Standard cylindrical inductive proximity sensors carry a nominal sensing distance Sn rated against a 1 mm thick Fe360 mild-steel test target, the same test plate defined by the IEC inductive-sensor standard, and that single number is where most selection errors start [S2][S4].
When the actual target is aluminum, brass, copper or 302 stainless steel, the effective range collapses to roughly 0.40 to 0.85 of the catalog rating, so a sensor marked 10 mm Sn on mild steel may only reach about 4.5 mm on aluminum [S4]. Understanding that ratio, called the correction factor, is the difference between a panel that works on the bench and one that misfires in production [S1][S2].
Why mild steel is the universal reference target
Under the IEC standard that defines proximity-sensor test methods, the reference target is a square piece of carbon steel grade Fe360, 1 mm thick, and large enough to fully cover the sensing face; all manufacturer "Sn" ratings are measured against that exact plate [S2]. Mild steel generates strong eddy currents that absorb energy from the sensor's high-frequency electromagnetic field, damp the oscillator, and trip the detection circuit at the rated distance [S3].
That is why inductive proximity sensor datasheets and the broader proximity sensor encyclopedia entry both anchor every number to Fe360: it gives a reproducible baseline so a user comparing an Eaton M12 to a Balluff BES M18 can trust the "10 mm" label means the same thing on both boxes [S1][S2].
Correction factors for non-ferrous and stainless targets
The type of metal being detected has the largest influence on real range, and the typical correction factors published across the industry are remarkably consistent [S4]. For a sensor rated 10 mm Sn on mild steel, the achievable distance on common engineering metals falls into a tight band: stainless steel 302 about 0.85, brass about 0.50, aluminum 0.45 to 0.47, and copper about 0.40, which means the same sensor delivers anywhere from roughly 4.0 mm on copper up to 8.5 mm on stainless [S4].
Ferrous-only sensors, by design, ignore aluminum entirely and are commonly used in machining cells where steel tool pallets must be detected while aluminum fixtures swing past, an application pattern Balluff and Eaton both highlight in their selection guidance [S1]. When a line must detect mixed alloys (steel, stainless, aluminum, brass) at the same range on one fixture, "Factor 1" or "all-metal" sensors are the specialized alternative, with their own tradeoff in coil design and cost [S4].
Target size, shielding, and temperature: the other Sn killers
Three further variables quietly eat into Sn, and ignoring any of them causes field failures even when the metal type is right [S4][S5].
First, target size: the target should be at least three times the diameter of the sensing face for the full rated distance, so an M18 sensor nominally needs a target around 54 mm across; smaller targets produce weaker eddy currents and may not trigger the oscillator at the rated Sn [S4].
Second, shielded vs unshielded geometry: shielded (flush-mountable) sensors concentrate the field forward and accept surrounding metal, while unshielded (non-flush) sensors throw a longer, rounder field but cannot be embedded in metal; unshielded versions of the same body size typically deliver roughly 1.5 to 2 times the Sn of a shielded unit [S4].
Third, temperature and supply voltage: most inductive sensors drift a few percent over their operating temperature window, and Pepperl+Fuchs notes that simply enlarging the target beyond a certain point stops increasing the operating distance, so "bigger target" is not a free upgrade once the 3:1 face-diameter rule is met [S5].
Selection workflow: four steps that prevent rework
A reliable selection sequence compresses to four checks, and walking them in order keeps the BOM honest [S4].
Step 1, identify target material and pull its correction factor from the datasheet; Step 2, divide the required mechanical detection range by that factor to back-calculate the minimum Sn the sensor must carry, then upsize to the next standard body; Step 3, confirm environment, IP rating, temperature window, and whether weld-spatter or coolant exposure will be present; Step 4, pick shielded if the sensor will be flush-mounted in metal and unshielded if the long range matters and the surrounding area is clear [S4].
For cylindrical threaded-barrel sensors, the CENELEC-standardized form factor covers about 70% of all inductive-sensor purchases, so starting with a standard M12, M18 or M30 body, in either shielded or unshielded flavor, is the lowest-risk default for general factory automation [S3].
Common mistakes when a sensor will not trigger
Four mistakes account for most field returns on inductive sensors, and each has a mechanical, not electronic, root cause [S4][S5]. Specifying Sn without applying the aluminum or stainless correction factor, undersizing the target below the 3:1 face-diameter rule, flush-mounting an unshielded sensor into a steel bracket, and ignoring temperature drift on a hot machine all produce the same symptom: the sensor never reaches the operate point, even though bench testing with a steel plate "worked" [S4].
The fix is rarely a higher-Sn sensor: the fix is matching the correction factor, upsizing the target, swapping to a shielded body, or moving the sensor 10 to 20 mm farther from the bracket edge so the unshielded field has room to breathe [S4][S5].
Standards, datasheets, and the data to verify before ordering
Every credible Sn number is traceable to the same chain: the IEC test target (1 mm Fe360 mild steel, square, face-covering), a CENELEC-standardized cylindrical body, and a manufacturer datasheet that lists the target material, target size, supply voltage, and operating temperature used to derive Sn [S2][S3]. Balluff's "Material does matter" piece and Eaton's inductive-sensor product page both reinforce that the rated Sn is conditional on those test conditions, and Pepperl+Fuchs' operating-distance knowledge base adds the empirical rule that increasing target size past the face-saturation point stops increasing range [S1][S2][S5].
Trackable signals for the next planning cycle: vendors publishing Factor 1 / all-metal correction curves for new weld-field and EV-battery fixtures, and a tighter datasheet convention showing the actual measured range on 304 stainless and 6061 aluminum, not just the Fe360 Sn, would let process engineers stop applying correction factors by hand [S1][S4][S5].
Background reading: SABB vs Spherical Roller: Load Capacity, Speed, and Misalignment.