Industrial Sensor Glossary: 40 Terms Explained for Engineers and Buyers
Industrial sensor terms describe different parts of a sensing task: the quantity measured, measurement performance, output connection and operating limits. This glossary explains 40 common terms, the distinctions most often confused, and what each means when you read a datasheet or compare sensors.
A sensor specification answers one question at a time: what it detects, how well it measures, how its output connects, or where it can operate. Problems start when one answer is used in place of another. Fine resolution does not prove accuracy; a PNP output does not specify NO/NC behavior; an IP rating does not establish machine safety.
Showing all 40 terms.
Which terms describe what a sensor measures—and how well?
Start with the quantity and measuring interval, then separate error, variation and detectable change. These describe different aspects of performance.
1. Measurand
The quantity you intend to measure, such as pressure at a specified port, the temperature of a surface, or distance to a particular target. Defining it also means defining the relevant conditions: surface temperature is not automatically the temperature inside a product.
In an application: “Detect a part” is a task; “record the position where the output switches during approach” defines a measurable event.
2. Sensing principle
The physical effect used to detect a target or convert a quantity into a signal. Inductive sensing responds to conductive targets; photoelectric sensing evaluates light; capacitive sensing responds to changes in capacitance.
Why it matters: A suitable output interface cannot compensate for a sensing principle that does not work with the target. Material, transparency, reflectivity, mounting and contamination belong in the selection decision.
3. Measurement range
The interval over which the sensor measures with its stated performance under stated conditions. A displacement sensor specified for 50–150 mm is not necessarily able to measure a target at 20 mm.
Do not confuse it with: an overload, survival or absolute-maximum limit. Surviving an input does not mean measuring it correctly.
4. Span
The difference between the upper and lower endpoints of the relevant interval. Both 0–100 mm and 50–150 mm have a 100 mm span, but they cover different positions.
Read the percentage basis: An illustrative error limit of ±0.5% of a 100 mm span means ±0.5 mm. A limit of ±0.5% of reading at 20 mm means ±0.1 mm. “FS,” “full scale” and “span” must be interpreted using the manufacturer's definition.
5. Accuracy
How close a measurement is to the true quantity value. In formal metrology, accuracy is qualitative, not itself a numerical quantity. Datasheets commonly use “accuracy ±…” as shorthand for a specified error limit or performance envelope.
Read the footnote: Identify the reference, temperature range, error components and percentage basis. Measurement error is the difference from a reference value; measurement uncertainty characterizes the dispersion of values attributed to the measurand. They are not interchangeable numbers.
6. Precision
How closely repeated measurements agree under specified conditions. It describes scatter, not whether the group of results is centered on the right value.
Ask what stayed the same: Repeated readings from one setup and measurements taken by different operators or instruments examine different conditions. A precision claim needs those conditions and a defined statistic.
7. Repeatability
Precision under repeatability conditions: the same procedure, measuring system and other specified conditions over a short period. For an analog sensor, compare repeated measured values. For a proximity switch, compare repeated switching positions or times—not just a list of identical ON bits.
Compare like with like: Observed range, standard deviation and a percentage of rated sensing distance are different ways to report variation. Do not treat an undefined ± value as a standard deviation.
8. Resolution
The smallest change in the measured quantity that causes a perceptible change in the indication. Display increments, transmitted data steps and usable sensor resolution can differ because of noise and processing.
A common trap: A display reading to 0.01 mm does not establish ±0.01 mm accuracy. In safety-light-curtain specifications, “resolution” refers to object-detection capability, not a measurement display increment.
Small steps. Tight grouping. Still offset.
The mean is 10.40 mm, giving a +0.40 mm difference from the reference. The three readings span only 0.02 mm. Their tight grouping and 0.01 mm increments do not remove that offset.
9. Sensitivity
For a measuring system, the change in indication divided by the corresponding change in input. A linear 0–10 V output mapped over a 100 mm span has a nominal slope of 0.1 V/mm.
Watch the context: A photoelectric sensor's “sensitivity” knob may instead adjust gain or a switching threshold. Turning it up is not the same as improving measurement accuracy.
10. Linearity
How closely the input–output characteristic follows a specified straight line. A nonlinearity specification gives the departure from that line.
Before comparing: Identify whether the reference is an endpoint line, best-fit line or another defined method. A small nonlinearity figure does not automatically include offset, temperature effects or every other source of error.
