device called a gauss meter, an electronic device that is very sensitive and capable of measuring the lines of flux in a magnet’s working air gap using a hand-held probe. As the probe is moved around the magnet the values will increase/decrease accordingly.
While a gauss meter can be a useful tool, this type of measurement is subject to several potential problems.
- Sensitivity - gauss meters are very sensitive tools, so sensitive in fact that it is near impossible to gain a reliable, repeatable value for any given magnet. Even with controlled usage, small movements of the probe in the magnet’s working air gap will generate a different result. This makes monitoring the strength of a magnet over time difficult.
- Calibration - gauss meters are electronic devices and are susceptible to improper calibration. Most gauss meters need to be re-calibrated between each measurement making repeated testing a challenge.
Pull Testing
The other option for measuring the strength of a magnet is through pull testing. In a pull test, the tester places a known ferrous object (typically a sphere or steel ball of a given diameter) directly on the magnet’s working surface. The object is then attached to a scale and pulled away from the magnet until the object breaks away from the magnet’s surface and a pull strength is measured. Strength is typically measured in pounds and/or ounce depending on the type of magnet.
It is also possible to measure the magnet’s gradient, or the change in magnetic strength as the gap from the magnet’s working surface is increased. This is accomplished by placing a non-ferrous spaced, such as aluminum, between the sphere and magnet. The gradient values are important when a magnet is required to capture metals from a distance away from the magnet’s surface.
Our Recommendation – Pull Testing
After more than 30 years conducting customer plant audits, we have found the pull test method meets the most stringent quality programs such as HACCP, TQM, BRC, etc. and the data is useful to operators in prioritizing future magnet upgrades.
Additionally, operators can be easily trained in pull testing allowing for self-auditing. This autonomy is beneficial where magnets are considered a critical component of the facility’s quality program.
Importance of Documentation
While both gauss and pull testing can be useful in maintaining an effective metal management system, inspecting magnets at regular intervals, and properly documenting test results, visual inspection notes, metal capture data, environmental conditions and any concerns, is essential regardless of the type of testing performed.