What Size Test Pieces Should You Use for Metal Detector Testing?

2026-10-28 · Jordan Wheeler

Every metal detector check depends on one small object: the test piece. Choose sizes that are too large and your checks prove very little. Choose sizes the detector can't reliably find in your product and you'll fight constant failures and false rejects.

So what size should you use? There's no single answer that fits every line, but there is a clear way to decide, and auditors expect you to be able to explain it.

Why there's no universal size

The smallest piece of metal a detector can reliably find depends on:

• Aperture size. Smaller openings give better sensitivity. A detector with a large aperture for bulk product can't match one sized tightly around small packages.
• Product effect. Wet, salty, and conductive products like meat, cheese, and sauces create their own signal, which limits how small a test piece the detector can find.
• Metal type. Ferrous metal is usually the easiest to detect, non-ferrous is next, and stainless steel is usually the hardest. That's why stainless test pieces are typically the largest of the three.
• Position and orientation. The center of the aperture is the least sensitive spot, and long, thin pieces of metal are harder to detect in some orientations.
• Line speed and product temperature. Both can affect performance.

Because of all this, two plants making similar products on different lines can end up with different test piece sizes, and both can be correct.

How to choose your sizes

A sound approach looks like this:

1. Start with your hazard. The FDA treats hard or sharp foreign objects between 7 and 25 mm as a potential injury hazard. Your detection capability should be well below that.
2. Check customer and retailer requirements. Many brand owners and retailers specify maximum test piece sizes for each metal type. Those requirements override your own preferences.
3. Find the best achievable sensitivity in your product. Working with your equipment supplier, set the detector up for your real product and find the smallest ferrous, non-ferrous, and stainless steel pieces it reliably detects at the center of the product.
4. Set your standard at or just above that. Use the smallest sizes your detector can consistently find without unacceptable false rejects.
5. Write down your reasoning. Record the sizes for each line and product, and why you chose them.

If you can't explain why you use a particular size, an auditor will ask. "That's what the last company used" isn't a justification.

Are 3 mm or 4 mm test pieces the right size?

Sizes like 3 mm and 4 mm come up a lot, usually for stainless steel on larger apertures or in wet, salty, or frozen products, where smaller pieces can't be found reliably. On smaller apertures and dry products, many lines can do much better, often well under 3 mm for ferrous.

So 3 mm or 4 mm can be the right answer for one line and far too large for another. The test is the same either way. Is it the smallest size your detector reliably finds in your product? Does it meet your customer's specification? Is your reasoning written down? If your current sizes are larger than your line can achieve, an auditor may ask why you aren't testing with smaller ones.

Use certified test pieces

A test piece is only useful if you know its true size. Good practice is to:

• Use certified test pieces with a certificate showing the sphere size and its tolerance, traceable to national standards
• Use the right material, including type 316 stainless steel for stainless tests, since common grades behave differently
• Label each one clearly with metal type and size
• Choose a format that fits your process: cards, sticks, or pucks for packaged product, and special carriers for pipelines and gravity-fall systems

Look after them

Test pieces get dropped, worn, and lost. Make it part of your program to:

• Inspect them regularly for cracks, wear, or exposed spheres
• Replace any damaged piece right away, because a cracked card may not match its labeled size
• Store them securely and keep a log of who uses them, so a test piece never ends up in product
• Reconcile them at the end of each shift

Test the hardest case

When you verify, put the test piece where detection is hardest: in or with the product, through the center of the aperture, and at the leading and trailing edges. Taping a test piece to the top of a package puts it in the most sensitive position, which makes the detector look better than it really is.

How validation confirms your sizes

During a validation, we confirm that every size in your specification is detected and rejected three times in a row at the leading edge, center, and trailing edge of your actual products. If a size fails, we adjust the detector and retest until it passes, and document the as-found and as-left settings. The report gives you written proof that your chosen sizes are achievable on that line.

The bottom line

The right test piece size is the smallest one your detector can reliably find in your product, at or below what your customers require, using certified pieces in good condition. Most importantly, document your reasoning.

Need help confirming your sizes? Learn about our metal detector validation for Utah food plants, or contact us.