11. Hysteresis
Dependence on the previous input or direction of approach. In a proximity switch, it is the separation between the operate and release points.
Illustrative example: A target causes switching ON at a 5 mm gap while approaching, then OFF at 6 mm while retreating. The differential travel is 1 mm. This separation can limit chatter near a threshold; it is not random repeatability error.
12. Dead band
An input interval through which a quantity can change in both directions without a detectable change in indication. In control settings, a dead band may deliberately define a region where no new corrective action occurs.
Keep two ideas separate: A dead band in signal or control behavior is not necessarily the close-range blind zone of an ultrasonic or optical sensor. Nor does it automatically mean the same thing as hysteresis.
13. Response time
The time between a defined input change and a defined output response. A switch may specify separate ON and OFF delays; a measuring sensor may define response using a percentage of a step change or a tolerance band.
Read the endpoints: Sensor response time is not the entire delay until a PLC program or actuator reacts. Input filters, communications and controller processing add their own delays.
14. Settling time
Time after an input change for the output to enter and remain within a stated band around its final value.
Why it matters: A value may start moving quickly but take longer to settle. Compare the allowed band and filter settings before deciding when a measurement can be sampled. “Fast response” without a defined endpoint is incomplete.
15. Drift
A change in indication over time caused by changes in the measuring instrument's properties. In the formal definition, this is not a change caused by the measurand or a recognized influence quantity changing.
Separate causes: An offset that follows ambient temperature should first be evaluated as a temperature effect, not automatically labeled long-term drift. A stability claim needs a time interval and test conditions.
16. Temperature coefficient
How a specified characteristic changes with temperature, often expressed per °C. It may describe zero shift, span change, resistance change or another property.
Read the units: “% span/°C” and “% reading/°C” use different bases. Also separate the sensor's allowable ambient temperature from its compensated range and, for temperature sensors, the temperature being measured.
17. Calibration
Establishing, under specified conditions, the relationship between reference values and instrument indications, including their uncertainties, and using that relationship to obtain measurement results.
Calibration is not adjustment: It can reveal an error without changing the device. A useful record identifies the instrument, test points, reference, uncertainty and any corrections. If adjustment was performed, distinguish results before and after it.
18. Metrological traceability
A measurement result's documented connection to a reference through an unbroken calibration chain, with each stage contributing uncertainty.
What it does not establish: Traceability alone does not prove the uncertainty is small enough for your tolerance. It is also different from tracking a sensor's serial number, shipment or production batch.
Which terms determine how the sensor connects to a controller?
Separate the information carried by the output from its electrical implementation. A compatible connector shape alone establishes neither.
19. Analog output
An output whose voltage or current represents a measured quantity across a range, rather than only indicating a detection state. Common industrial examples include 0–10 V and 4–20 mA.
Specify the mapping: The controller needs the electrical range, engineering-unit endpoints and signal direction. An analog output label alone does not state measurement accuracy or resolution.
20. 4–20 mA current loop
A signal that normally maps 4 mA to the configured lower endpoint and 20 mA to the upper endpoint. The 4 mA “live zero” is not necessarily zero pressure, distance or temperature.
Check the voltage budget: The transmitter, receiver and cable all require voltage. Current regulation works only while sufficient supply headroom remains. In a powered system, near-zero current can indicate a fault, but does not identify its cause by itself.
21. 0–10 V output
A voltage signal representing a configured measurement range. Its interpretation depends on the voltage measured at the receiver relative to the specified signal reference.
Check the receiving input: Its impedance, reference arrangement and cable installation affect the interface. A 0–10 V input is not interchangeable with a 4–20 mA input merely because the displayed engineering units are the same.
22. Discrete output
An output representing states such as detected/not detected or above/below threshold. It may use a transistor, relay or another electrical implementation.
Distinguish two uses of “digital”: A discrete ON/OFF signal is not the same as a digital communication channel carrying a numerical measurement and diagnostics.
23. PNP output
A commonly used DC sourcing output: when active, it supplies current toward the connected load. The receiving circuit must provide the return path.
Use the circuit diagram: Confirm the PLC input's common connection, thresholds and load requirements. PNP specifies the output arrangement; it does not tell you whether detection turns the output ON or OFF.
