Sensor Basics and Selection

What a sensor is

A sensor responds to a physical quantity (position, temperature, pressure, light, force) and produces a signal that a controller can use. Industrial work distinguishes a few words that are often mixed up:

Term	Meaning	Example
Sensing element	The part that physically responds to the quantity	Thermocouple junction, strain gauge, photodiode
Sensor	The element plus its housing and basic output	An inductive proximity switch
Transducer	Converts one form of energy into another, often with a defined output	A pressure transducer with a 0 to 10 V output
Transmitter	A sensor with signal conditioning and a standard output for transmission over a distance	A 4 to 20 mA temperature transmitter
Switch	A sensor with an on/off (discrete) output at a set point	A pressure switch, a limit switch

Sensors can be discrete (on or off: present or absent, above or below a limit) or analog (a continuous value of the quantity); contact or non-contact; active (needing a supply) or passive (producing a signal on their own, such as a thermocouple). The pages of this guide cover each kind: Proximity Sensors, Photoelectric and Vision Sensors, Position, Displacement and Speed Sensors, Temperature Sensors, Pressure, Level and Flow Sensors and Force, Load Cells and Vibration; the wiring and installation come in Sensor Outputs and Wiring and Installing and Troubleshooting Sensors.

The signal chain

A measurement is only as good as the weakest part of this chain. The element can be wrong (a poorly mounted thermocouple), the conditioning can add noise or drift, the cable can pick up interference, and the input card can have too little resolution. It pays to think of the whole chain, not only the sensor’s data sheet.

Reading a specification

Term	Meaning
Range and span	The minimum to maximum value measured. The span is the difference between them (full scale = FS)
Accuracy	How close the reading is to the true value. Given as a percentage of full scale (% FS) or of reading (% rdg), and usually includes linearity, hysteresis and repeatability
Resolution	The smallest change that can be detected or displayed
Repeatability	How closely the same value gives the same reading under the same conditions: often far better than the accuracy
Linearity	How far the output departs from a straight line
Hysteresis	The difference in reading depending on the direction of approach (rising or falling input)
Sensitivity	Output change per unit input, for example mV per V per kg for a load cell
Response time	How long the output takes to follow a change (for example the time to 63 or 90 percent)
Drift and temperature coefficient	Slow change with time or temperature, often quoted as % FS per year or per °C
Operating conditions	Temperature, pressure, vibration, shock, ingress protection (IP), EMC and approvals

Accuracy, resolution and error budgets

Response time and sampling

Most sensors respond like a first-order system: after a step the output reaches 63 percent of the change in one time constant τ, 95 percent in 3τ and 99 percent in 5τ (see the figure). Thermal sensors in a protective sheath are slow; electronic sensors can be very fast. The controller must also sample fast enough: the sampling rate should be at least twice the highest frequency of interest (the Nyquist limit), and in practice five to ten times.

Standard output signals

Signal	Range	Pros	Cons
On/off (discrete)	PNP or NPN, 10 to 30 V; relay contact	Simple, cheap, robust	Only a threshold
4 to 20 mA	Current loop	Immune to cable resistance and noise, broken-wire detection (below 4 mA), power over the loop	Needs a loop supply and a receiver resistor
0 to 10 V, 1 to 5 V	Voltage	Simple to read	Voltage drop and noise on long cables, no broken-wire detection at 0 V
mV (thermocouple, bridge)	Millivolts	Direct from the element	Tiny: needs a conditioner and short, shielded wiring
Resistance (RTD, thermistor, potentiometer)	Ohms	Simple element	Lead resistance and self-heating
Pulse / frequency	Square wave, 0.1 Hz to MHz	Digital, noise-tolerant, easy to count	Needs a counter input
Serial / fieldbus (IO-Link, Modbus, HART, CANopen)	Digital data	Rich data, configuration and diagnostics over one cable	Needs the right master and set-up (see Industrial Communication)

A 4 to 20 mA signal of 12.5 mA is (12.5 − 4) ÷ 16 = 53.1 percent of range; 9.6 mA corresponds to 35 percent. The Analog Scaling Tool does the conversion for any range.

