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What is the Severn Gage?
The Severn Gage is a non-destructive inspection instrument made by Severn Engineering Instruments LLC, used to check the magnetic properties of weakly magnetic materials and welds. It is a simple, rugged tool intended for laboratory, shop, and field use, and it comes in two models with different measurement purposes.
Two models, two jobs
- Low Mu Permeability Indicator — measures the magnetic permeability of weakly magnetic materials such as stainless steel. Permeability describes how easily a material can be magnetized, so this model is for assessing the material itself.
- Ferrite Indicator — measures ferrite content in materials such as stainless steel welds. It is used to judge weld soundness, notably in stainless steel pipe welds for nuclear power plants and food handling equipment. It is available with two calibration types: Percent Ferrite and Ferrite Number.
Which one fits your situation
If your concern is the base material's magnetic response — for example, qualifying stainless steel stock — the Low Mu Permeability model is the relevant choice. If your concern is weld quality and you need a ferrite reading expressed as Percent Ferrite or Ferrite Number, choose the Ferrite Indicator and confirm which calibration matches the specification you are working to.
A practical next step: before ordering, write down the acceptance criterion you must meet (a permeability limit, or a ferrite percentage or Ferrite Number range) and confirm which model and calibration reports in those terms. That single check prevents buying the wrong version of an otherwise suitable gage.
How does the Low Mu Permeability Indicator differ from the Ferrite Indicator?
They measure different things, even though both are Severn Gage models. The Low Mu Permeability Indicator measures the magnetic permeability of weakly magnetic materials such as stainless steel. The Ferrite Indicator measures ferrite content, for example in a stainless steel weld, and is used to check weld soundness in applications such as stainless steel pipework in nuclear power plants and food handling equipment. Permeability describes how easily a material can be magnetized; ferrite content describes how much ferritic phase is present.
Choosing between them
- Choose the Low Mu Permeability Indicator when the question is about the base material: how magnetic a "non-magnetic" or weakly magnetic alloy actually is. This matters for instruments, sensors, and components where stray magnetism would affect performance.
- Choose the Ferrite Indicator when the question is about a weld or a similar localized region: whether the ferrite level is within the intended range. Weld ferrite is a common quality check because too little or too much affects cracking resistance and corrosion behavior.
- Calibration is a further difference. The Ferrite Indicator is offered with two calibration types, Percent Ferrite and Ferrite Number. Those scales are not interchangeable, so the choice should match the specification or acceptance criterion you are working to.
- Both are described as simple, rugged, non-destructive instruments for laboratory, shop, and field use, so the decision is mainly about the measurement question, not the setting.
A practical example: a fabricator welding stainless steel pipe might use the Ferrite Indicator on the weld and, if the procedure also calls for verifying the base metal, use the Low Mu Permeability Indicator on the parent material. A next step is to check which scale or unit your drawing, welding procedure, or customer specification names, then match that to the model and calibration.
Which Severn Gage model should I choose for inspecting stainless steel welds?
For stainless steel welds, choose the Ferrite Indicator model, not the Low Mu Permeability Indicator. The Severn Gage line splits by what you need to measure: the Ferrite Indicator measures ferrite content in a material such as a stainless steel weld, while the Low Mu Permeability Indicator measures the magnetic permeability of weakly magnetic materials like stainless steel itself.
Why the Ferrite Indicator fits weld inspection
Weld soundness in austenitic stainless steel is commonly judged by ferrite content, because the wrong balance of ferrite can leave the weld prone to cracking or, in corrosive service, to problems that show up later. The Severn page describes the Ferrite Indicator as being used extensively to check the soundness of welds in stainless steel pipes in nuclear power plants and food handling equipment — both settings where a documented ferrite reading matters.
The Ferrite Indicator is available in two calibration types:
- Percent Ferrite — reads ferrite as a percentage.
- Ferrite Number — reads ferrite on the Ferrite Number scale.
Choosing between Percent Ferrite and Ferrite Number
| Calibration | Best for | Trade-off |
|---|---|---|
| Percent Ferrite | General shop and field checks where a percentage reading is expected | Less standard for weld procedure specifications that call out Ferrite Number |
| Ferrite Number | Weld procedures, nuclear and other regulated work that specify FN | More specific to weld documentation |
The practical rule: if your welding procedure specification, client specification or inspection code calls for a Ferrite Number, order that calibration. If it calls for percent ferrite, order that. If you are unsure, ask the specification owner before ordering, because the two scales are not interchangeable in documentation.
