Shielded inductive proximity sensors embed flush in surrounding metal; unshielded versions require a metal-free zone around the head but reach 1.5 to 2 times the nominal sensing distance of a shielded unit of the same barrel diameter [S2].
The two constructions differ by a single design choice: a metal shroud around the coil that contains the magnetic field. Everything else, including range, mounting, and side-detection behaviour, follows from that one decision, and CENELEC-standardised cylindrical threaded-barrel housings (M8, M12, M18, M30) account for roughly 70% of inductive sensor purchases, so the shielded/unshielded question lands on most drawings [S1][S3].
Field Geometry: Why the Same Coil Behave Differently
A shielded inductive sensor wraps the coil in a ferrite core and metal shroud that redirect the oscillating field so it radiates only from the front face; an unshielded sensor leaves the coil open so the doughnut-shaped field expands forward and sideways, which is what enables longer reach and side-entry detection [S1][S2][S7].
Reference inductance numbers in vendor test data put an unshielded M18-style coil at about 0.893 µH versus 0.845 µH for a shielded equivalent, a small but real difference that contributes to the longer nominal range [S2]. The fast visual rule from the field: if the plastic nose protrudes past the threads, it is unshielded; if the threads run all the way to the face, it is shielded [S3][S4].
Nominal Range: Side-by-Side Numbers for the Common Barrels
For the same diameter, expect unshielded Sn to run 1.5 to 2x shielded Sn, the rule every selection guide repeats [S2][S8]. The M18 case is the cleanest illustration: shielded about 5 mm, unshielded about 8 mm; M30 runs about 10 mm shielded and 15 mm unshielded, both per the same vendor comparison table [S2].
These are nominal Sn on a standard mild-steel target. Real range on stainless, aluminium, or copper drops further because the eddy-current damping effect that the sensor detects varies with target conductivity and permeability, and correction factors of 0.3 to 1.0 must be applied to Sn for non-ferrous targets [S9]. Engineers who skip the correction end up with a sensor that misses the target on day one.
Mounting Clearance: The Core Decision Driver
Flush-mount (shielded) sensors can sit in a metal bracket with no side clearance because the field is contained to the front face; non-flush (unshielded) sensors need a metal-free zone around the protruding nose, otherwise surrounding metal either false-triggers the output or pulls the effective range down [S2][S5][S6].
AutomationDirect's own support note is blunt about the failure mode: an unshielded sensor mounted flush in metal "would always be activated" [S5]. Practical rules from the same set of sources: keep the unshielded nose at least one sensor diameter clear of surrounding steel on the sides, and avoid embedding it in a sealed metal gland, since the field will couple into the housing [S3][S6]. The general-purpose shielded M12/M18/M30 in a steel plate is the workhorse because it survives being dropped into a crowded machine without range loss [S1][S5].
Decision Matrix: Pick by Mounting First, Range Second
Use the matrix below as a hard filter: if you cannot guarantee a metal-free zone around the head, the shielded part is the only safe choice, and the range loss is the price of mounting density [S2][S3][S5][S8].
Shielded wins when the sensor must recess into a steel fixture, when multiple sensors sit close together, or when a standardised M12/M18/M30 in a CENELEC housing drops in with no mechanical rework. Unshielded wins when the bracket is plastic or aluminium, when the target sits further than the shielded Sn allows, when side-approach detection is wanted, or when there is room to keep the recommended one-diameter side clearance [S1][S2][S3][S6][S8]. For a related reference on the same shielded/unshielded logic applied to a different sensor family, see the capacitive sensor guide.
Failure Modes and Field Fixes
The three recurring failure modes on the shop floor are: unshielded sensor embedded in metal and latched on (always-activated state); shielded sensor mounted with rear metal too close and range collapses from the back; two sensors of either type packed too tightly and they cross-talk [S5][S7].
Fixes are mechanical. For an unshielded sensor in metal, add a plastic spacer or relocate so the nose clears the bracket; for a shielded sensor losing range, check the back-side clearance (many datasheets specify a minimum free zone behind the face, often equal to the barrel diameter) and respect it; for cross-talk, stagger the sensors, fit a shielding sleeve, or use a sensor with an alternative-frequency oscillator [S6][S7]. If the target itself is the problem (aluminium, stainless, copper) the answer is range correction, not a different sensor family [S9].
Cost, Availability, and Integration Footprint
Shielded cylindrical sensors in M8/M12/M18/M30 are the highest-volume, lowest-cost SKUs because CENELEC harmonisation fixed body size, sensing distance, and output levels, so distributors stock deep and lead times are short [S1].
Unshielded equivalents of the same diameter are also stocked but the range is narrower; the longest-range parts (M30 unshielded at 15 mm Sn and above) tend to be specials rather than catalogue staples [S2]. Wiring effort is identical for both, same PNP/NPN, same 3-wire or 4-wire pinout, so the integration cost difference is mechanical, not electrical, which keeps total installed cost close as long as you avoid rework. For a complementary look at how a similar flush-vs-non-flush decision plays out in the proximity family, see flush vs non-flush inductive proximity sensor selection.
Selection Checklist Before You Commit
Confirm three things before issuing the PO: target material and required corrected range; mounting environment (flush in metal, standoff, or free air) and the metal-free zone you can guarantee; and the sensor's IP rating against washdown, oil, or chip exposure, since unshielded sensors with an exposed coil cap are typically IP67 at best [S2][S8].
If those three are nailed down, the choice is forced: shielded for embedded metal mounting, unshielded for plastic brackets and longer reach. Watch the CENELEC barrel standards so spare parts stay swappable across machines, and keep a few unshielded M18 spares on the shelf for the 1.5x range jobs that always turn up after commissioning [S1][S2]. For the wider decision pattern that governs every proximity-sensor purchase, see the inductive sensor specification guide.