A magnetic reed proximity switch is a 2-wire contact device that closes when a permanent magnet enters its actuation zone, while an inductive proximity switch is a 3-wire solid-state device that triggers on eddy-current loss when any conductive metal enters its high-frequency electromagnetic field [S1][S2][S4].
The two are not drop-in replacements: reed switches detect through plastic, wood, and stainless-steel non-magnetic housings and read stationary magnets, whereas inductive units ignore non-conductive material and require the target to enter the rated sensing distance, typically 0.8 mm to 15 mm for M12/M18 tubular bodies [S1][S2][S5].
Operating Principle and Target Physics
An inductive proximity switch drives a coil at a high frequency, watches the oscillation amplitude, and switches its output when eddy currents in a nearby conductive metal damp the tank circuit below a threshold [S6][S7]. Because the energy transfer relies on conductivity and permeability, ferrous steel gives the longest range, stainless steel gives roughly 60 to 80 percent of that range, and aluminum or copper sit at the low end of the correction factor curve [S1][S7].
A magnetic reed switch, by contrast, contains two ferromagnetic reed blades sealed in a glass envelope filled with inert gas; the blades flex together and close the contact when an external magnetic field exceeds the pull-in Gauss rating, commonly 10 to 40 mT for small form factors [S4][S2]. The same glass envelope construction is documented in the standard magnetic-reed form factor, and reed variants paired with a bias magnet can also be packaged as an inductive sensor-style cylinder when the application calls for a magnet-on-piston cylinder-end position read [S2].
Sensing Range, Target Selectivity, and Output Wiring
Reed switches generally out-pull inductive switches on range for their size: a typical M12 magnetic cylinder sensor hits 5 mm to 10 mm reliable actuation with a bonded magnet target, while an M12 inductive switch on mild steel is usually rated at 2 mm to 4 mm [S1][S2]. A farm-equipment retrofit thread on EEVblog describes exactly that gap, where the original equipment used a 3-wire NPN magnetic cylinder sensor with 5 mm reliable range through a hydraulic ram, and an inductive substitute would have been the wrong fit because the cylinder body is non-ferrous and the target is a permanent magnet, not a metal flag [S5].
Output topology is the second hard difference. A reed switch is a passive contact: 2 wires, no polarity, works on AC or DC, and is wired exactly like a mechanical limit switch, including the need for an external load and a snubber on inductive loads [S3][S4]. An inductive proximity switch is an active 3-wire or 4-wire device with PNP/NPN push-pull or NO/NC contact logic, requires supply voltage (commonly 10 to 30 V DC), and supplies its own switched output to a PLC or relay coil [S3][S7]. For engineers new to the family, the proximity sensor reference page lays out the same four-block architecture: oscillator, demodulator, trigger, and output stage.
Material Behaviour and Environmental Limits
Inductive units are immune to dust, oil, and non-magnetic contamination, but they fail outright on non-conductive targets, including plastic, wood, liquids, and living tissue, and they will false-trigger on any nearby ferrous bracket if the rated distance is exceeded [S1][S2]. Reed switches ignore virtually all of that, sensing only flux density, so they read through 304/316 stainless walls, plastic tank walls, and aluminum extrusion housings, and they survive millions of cycles if the contact load stays inside the published VA rating [S2][S4].
On the failure-mode axis, the trade is reversed: an inductive switch has no moving parts and typically outlives the machine, while a reed switch has glass-sealed contacts that can weld shut, fatigue, or fracture under mechanical shock, and its published life is often quoted in the 10^6 to 10^8 operations envelope depending on load current [S4]. For hazardous-area or washdown cabinets, the magnetic sensor reference notes that reed-based cylinder switches are also available in IP67/IP69K sealed bodies, with the magnet mounted on the piston and the switch clamped to the extrusion.
Selection Criteria and Decision Matrix
For a quick specification, compare the two against four criteria drawn from the research. Range: reed 5 to 10 mm on a paired magnet, inductive 0.8 to 15 mm on steel only [S1][S2]. Target selectivity: reed reads only magnets, inductive reads any conductive metal entering the rated zone [S1][S5]. Wiring: reed 2-wire passive contact, inductive 3-wire active solid-state with PNP/NPN selection [S3][S7]. Lifetime: reed limited by contact wear and glass fragility, inductive limited by electronics and surge protection [S2][S4].
Use that matrix as the gate. If the moving part is a hydraulic or pneumatic cylinder and you can bond a magnet to the piston, the magnetic reed switch family is the engineering default, with the inductive option reserved for cases where the target is the cylinder rod itself or an external metal flag. If the target is a steel part on a conveyor, machine tool slide, or robot arm link where no magnet can be attached, the inductive proximity switch is the only correct answer, and the proximity probe form factor applies when sub-millimetre eddy-current resolution is required.
Use Cases and Application Boundaries
Cylinder end-of-stroke position is the textbook reed application, especially on pneumatic clamps, packaging indexing tables, and agricultural hydraulic rams, where the magnet rides on the piston and the switch reads through the aluminum or stainless barrel [S2][S5]. Inductive switches dominate discrete part detection on metal targets, including gear-tooth speed sensing, valve-position feedback, and tool-presence detection on CNC spindles, where the metal target already exists and you do not want to add a magnet [S6][S7].
Where the two compete head-to-head, the deciding factor is usually target access. If the target is hidden behind a non-magnetic wall, reed wins. If the target is a conductive metal that you can see and reach, inductive wins on lifetime and switching speed. Where the application needs a non-contact read but the engineer wants the simplest 2-wire wiring possible, reed still has a place even in modern PLC cabinets, particularly on retrofits where the existing 24 V DC input card accepts a contact closure directly [S3][S4]. For panel builders and skid integrators looking at the related field of magnetic material selection, the same bias-magnet physics applies when sizing an actuator magnet for a long-throw reed sensor.
Standards, Sourcing, and Reliability Notes
Both device families are normally specified to IEC 60947-5-2 for proximity switches with defined operating distances, switching frequency, and EMC behaviour, while reed contact ratings are also cross-referenced to IEC 60947-5-1 for low-voltage switchgear contactors and to UL 508 on the North American side [S2][S4]. IP67 is the typical washdown rating for both inductive and magnetic cylinder switches, and IP69K is commonly available on the magnetic cylinder form factor for food and beverage skids [S2].
Sourcing and second-source risk diverge sharply. Inductive M12/M18/M30 tubular bodies are a commodity: at least five major brands offer form-fit-function equivalents in 2 mm, 4 mm, 5 mm, 8 mm, and 10 mm sensing distances, and a NAMUR IEC 60947-5-6 output is widely available for intrinsic-safety panels. Reed cylinder switches are narrower, dominated by a smaller set of cylinder-sensor specialists, and the magnet must be ordered as a matched pair or the rated range will not be met. Track the next revision of IEC 60947-5-2 and the harmonised UL 60947-5-2 listing for any tightening of the test target used to define the rated operating distance, since that single value drives both the stock SKU and the mounting clearance on the machine.
This topic is covered further in Warm Box vs Hot Box: Curing Temperature and Catalyst Logic.