Choosing a sensor

• What do you need to know? Presence, a position, a value, a rate? Discrete or analog?
• Range and accuracy of the quantity, with the error budget above. Do not over-specify.
• The target and the medium: material, size, color, temperature, corrosiveness, pressure, cleanliness.
• The environment: temperature, vibration, washdown (IP67, IP69K), hazardous area (ATEX), EMC, space and mounting.
• Speed: response time and the switching or sampling frequency needed.
• The output and the controller: a signal the input module accepts; wiring length; power supply.
• Life and cost: contact sensors wear; non-contact ones do not. Count spares, calibration and the cost of downtime, not only the price.
• Safety: a sensor used for a personal-safety function must be a certified safety device (see Safety Sensors and Devices).

To measure…	Common sensors	Page
Presence of a metal part	Inductive proximity sensor	Proximity Sensors
Presence of any object, at a distance	Photoelectric, capacitive, ultrasonic	Photoelectric and Vision Sensors
Position or length	Potentiometer, LVDT, magnetostrictive, laser, linear encoder	Position, Displacement and Speed Sensors
Angle or speed	Encoder, resolver, Hall sensor, tachometer	Encoders, Resolvers and Feedback
Temperature	Thermocouple, RTD, thermistor, infrared	Temperature Sensors
Pressure, level, flow	Pressure transmitter, ultrasonic or radar, magnetic or vortex flowmeter	Pressure, Level and Flow Sensors
Force, weight, vibration	Load cell, strain gauge, accelerometer	Force, Load Cells and Vibration
People near a hazard	Light curtain, safety scanner, interlock	Safety Sensors and Devices

Calibration and drift

• Calibration compares the sensor with a reference of known accuracy, ideally ten times better, traceable to a national standard, and adjusts zero (offset) and span (gain).
• Zero and span are the two basic adjustments; non-linearity cannot be corrected by them, only by a multi-point calibration or a lookup table.
• Intervals: from monthly (critical process) to several years (stable discrete sensors). Record the as-found values so the interval can be tuned.
• In-situ checks: compare redundant sensors, or check against a known point (an ice bath at 0 °C, an unloaded scale, vented pressure).
• Drift is a fact of life: temperature, mechanical stress, aging and contamination all move the zero; choose sensors with good long-term stability for important measurements.

Related

• Sensors and Field Devices: the overview of this guide.
• Sensor Outputs and Wiring and Installing and Troubleshooting Sensors: getting the signal to the controller.
• I/O Wiring and Signals and the Analog Scaling Tool.
• Sensors and Signal Conditioning: the electronics for thermistors, bridges and 4-20 mA loops.
• Encoders, Resolvers and Feedback for motion feedback devices.

FAQ

What is the difference between accuracy and resolution?

Resolution is the smallest change a device can show. Accuracy is how close its reading is to the true value. A display with many digits can still be inaccurate.

Percent of full scale or percent of reading?

A percent-of-full-scale accuracy is a fixed error across the range, so relative error grows at low readings. A percent-of-reading accuracy scales with the value. Compare sensors on the same basis.

Do I need a 4-20 mA signal or is 0-10 V enough?

For short runs in a quiet cabinet, 0 to 10 V is fine. For longer cables, noise, or when a broken wire must be detected, use 4 to 20 mA.

How often must sensors be calibrated?

It depends on the stability and the risk: critical process and quality measurements often yearly or more often; discrete sensors rarely need calibrating, only checking and replacing when faulty.

References

• Bentley, J. P., Principles of Measurement Systems, Pearson.
• Fraden, J., Handbook of Modern Sensors, Springer.
• ISO/IEC Guide 98-3 (GUM): Guide to the expression of uncertainty in measurement.

Last reviewed: October 2026. Values are worked examples; use the manufacturer’s data for the real sensor.