Sn, Sr, Su and Sa in Proximity Sensors Explained

What do Sn, Sr, Su and Sa mean?

Think of them as different kinds of evidence, not four selectable range settings. The definitions below concern conventional inductive proximity sensing with the specified reference target.

Definitions and conventional relationships: Pepperl+Fuchs operating-distance guide.

Read the definition, not just the word “range”

A supplier may write “sensing range,” “operating distance” or “set distance” instead of listing all four symbols. These labels do not always map one-to-one across catalogs. Find the target, reference conditions, tolerance and intended use of each field. An empty Sa field is a reason to request clarification, not permission to install at Sn.

IEC 60947-5-2:2019 covers several proximity-switch technologies. That broad scope does not make the inductive distance relationships below a universal formula for capacitive, ultrasonic, magnetic or photoelectric sensors.

Why does 0.81 × Sn appear in the distance calculation?

In the conventional inductive relationship, Sr can be 10% below Sn, and Su can be a further 10% below that unit's Sr. The lower limit is therefore 0.9 × 0.9 = 0.81, not “subtract 10% twice from Sn.”

The combined Su interval is an envelope across allowed unit and condition variation. It does not mean one sensor must move through that whole interval, or that its upper end is assured detection. Balluff's inductive-sensing reference, “Switching distances”, shows this distinction.

Worked example: Sn = 8 mm

Illustrative calculation using the relationships above:

• Sr: 0.9 × 8 to 1.1 × 8 = 7.20–8.80 mm.
• Combined Su envelope: 0.81 × 8 to 1.21 × 8 = 6.48–9.68 mm.
• Conventional assured-zone upper limit: 0.81 × 8 = 6.48 mm.

If an individual unit instead has a measured Sr of 8.40 mm, its corresponding 0.9 Sr–1.1 Sr interval is 7.56–9.24 mm. Do not replace that unit's Sr with Sn when interpreting its test results.

A real datasheet separates the rating from the operating distance

The published ifm IG6213 / IGC2008SFRKG/10M datasheet lists 8 mm sensing range, Sr of 8 mm ±10%, and operating distance of 0–6.48 mm. It also specifies non-flush mounting, 10–36 V DC and −25 to +80 °C. The cited document is revision IG6213-00, dated 9 December 2013; it is a document-reading example, not a current availability recommendation or an xsz sensor rating.

Other catalogs may recommend a different set-distance range. OMRON's terminology guide describes set distance as approximately 70–80% of rated distance. Do not overwrite a manufacturer's installation recommendation with a generic 81% calculation.

Does the standard-target distance apply to your part?

Only if the relevant conditions match. The practical question is not simply whether the part is metal; it is whether the quoted sensing performance covers its material, dimensions, approach and surroundings.

The standard target is not an arbitrary piece of steel

The conventional reference is a smooth, square, 1 mm-thick steel target, FE 360 / St37. Its side is the larger of the defined sensing-face inner-circle diameter and 3 × Sn. For a defined diameter of 18 mm and Sn = 8 mm, that gives 24 × 24 × 1 mm. Confirm the actual reference dimensions in the model's data.

A screw head, curved shaft or narrow tab is a different target. The manufacturer's standard-target explanation shows why target dimensions belong beside a quoted range.

Material factors do not correct every target difference

A material correction factor relates a specified metal's response to the reference metal. It is not automatically a guaranteed Sa multiplier for every alloy, thickness or shape.

Factor 1 designs can reduce material-dependent range differences, but they do not remove target-size limits. For example, ifm's technology overview explicitly identifies small/thin-target limitations for its Kplus technology. Do not assume that “thinner always means shorter range” across all designs and metals.

If the target differs, ask for model-specific curves or a documented test with your smallest production part. Use the target-size guide for geometry questions, or the aluminum-detection guide for nonferrous applications.

Mounting metal and approach direction are separate checks

Flush and non-flush models need different metal clearances. Follow the exact installation drawing for the bracket, sensing face and adjacent sensors; a shorter target gap does not fix an incorrectly embedded non-flush device.

For axial approach, the target moves toward the face. For lateral approach, it moves across the face at a given gap, and the switching position also depends on overlap and the response curve. Balluff's “Axial and radial damping” explanation illustrates the difference. Do not treat Sa as a lateral position tolerance or copy universal sensor-spacing multipliers from another model.

How do you turn the distance rating into a mounting gap?

Define two mechanical limits: the largest gap where detection is required, and the smallest gap reached during movement. The first must remain within the supported detection zone; the second must preserve physical clearance. A single nominal-gap dimension answers neither question completely.