A concrete example
A fabricator welding 304L stainless pipe for a food plant might check each production weld with the Ferrite Indicator to confirm the ferrite level is within the range the procedure allows. The same fabricator would use the Low Mu Permeability Indicator only when checking whether a finished stainless component is weakly magnetic enough for a sensitive application — a different question.
Next step
Confirm two things before ordering: the calibration type your specification requires (Percent Ferrite or Ferrite Number), and whether you also need permeability measurement on finished parts. If both apply, the two models cover separate jobs rather than substituting for each other.
What is the difference between Percent Ferrite and Ferrite Number calibration?
Percent Ferrite and Ferrite Number are two different scales for expressing roughly the same thing: how much ferrite is present in a stainless steel weld or similar weakly magnetic material. The difference is mainly in how the value is defined and what it is meant to be compared against.
Percent Ferrite
- Expresses ferrite as a percentage, by volume, of the material.
- Intuitive for general shop and production work: "about 8% ferrite" is easy to communicate.
- Historically derived from metallographic point counting and similar laboratory estimates, so it is tied to what you would see under a microscope.
- Useful when specifications, procedures or a customer simply state a target in percent.
Ferrite Number (FN)
- A standardized, instrument-based scale that was created because different measurement methods gave different "percent" answers for the same weld.
- Deliberately not identical to percent ferrite; FN values are usually higher than the corresponding percent value in the mid range, and the relationship is not a simple conversion.
- Backed by welding and inspection standards, so it is the scale normally called out when a weld procedure or code specifies a ferrite requirement.
- The preferred scale for comparing readings between instruments, operators and organizations.
Practical comparison
| Point | Percent Ferrite | Ferrite Number |
|---|---|---|
| What it represents | Estimated volume percent ferrite | Standardized scale value |
| Typical origin | Metallographic/laboratory estimates | Calibrated instrument scale |
| Best for | General shop communication, percent-based specs | Code, procedure and cross-instrument comparison |
| Relationship | Not numerically equal to FN | Not numerically equal to percent |
What this means for a Severn Gage buyer
The Severn Gage is offered as a Ferrite Indicator with two calibration types, Percent Ferrite and Ferrite Number, alongside a Low Mu Permeability Indicator for weakly magnetic materials such as stainless steel. Choosing between the two ferrite calibrations is therefore mostly about the paperwork you have to satisfy, not about the hardware.
A useful decision rule: if your welding procedure, inspection code or customer drawing states a ferrite requirement, use the scale that document uses. If it says "FN," order the Ferrite Number calibration; if it says "percent ferrite," order the Percent Ferrite calibration. If you work across both worlds, note that readings from the two scales should not be converted by a simple formula, so keep each measurement on its own scale and record which one you used.
A concrete scenario: a fabricator welding stainless pipe for a food-processing line may be told by the end user to keep ferrite in a certain range. If the specification is written in FN, a Percent Ferrite reading, even a plausible-looking one, will not directly demonstrate compliance. Matching the calibration to the specification avoids that argument entirely.
Why is ferrite content important for weld soundness in nuclear power plants and food handling equipment?
Ferrite content matters because it controls two failure modes that are otherwise invisible until a joint is in service: solidification cracking during welding, and corrosion or phase instability afterwards.
In austenitic stainless steel welds, a small amount of retained delta ferrite is generally beneficial. It helps resist hot cracking as the weld freezes, and it keeps the weld metal from being fully austenitic, which is the condition most prone to cracking. Too little ferrite raises cracking risk; too much can reduce toughness and, in some service environments, promote sigma-phase formation at elevated temperature. Weld soundness therefore depends on staying inside a workable range, not simply on maximizing or minimizing ferrite.
Nuclear power plants
Nuclear piping and components are held to strict weld acceptance criteria because a crack can mean both a safety issue and a costly outage. Stainless steel pipe welds are commonly checked against a specified ferrite range, often expressed as Ferrite Number, to confirm the welding procedure and consumables produced the intended microstructure. The measurement is a fast, non-destructive check that the joint is not an outlier before it goes into service.
