Industrial Sensor Selection FAQ

Common Industrial IoT sensor types include:

• Temperature: Thermocouples, RTDs, thermistors, infrared
• Pressure: Absolute, gauge, differential
• Vibration: Accelerometers, velocity sensors
• Flow: Turbine, ultrasonic, magnetic, Coriolis
• Level: Ultrasonic, radar, capacitive, float
• Electrical: Current and voltage sensors
• Environmental: Humidity, moisture, gas detectors
• Position: Proximity, encoders, limit switches
• Imaging: Thermal, visual cameras

The specific sensors needed depend on your monitoring requirements and application.

Learn more: Sensor Selection for Industrial IoT

Thermocouples:

• Wider temperature range: -200°C to 2300°C (type dependent)
• Faster response
• Lower cost
• Less accurate: ±0.5-2°C

RTDs:

• Higher accuracy: ±0.1°C
• Better stability and linearity
• Limited range: -200°C to 850°C
• Slower response, higher cost

Use thermocouples for high temperatures, fast response needs, or cost-sensitive applications. Use RTDs where accuracy and stability are critical.

Required accuracy depends on application criticality and decision thresholds:

• Process control: Accuracy should be better than half the control tolerance (controlling to ±2°C = sensors ±1°C or better)
• Trend monitoring: Accuracy can be looser since you're detecting changes, not absolute values
• Resolution: Should be 5-10x better than accuracy

Over-specifying wastes money; under-specifying compromises decisions. Match sensor capability to actual requirements.

Response time is how quickly a sensor detects and reports changes. It matters for:

• Monitoring rapidly changing conditions
• Closed-loop control applications
• Safety systems requiring fast detection

Typical response times:

• Temperature: milliseconds (exposed TC) to minutes (large mass RTD)
• Pressure and flow: milliseconds
• Vibration: sub-millisecond

For control, sensor response must be faster than process dynamics. For trending, slower response is often acceptable.

Common sensor outputs include:

• 4-20mA: Most common, noise-resistant, long cable runs
• 0-10V: Simple but susceptible to voltage drops
• Digital protocols: Modbus RTU/TCP, HART, Profibus, EtherNet/IP
• Wireless protocols: WirelessHART, ISA100.11a, LoRa, Zigbee
• Direct digital: Switches, encoders

Match output to your data acquisition system. 4-20mA is nearly universal but provides one value per sensor. Digital protocols enable multiple values, diagnostics, and configuration.

IP (Ingress Protection) ratings indicate protection against solid objects and liquids. Format: IP + two digits (first for solids 0-6, second for liquids 0-9).

Common ratings:

• IP65: Dust-tight, protected against water jets
• IP67: Dust-tight, protected against temporary immersion
• IP68: Dust-tight, protected against continuous immersion

Guidelines:

• Most indoor industrial: IP65 suffices
• Washdown areas: IP67 or higher
• Outdoor or submerged: IP68
• Hazardous areas: Additional certifications required (ATEX, IECEx)

Hazardous area sensors must be certified for the specific zone classification and gas/dust group.

Key certifications:

• ATEX: Europe
• IECEx: International
• NEC/CEC Class/Division: North America

Protection methods:

• Intrinsic safety (limiting energy)
• Explosion-proof enclosures
• Increased safety construction

Match sensor certification to your area classification and gas group. Work with qualified personnel for installation. Budget 2-5x more than standard sensors.

Wired sensors:

• Continuous high-frequency data
• Higher reliability
• No battery concerns
• Higher installation cost: $500-2000+ per point in cabling

Wireless sensors:

• Quick, cheap installation
• Flexible for retrofits and temporary monitoring
• Require battery management
• Limited data rates

Use wired for critical applications and high-frequency needs. Use wireless for retrofits and distributed monitoring. Most facilities use both.

Learn more: Wired vs Wireless Sensors

Proper sizing and location ensure representative measurements:

Temperature:

• Adequate insertion depth (8-10x thermowell diameter)
• Locate in flowing fluid
• Avoid dead legs and direct heat sources

Flow:

• Provide required straight pipe runs (varies by meter)
• Install per manufacturer specifications

Vibration:

• Mount directly on bearing housings
• Use proper mounting hardware
• Maintain consistent orientation

Calibration frequency depends on sensor type, application criticality, and regulatory requirements:

• Critical process sensors: Quarterly to annually
• Quality-related sensors: Per quality system requirements (often annually)
• Safety sensors: Per safety system requirements
• Trending sensors: Annually to biennially

Start with manufacturer recommendations and adjust based on drift data. Track calibration results to identify sensors needing more frequent attention. Some applications require traceable calibration to national standards.

Smart sensors include onboard processing, diagnostics, and communication capabilities beyond simple measurement.

Benefits:

• Self-diagnostics (detecting sensor problems before failure)
• Automatic compensation (temperature, pressure effects)
• Local processing (filtering, averaging, peak detection)
• Digital communication with multiple data points
• Remote configuration

Use smart sensors when diagnostics are valuable, multiple measurements are needed per point, for critical applications, or when remote configuration saves labor. They cost more but reduce total cost of ownership for appropriate applications.

Evaluate sensor vendors on:

• Product quality and reliability track record
• Technical support capabilities
• Delivery lead times
• Pricing and total cost of ownership
• Documentation quality
• Calibration services
• Warranty terms
• Local representation
• Industry experience

Request references from similar applications. Consider vendor stability—will they support products long-term? For critical applications, qualify multiple vendors to ensure supply chain resilience.