24. NPN output
A commonly used DC sinking output: when active, it provides a current path toward the negative supply/common. The receiving circuit must supply the current path from the positive side.
Replacement consequence: Swapping NPN for PNP can require a different input arrangement even if supply voltage, housing and sensing distance match. Match the sensor and input diagrams, not terminology alone.
25. NO/NC output
Normally open and normally closed describe switching behavior relative to a defined normal state. For a typical proximity switch, NO is active on target detection; NC is active without the target. The product's state table is authoritative.
Independent choices: PNP can be NO or NC, and so can NPN. Optical light-operate/dark-operate terminology needs the sensing mode's light-state interpretation. A transistor “NC” output is not proof of a fail-safe circuit.
26. IO-Link
A standardized point-to-point communication technology between a device and an IO-Link master. It can carry process values, parameter settings and diagnostic information.
Look beyond the connector: The device's IODD describes identification, data and parameters. Confirm master-port compatibility, power requirements, process-data interpretation and supported cycle time; “IO-Link” does not make unlike sensors functionally interchangeable.
27. Switching frequency
The repeated switching rate supported under a specified test arrangement. Target dimensions, spacing and sensing conditions are part of that rating.
Not a universal reciprocal: Do not obtain response time by simply calculating 1/frequency. A production counter also needs enough ON time and OFF time for the sensor and controller to recognize every event.
28. Load current
The current through the load connected to an output. The sensor may specify a maximum switching current and, for some interfaces, a minimum operating load.
Separate the figures: No-load supply consumption is not output load capacity. A short-circuit-protection claim also does not authorize exceeding the normal load rating or ignoring the specified load type.
29. Leakage current and residual voltage
Leakage current is current that can flow through an output in its OFF state. Residual voltage is voltage remaining across the conducting output or sensor in its ON state.
Two separate checks: OFF-state current must not falsely activate the receiving input; ON-state voltage drop must still leave enough voltage for it to operate. These checks are especially important for a two-wire sensor connected in series with its load.
30. Pinout
The assignment of functions to connector pins, terminals or cable conductors: supply, return, outputs, teach input or communications.
Read the drawing's viewpoint: Mating-face and wiring-side views can appear mirrored. The same M8/M12 connector size does not guarantee the same assignments. Isolate power before wiring changes and follow the exact device and controller documentation.
Which terms change the sensing distance or installation?
A catalog number describes a defined setup. These terms explain why a different target, mounting arrangement or process medium can change the result.
31. Switching distance
The target distance at which a switching event occurs under defined conditions. The target, reference surface and direction of movement matter.
Read the qualifier: Nominal/rated, effective, usable and assured operating distances are not interchangeable labels. The rated distance is not automatically the best installation gap for every target or operating condition.
32. Flush/non-flush mounting
For proximity sensors, these terms describe the permitted relationship between the sensing face and surrounding mounting material. Shielded inductive sensors generally permit flush metal mounting; unshielded designs require clearance around the active region.
Use the dimensional drawing: “Flush” does not mean zero clearance to an opposing target or neighboring sensor. Follow the specific recess, projection and spacing requirements.
33. Correction factor
In inductive-sensor selection, a material correction or reduction factor relates the operating distance for another target material to the reference-target distance.
Not a universal material constant: Values depend on the sensor design as well as the target. A factor from one model is not automatically valid for another. Target size and thickness still matter; a material factor alone is not an installed-distance guarantee.
34. Background suppression
A photoelectric sensing function that discriminates by distance so that objects beyond a specified or set cutoff are rejected. Fixed-field and adjustable-field designs are common.
What to confirm: The target must remain inside the usable field and sufficiently separated from the background. Performance still depends on the target surface, angle and the model's transition behavior near the cutoff. BGS does not promise to ignore every shiny background.
35. Turndown ratio
A ratio describing how far an instrument's usable or configurable range can be reduced. In process transmitters, it commonly relates the nominal span to the configured span; in flow measurement, “rangeability” may describe a maximum-to-minimum usable flow ratio.
Read the definition before dividing: It is not simply the displayed upper endpoint divided by the lower endpoint. A smaller configured span can change the relative error specification, so check performance at the intended setting.
36. Thermal time constant
For a first-order thermal response, the time to complete about 63.2% of a temperature step. It is not the time to finish the entire change.
