Sensor Kompendium: Industrial Sensors, Signals & Measurement
Industrial measurement begins with a measurand such as pressure, temperature, position, flow or acceleration. A sensor converts that physical quantity into an electrical or digital response, which is conditioned, transmitted and interpreted by a PLC, controller, DAQ or measuring instrument.
Correct sensor selection depends on the complete measurement chain: sensing principle, range, environmental limits, mechanical installation, output type, wiring, receiving input, calibration and the uncertainty required from the final result.
A sensor is one part of the complete measurement path
A useful measurement system can be separated into five functions. The measurand is the physical quantity being determined. The sensor responds to that quantity. Signal conditioning provides functions such as excitation, amplification, filtering, linearisation, isolation or compensation. The output carries the information, and the receiving system converts that information into engineering units or control logic.
Choose the sensing principle from the measurand and the real operating conditions
Sensors are most usefully grouped first by the quantity they measure, then by sensing principle and installation method. Different technologies can measure the same quantity but have very different range limits, dynamics, environmental sensitivities and interface requirements.
| Measured quantity | Common principles | Key engineering checks | Library reference |
|---|---|---|---|
| Pressure | Piezoresistive, strain-gauge, capacitive, resonant and piezoelectric devices; gauge, absolute and differential arrangements. | Pressure reference, range, overload, media compatibility, process connection, temperature and dynamic response. | Pressure Sensors |
| Temperature | RTDs such as Pt100/Pt1000, thermocouples, thermistors and semiconductor temperature sensors. | Range, accuracy class, wiring resistance, cold-junction compensation, immersion, response time and self-heating. | Temperature Sensors |
| Position & distance | Encoders, LVDTs, magnetostrictive, laser, ultrasonic and draw-wire displacement measurement. | Stroke or angle, resolution, repeatability, target surface, alignment, speed, mounting and output type. | Position & Distance Sensors |
| Presence & proximity | Inductive, capacitive, photoelectric, ultrasonic and magnetic switching sensors. | Target material, sensing distance, hysteresis, switching frequency, contamination, mounting and PNP/NPN compatibility. | Proximity Sensors |
| Flow & level | Electromagnetic, Coriolis, vortex and ultrasonic flow; radar, guided-wave radar, hydrostatic and point-level detection. | Fluid properties, pipe or vessel geometry, straight runs, density, conductivity, vapour, foam, pressure and temperature. | Flow & Level Sensors |
| Vibration & acceleration | Piezoelectric, IEPE, MEMS capacitive and velocity/displacement measurement depending on frequency range. | Frequency response, amplitude range, mounting stiffness, transverse sensitivity, cabling, power and signal conditioning. | Vibration & Acceleration Sensors |
MEMS and piezoelectric accelerometers serve different measurement ranges
MEMS accelerometers commonly use a micro-machined proof mass and capacitive or piezoresistive sensing. They can measure static acceleration, low-frequency motion and inclination relative to gravity when the device and signal chain support DC response. Piezoelectric accelerometers are widely used for dynamic vibration because their sensing element produces charge in response to mechanical stress; IEPE variants integrate electronics that operate from a constant-current supply.
MEMS acceleration
Useful where DC or very low frequency response matters, including tilt, inertial measurement, machinery motion and condition monitoring at low frequencies.
- Check offset and temperature drift.
- Check noise density and bandwidth together.
- Mounting and axis alignment affect the measured components of gravity and motion.
Piezoelectric / IEPE vibration
Well suited to dynamic acceleration and machine vibration over broad frequency ranges when a compatible conditioning or IEPE input is available.
- Not generally used for true static acceleration.
- Mounting stiffness affects high-frequency response.
- Cable, constant-current supply and input coupling must match the sensor.
For sensor construction, mounting, frequency response and interface details, use the Vibration & Acceleration Sensors reference.
Sensor outputs differ in how they carry information and how faults appear
The output circuit is part of the measurement system. A 4–20 mA loop represents an analogue value as current, 0–10 V represents it as voltage, PNP/NPN outputs provide discrete switching states, and pulse or frequency outputs encode events, rate or quantity in transitions that must be counted or timed.
| Output | Carries | Main interface checks | Reference |
|---|---|---|---|
| 4–20 mA | Analogue process value as loop current; 4 mA provides a live zero. | Loop supply, transmitter voltage requirement, load resistance, polarity, grounding and PLC analogue input type. | 4–20 mA Current Loop |
| 0–10 V | Analogue process value as voltage referenced to a common or differential input. | Input impedance, common reference, cable drop, noise, grounding and permissible common-mode voltage. | 0–10 V Sensor Output |
| PNP / NPN | Discrete ON/OFF state through sourcing or sinking transistor outputs. | PLC input polarity, supply voltage, load current, normally-open/closed logic and leakage current. | PNP vs NPN |
| Pulse / frequency | Count, speed, flow quantity, rate or position information in edges or repetition frequency. | High-speed counter capability, pulse width, input threshold, maximum frequency and electrical output type. | Pulse & Frequency Outputs |
2-, 3- and 4-wire sensors
Wire count alone does not define the signal. Identify which conductors supply power, carry the signal, provide a return or form a separate measurement pair.
POWER / SIGNAL / RETURN4–20 mA vs 0–10 V
Current and voltage interfaces differ in wiring, cable sensitivity, live-zero behaviour and receiving-input requirements.
ANALOGUE INTERFACE SELECTIONSignals overview
Choose the electrical interface only after the sensing requirement and controller input capabilities are both known.