Food handling equipment
Food-contact stainless steel must resist corrosion and be cleanable. Welds that are low in ferrite are more prone to cracking; welds with excessive ferrite or poorly balanced composition can be more susceptible to localized corrosion in product-contact areas. A ferrite check helps verify that the weld will hold up to repeated cleaning and sanitizing rather than becoming a crevice where residue collects.
Why a portable indicator is used
The Severn Gage is described as a simple, rugged, non-destructive instrument for laboratory, shop and field use, offered in two models: a Low Mu Permeability Indicator for weakly magnetic materials such as stainless steel, and a Ferrite Indicator for ferrite content in stainless steel welds, with calibrations in Percent Ferrite and Ferrite Number. That split reflects a practical distinction: permeability measurement answers "how magnetic is this material," while ferrite measurement answers "how much ferrite is in this weld." For weld acceptance work, the Ferrite Number calibration is the relevant one, since it is the scale typically written into welding procedures and specifications.
A useful next step is to check which scale your welding procedure or acceptance standard calls for — Percent Ferrite or Ferrite Number — before choosing a model, since the two are not interchangeable readings. For a nuclear or food-plant inspection, confirm the required range and the instrument's calibration type, then verify the reading against the procedure's acceptance criteria rather than treating any single number as universally "good."
How do I use the Severn Gage for non-destructive inspection in the field?
The Severn Gage is a hand-held instrument for two related field checks: measuring the magnetic permeability of weakly magnetic materials (the Low Mu Permeability Indicator model) and measuring ferrite content in stainless steel welds (the Ferrite Indicator model). Field use follows a simple pattern: confirm the right model and calibration for your material, prepare the surface, take readings at defined points, and record them against your acceptance criteria.
Choosing the right model and calibration
| Your task | Model | Calibration to specify |
|---|---|---|
| Check whether a weakly magnetic material such as stainless steel has acceptable permeability | Low Mu Permeability Indicator | Permeability measurement |
| Check ferrite content in a stainless steel weld | Ferrite Indicator | Percent Ferrite or Ferrite Number |
The Ferrite Indicator is offered in two calibrations, Percent Ferrite and Ferrite Number, so the first field decision is which scale your procedure or welding specification calls for. The two are not interchangeable as a default; match the instrument to the drawing, weld procedure or client specification before you travel to site.
A practical field sequence
- Confirm the inspection requirement: permeability value or ferrite content, and which calibration scale applies.
- Check the instrument against its reference or calibration standard before and after the shift, following the manufacturer's instructions.
- Clean the test location. Scale, paint, heavy oxide and rough weld caps can hold the probe off the surface and distort readings.
- Place the probe firmly and consistently on the prepared spot. Keep the orientation and contact pressure the same from reading to reading.
- Take readings at the locations your procedure defines — for welds, that usually means a set pattern along the cap and, where specified, the root or heat-affected zone.
- Record each value with its location, then compare against the acceptance criteria in the applicable specification.
Field trade-offs to plan for
The instrument is described as simple, rugged and suited to laboratory, shop and field use, which is the main appeal for site work: fewer setup steps and less to go wrong outdoors. The trade-off is that a contact gauge gives you a local reading, not a map of the whole component. On a long pipe weld or a large fabricated vessel, you are sampling, so the number and placement of readings matter as much as the instrument itself. Agree the sampling plan before the inspection rather than deciding on the spot.
Surface condition is the other practical limit. A reading taken over a rough, contaminated or poorly prepared area is a reading of the surface, not the material beneath it. Budget time for cleaning and for re-checking any value that looks out of trend.
When to bring in a specialist
For nuclear power plant piping and food handling equipment — two applications the manufacturer highlights — acceptance criteria are typically tight and documented. If your procedure requires a specific ferrite number range, a qualified welding inspector or NDT level II/III should own the interpretation. Ferrite content in stainless steel welds is linked to both cracking resistance and corrosion behaviour, so a marginal reading is a decision for the responsible engineer, not a pass/fail call at the bench.
For instrument-specific operating steps and calibration intervals, use the manufacturer's documentation at Severn Engineering Instruments LLC. If your work also involves general weld acceptance criteria, the applicable code or client specification is the controlling document.
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