Example: a nominal 5.0 mm gap is not always 5.0 mm

Illustrative design calculation, not measured machine data: assume the target and mounting conditions match a documented 6.48 mm operating-distance upper limit. Three independent mechanical allowances can open or close the gap. Their maximum magnitudes are conservatively added here; no statistical distribution is assumed.

Largest gap = 5.00 + 0.30 + 0.20 + 0.30 = 5.80 mm
Smallest gap = 5.00 − 0.30 − 0.20 − 0.30 = 4.20 mm

The largest gap is 0.68 mm below the assumed 6.48 mm boundary. That is arithmetic headroom, not a standard reserve requirement or an automatic pass. The smallest gap still needs a collision-clearance check, and any unaccounted variation or measurement uncertainty needs its own treatment.

If the actual target is a small aluminum tab, hold the selection: the assumed standard-target boundary has not established its performance. Obtain application-specific evidence before using this comparison. Likewise, do not subtract temperature drift twice if the documented operating-distance limit already includes the relevant temperature range.

A longer-range replacement is not automatically better. It can change the required mounting space and what unintended metal is detected. Match the full application, not just Sn and housing size; use the sensor replacement checklist when changing models.

What should you verify before releasing the installation?

Use an application test to check the real geometry and controls. A finite sample test records what happened under its conditions; it does not redefine Sa, certify every future unit or replace missing manufacturer limits. Prevent unintended motion and follow machine isolation procedures before wiring or mechanical adjustments.

Separate actuation, release and controller response

Hysteresis is the separation between operation and reset positions. Repeatability concerns the spread when the same event is repeated. Neither is the nominal range. For an NO output, target actuation normally turns the output on; for an NC output, it normally turns it off. Record the actual logic rather than calling every detection event “ON.” OMRON's term definitions distinguish these behaviors.

1. Define the required states and geometry.

   Record the exact model, bracket, active-face reference, target path and required detection/release positions. Include the minimum collision clearance and the maximum gap where detection must hold.

2. Use identified production extremes.

   Include the smallest feature, allowed materials, thicknesses and poses. Keep the final surrounding metal and adjacent-sensor conditions in the test.

3. Record both approach and retreat.

   In a controlled setup, measure repeated actuation and release points without contacting the face. For lateral motion, record positions along the actual travel path, not just the face-to-target gap.

4. Challenge the specified operating conditions.

   Cover the application's temperature, supply, vibration and maintenance conditions using an appropriate test plan. Log the test method, instrument uncertainty, settings, misses and unstable transitions.

5. Confirm the event reaches the controller at speed.

   Compare the sensor output with the accepted PLC event. Check response time, input filtering and scan or communication timing, then verify the target-present and target-absent sequence at production speed.

A target can be close enough but pass too quickly for the complete signal chain. Conversely, “target absent” does not necessarily mean the sensor will reset immediately outside Sa. Check the documented response and release behavior separately. For position-critical work, continue with the repeatability measurement guide.

If the result is unstable, change one variable at a time

• Real part fails; reference target works: investigate target geometry and material before changing the nominal-range requirement.
• Behavior changes after tightening the bracket: compare face position, mounting-metal clearance and mechanical movement.
• Behavior changes only with a neighboring sensor active: investigate mutual interference while keeping the target fixed.
• Low-speed detection works; PLC misses at speed: capture the output pulse and input event before blaming sensing distance.

These observations narrow the next test; they are not diagnoses by themselves.

“Assured” does not mean personnel-safety-rated. Sa alone establishes no PL, SIL or machine stopping distance. A standard process sensor must not be treated as a protective device merely because its datasheet uses the word “assured.”

What evidence should accompany a sensor quotation?

Instead of asking only for “an 8 mm proximity sensor,” connect the required machine gap to the exact product and its documented conditions. A short request can cover the essentials:

• Product identity: complete model and suffix, datasheet revision, supplied cable/connector and output configuration.
• Distance basis: rated distance plus the documented assured, operating or recommended set distance; standard target and applicable voltage/temperature conditions.
• Actual application: material grade, smallest target dimensions and thickness, approach direction, and minimum/maximum gap.
• Installation and timing: flush/non-flush drawing, surrounding metal, adjacent sensors, target speed and required PLC event.
• Unresolved differences: written model-specific guidance or application-test evidence wherever the real target or setup differs from the rating conditions.

Keep the decision in order: compare with Sn, read the documented operating limit, check the target and mounting, then verify the complete installed detection task. A room-temperature bench measurement cannot substitute for the rest of that chain.