OUTPUT / PLC / DAQField wiring must preserve both signal integrity and the intended circuit reference
Industrial sensor wiring is not interchangeable just because connectors or wire counts look similar. Confirm the supply, signal direction, common reference, shield termination, input type and permissible current or voltage for every device in the channel.
Check operating voltage at the sensor under load, including cable drop and loop resistance. A nominal 24 V supply does not guarantee the device receives 24 V.
0 V, protective earth, functional earth and cable shield can have different electrical purposes. Do not combine them without understanding the manufacturer and system requirements.
PLC and DAQ channels can be current, voltage, sourcing, sinking, differential, single-ended, high-speed or isolated. Match the sensor output to the actual channel configuration.
Shielding and cable routing reduce interference only when termination, grounding and separation from noisy power circuits are appropriate to the signal and installation.
Convert the electrical signal to the measurand with an explicit transfer relation
Linear transmitters are commonly scaled between a lower range value and an upper range value. The receiving system should preserve the raw input long enough to distinguish a sensor or wiring fault from a scaling or software error.
| Signal | Normalised fraction | Engineering value |
|---|---|---|
| 4–20 mA | (I − 4 mA) / 16 mA | LRV + fraction × (URV − LRV) |
| 0–10 V | V / 10 V | LRV + fraction × (URV − LRV) |
| Frequency | (f − fmin) / (fmax − fmin) | LRV + fraction × (URV − LRV) |
Accuracy, resolution and repeatability describe different properties
A measurement result cannot be judged from a single datasheet number. Measurement accuracy concerns closeness of agreement between a measured quantity value and a true quantity value of the measurand; repeatability concerns agreement between repeated results under repeatability conditions; resolution concerns the smallest distinguishable change; uncertainty quantifies the dispersion attributed to the reported result.
| Term | What it describes | Typical mistake |
|---|---|---|
| Accuracy | Closeness of agreement between a measured quantity value and a true quantity value of the measurand. | Treating accuracy as identical to precision, repeatability or resolution. |
| Repeatability | Measurement precision under repeatability conditions: the same procedure, operators, measuring system, operating conditions and location over a short period. | Assuming a repeatable offset is therefore accurate. |
| Resolution | Smallest distinguishable change in input or reported value. | Equating ADC bit depth with complete-system accuracy. |
| Sensitivity | Change in output divided by the corresponding change in input around the specified operating point or range. | Using sensitivity as a synonym for detection limit or accuracy. |
| Linearity | Deviation of the transfer characteristic from a specified reference line. | Comparing percentages without checking the reference line and denominator. |
| Hysteresis | Difference in output at the same input depending on direction or prior history. | Ignoring approach direction during calibration or test. |
| Drift | Change in metrological properties with time. | Confusing long-term drift with a specified temperature coefficient. |
| Response / bandwidth | How the sensor follows changing input in time or frequency. | Selecting from static accuracy while ignoring the dynamics of the event being measured. |
Accuracy vs resolution
Separate error limits, repeatability, quantisation and displayed digits before comparing sensors or data-acquisition channels.
Signal conditioning
Excitation, gain, filtering, isolation, linearisation and compensation can determine whether the sensor output remains useful at the receiver.
A calibrated sensor is not automatically a low-uncertainty measurement system
Calibration establishes, under specified conditions, relations between reference quantity values with their uncertainties and corresponding indications, and uses those relations to obtain a measurement result from an indication. Adjustment changes the measuring system. Verification provides objective evidence that specified requirements are fulfilled. Metrological traceability relates a measurement result to a reference through a documented unbroken chain of calibrations, each contributing to measurement uncertainty.
Measurement uncertainty
Build an uncertainty budget from relevant contributions such as reference uncertainty, repeatability, calibration fit, resolution, temperature, drift, installation and data processing.
Calibration & traceability
Record the reference, conditions, results, corrections and uncertainty. Keep calibration separate from adjustment and from simple functional checks.
Define the measurement task before choosing the technology or output
- Define the measurand. State exactly what physical quantity must be measured and where the measurement point is located.
- Set the real range. Include normal operation, startup, shutdown, overloads, fault conditions and any negative or bidirectional values.
- Set performance requirements. Separate allowable error, repeatability, resolution, response time, bandwidth, drift and required uncertainty.
- Check the environment. Temperature, pressure, humidity, chemicals, washdown, dust, shock, vibration and hazardous or safety requirements can eliminate otherwise suitable technologies.
- Check mechanical installation. Process connection, insertion depth, target geometry, orientation, stiffness, alignment and maintenance access can change the measurement.
- Choose the interface. Match supply, current/voltage/switching/pulse output, wire count, grounding, shielding and controller input.
- Plan calibration and verification. Decide how the installed channel will be checked, how often and against what reference before the system is commissioned.
Use the detailed references when the engineering decision depends on one topic
Sensor & Measurement Terminology
Measurand, range, span, accuracy, precision, uncertainty, repeatability, resolution, sensitivity, linearity, hysteresis, drift, response and calibration.
OPEN →SI Units & Unit Conversions
Base and derived SI units, prefixes, pressure and temperature references, engineering notation and practical conversion rules.
OPEN →Industrial Sensor Standards
Standards commonly encountered in industrial sensing, environmental protection, electrical interfaces and measurement practice.
OPEN →Sensor Selection Checklist
A structured check covering measurand, range, performance, environment, mounting, process connection, output, wiring and controller compatibility.
OPEN →