Compare the test medium: Air, moving liquid, immersion depth, sheath construction and mounting affect heat transfer. A time measured in one medium cannot simply be assumed for another, and electrical output speed does not remove thermal lag.
Which ratings describe protection—and where do their limits stop?
Ingress protection, enclosure type, disturbance immunity and safety outputs address different hazards. None is a general-purpose certificate of application suitability.
37. IP code
The IEC enclosure-protection classification. Its first characteristic numeral addresses access to hazardous parts and solid-object ingress; its second addresses water ingress under defined tests.
Do not read “waterproof” into it: Immersion and water-jet conditions differ. Also confirm connector sealing, installation conditions, temperature and chemical exposure. An IP code by itself is not a chemical-resistance or explosion-protection rating.
38. NEMA enclosure type
An enclosure classification with requirements defined for the stated Type, including environmental conditions that are not identical to those represented by an IP code.
No simple two-way conversion: An IP rating alone does not establish a NEMA Type. Request the required Type evidence directly, including applicable corrosion and installation conditions, rather than treating similar-looking comparison tables as certification.
39. EMC immunity
The ability to function without unacceptable degradation in the presence of electromagnetic disturbances. Immunity concerns what the sensor withstands; emissions concern disturbances it produces.
Read the test conditions: Disturbance type, level, ports, cable arrangement and performance criterion matter. A compliant component does not guarantee an interference-free machine after the installation, grounding or wiring arrangement changes.
40. OSSD
Output signal switching device: the part of electro-sensitive protective equipment that interfaces with the machine control system and goes to its OFF state when the sensing device is actuated in normal operation.
Not an ordinary process-output substitute: Follow the protective device and safety-controller manuals for output compatibility and fault behavior. An OSSD label alone does not establish the required Performance Level (PL) or Safety Integrity Level (SIL) of the complete safety function.
How do you turn a glossary definition into a better specification?
Write the parameter together with its meaning, conditions and required outcome. That prevents a supplier or colleague from answering a different question with a technically correct number.
| Instead of | Specify or ask |
|---|---|
| “High accuracy” | The allowable error at the required measurement points, its reference basis and the operating temperature range—not only the display digits. |
| “Fast sensor” | The event to detect, shortest ON/OFF durations, sensor response settings and the controller's input-capture requirements. |
| “Same output” | The electrical interface, PNP/NPN arrangement where applicable, state logic, pinout and receiving-input requirements. |
| “Waterproof” | The actual water exposure, connector condition, cleaning chemicals and relevant enclosure rating with its stated conditions. |
The useful habit: Read the definition, identify the specification's reference conditions, then compare those conditions with the machine. Use the glossary to resolve the term; use the device manual and an application assessment to decide whether a particular sensor fits.
Sources and method references
Definitions are paraphrased for industrial-sensor readers. Manufacturer terminology can differ from formal metrology; product-specific numerical claims must retain the conditions in their own documentation. Examples in this guide are illustrative, not xsz sensor test results.
- JCGM, International Vocabulary of Metrology: accuracy, precision and resolution—the distinct concepts behind terms 5–8 and the numerical example.
- JCGM: sensitivity, dead band and instrumental drift—formal meanings used in terms 9, 12 and 15.
- JCGM: calibration and metrological traceability—including uncertainty and the distinction from adjustment.
- NI: Sensor Terminology—range, linearity, hysteresis and response terminology in sensor literature.
- OMRON: Proximity Sensors—Explanation of Terms—switching events, frequency, mounting and output characteristics. Its product-family examples are not universal limits.
- Pepperl+Fuchs: Operating Distance—reference targets and model-dependent material reduction factors. Banner Engineering: Photoelectric Sensing Modes—background-suppression principles.
- Banner Engineering: PNP vs NPN—sourcing and sinking output arrangements.
- Analog Devices AN-1312—current-loop operation and the available voltage budget.
- IO-Link Community: IODD—device descriptions, process data, parameters and point-to-point communication.
- WIKA: Process Transmitters—configurable measurement range and span adjustment. OMEGA AD590 User's Guide, printed page 9—thermal time constant and the effect of the medium.
- IEC: IP ratings and NEMA: Enclosure FAQ—protection scope and why the classifications are not interchangeable.
- IEC Electropedia: OSSD, IEV 428-05-08—the protective-equipment output definition.