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EnglishWhat Percent Ferrite and Ferrite Number Calibrations Mean on the Severn Ferrite Indicator
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The Severn Ferrite Indicator is offered with two calibration types: Percent Ferrite and Ferrite Number (FN). Both measure the ferrite content of a material such as a stainless steel weld — the difference is the scale used to express that content. Choose the calibration that matches the acceptance criteria or reporting format your inspection standard requires. If your procedure or customer specifies ferrite "percent," order the Percent Ferrite version; if it specifies FN, order the Ferrite Number version.
The two calibrations at a glance
| Calibration | What it expresses | Typical use |
|---|---|---|
| Percent Ferrite | Ferrite content as a percentage | Procedures or reports written in percent ferrite |
| Ferrite Number (FN) | Ferrite content on the FN scale | Procedures or reports written in FN |
Both are read from the same instrument type — the Severn Gage Ferrite Indicator — so the choice is about the scale your work is judged against, not about two different measurement principles.
Why two scales exist
Ferrite content in stainless steel welds is measured to judge weld soundness. The Severn Ferrite Indicator is described as being used extensively to measure the soundness of welds to stainless steel pipes in nuclear power plants and food handling equipment — applications where the ferrite level in the weld affects both cracking resistance and corrosion behavior.
Because different standards, procedures, and customers express the same physical property in different units, the indicator is supplied in two calibrations rather than one. Percent Ferrite and Ferrite Number are both ways of stating how much ferrite is present; they are not interchangeable numbers, so a reading on one scale does not simply equal the same figure on the other.
How to decide which one you need
Work backward from the document that will receive the reading:
- Find the acceptance criterion. Look at the welding procedure specification, inspection procedure, or purchase order. It will state a limit in either percent ferrite or FN.
- Match the calibration to that unit. If the limit is written as a percentage, you need the Percent Ferrite calibration. If it is written as FN, you need the Ferrite Number calibration.
- Check the reporting format. Even if the criterion is loose, the inspection report usually has a fixed field for the unit. Match it.
- Confirm before ordering. The Severn Gage is sold in two models — the Low Mu Permeability Indicator and the Ferrite Indicator — and the Ferrite Indicator itself comes in the two calibrations above. Make sure both the model and the calibration are what you intend.
Common sticking points
- Assuming the numbers convert directly. A percent ferrite reading and an FN reading of the same weld are related but not identical values. Do not substitute one for the other in a report.
- Confusing the Ferrite Indicator with the Low Mu Permeability Indicator. The Low Mu Permeability Indicator measures the magnetic permeability of weakly magnetic materials such as stainless steel. The Ferrite Indicator measures ferrite content in a material such as a stainless steel weld. They answer different questions.
- Ordering by instrument name only. "Severn Gage" covers more than one model and calibration. Specify the calibration explicitly.
Where this fits in weld inspection
For stainless steel pipe welds — including those in nuclear power plants and food handling equipment — the ferrite reading is part of judging weld soundness. The Severn Gage is described as a simple, rugged, non-destructive inspection instrument intended for laboratory, shop, and field use, so the same calibration choice applies whether the reading is taken in a lab or on site. The calibration you pick should follow the standard governing the weld, not the location of the measurement.