Sensor Selection for Industrial Automation

Understanding the Critical Role of Sensors in Modern Industrial Automation

In today's competitive manufacturing landscape, sensor selection represents one of the most consequential decisions that industrial engineers and technical managers face. The right sensor can mean the difference between a system that operates flawlessly for years and one plagued by downtime, maintenance headaches, and costly production losses. For industries across Atlantic Canada—from food processing facilities in Nova Scotia to pulp and paper operations in New Brunswick—understanding sensor fundamentals is essential for maintaining competitive advantage.

Industrial sensors serve as the nervous system of automated processes, continuously gathering data about physical parameters such as temperature, pressure, position, flow, and proximity. This information feeds directly into programmable logic controllers (PLCs) and distributed control systems (DCS), enabling real-time decision-making that would be impossible through manual observation alone. With the rise of Industry 4.0 and the Industrial Internet of Things (IIoT), sensor technology has evolved dramatically, offering unprecedented accuracy, connectivity, and diagnostic capabilities.

This comprehensive guide explores the key considerations for sensor selection in industrial automation applications, providing practical insights that engineers and technical managers can apply immediately to their operations.

Environmental Considerations and Operating Conditions

Before evaluating specific sensor technologies, engineers must thoroughly analyse the environmental conditions where sensors will operate. Maritime Canada's climate presents unique challenges that differ significantly from facilities in central or western provinces. The combination of high humidity, salt air exposure in coastal installations, and temperature extremes ranging from -35°C in winter to +35°C in summer places extraordinary demands on industrial equipment.

Temperature and Humidity Factors

Temperature specifications are among the most critical parameters to evaluate. Most industrial sensors specify an operating temperature range, typically -25°C to +70°C for standard models. However, process temperatures may differ substantially from ambient conditions. A sensor monitoring a steam line operating at 180°C requires significantly different construction than one measuring outdoor air temperature in Amherst during January.

Key temperature-related specifications to evaluate include:

• Operating temperature range: The ambient temperature range within which the sensor functions correctly

• Process temperature range: The temperature of the medium being measured or contacted

• Temperature coefficient: How much the sensor's accuracy drifts per degree of temperature change, typically expressed as percentage per °C

• Thermal shock resistance: The sensor's ability to withstand rapid temperature changes without damage

Humidity considerations are equally important, particularly in food processing, beverage production, and marine-adjacent facilities common throughout Nova Scotia. Sensors rated IP67 or higher provide excellent protection against moisture ingress, while specialized marine-grade coatings offer additional protection against salt-laden atmospheres prevalent along the Atlantic coast.

Mechanical and Chemical Exposure

Industrial environments subject sensors to vibration, shock, and potentially corrosive substances. A sensor installed on a reciprocating compressor experiences vastly different mechanical stresses than one mounted in a control panel. Vibration specifications, typically expressed in g-forces and frequency ranges, help engineers match sensor capabilities to application requirements.

Chemical compatibility requires careful analysis of all substances the sensor may contact, including cleaning agents used during washdown procedures. Stainless steel housings (typically 316L grade) and fluoropolymer seals provide broad chemical resistance suitable for most industrial applications, though specialized materials may be necessary for particularly aggressive environments.

Sensor Types and Technology Selection

The industrial automation market offers an extensive array of sensor technologies, each with distinct advantages and limitations. Understanding these differences enables engineers to select optimal solutions for specific applications.

Proximity and Position Sensors

Proximity sensors detect the presence or absence of objects without physical contact, making them invaluable for counting, positioning, and safety applications. The three primary technologies include:

Inductive proximity sensors detect metallic objects through electromagnetic field disturbance. They offer excellent reliability in dirty environments and typically provide sensing ranges from 1mm to 60mm depending on the target material and sensor size. Ferrous metals produce the strongest response, with correction factors of 0.4-0.5 applied for aluminium and 0.3-0.4 for copper and brass targets.

Capacitive proximity sensors detect both metallic and non-metallic materials, including liquids, plastics, and granular substances. This versatility makes them ideal for level detection in tanks and hoppers. However, their sensitivity to environmental factors such as humidity and dust accumulation requires careful application engineering.

Photoelectric sensors use light beams to detect objects at ranges from millimetres to over 100 metres. Through-beam configurations offer the longest ranges and highest reliability, while retro-reflective and diffuse reflective models provide simpler installation. Laser variants achieve precision of ±0.1mm for dimensional gauging applications.

Process Measurement Sensors

Process industries require accurate measurement of temperature, pressure, flow, and level. Each parameter offers multiple measurement technologies:

Temperature measurement commonly employs thermocouples, resistance temperature detectors (RTDs), and infrared sensors. Thermocouples offer the widest temperature range (Type K: -200°C to +1,260°C) and fastest response times, while RTDs (typically Pt100 or Pt1000) provide superior accuracy of ±0.1°C and long-term stability. Infrared sensors enable non-contact measurement of moving or inaccessible targets.

Pressure transmitters utilize piezoresistive, capacitive, or piezoelectric sensing elements. Modern transmitters offer accuracy specifications of ±0.025% of span with rangeability of 100:1, allowing a single transmitter to cover multiple process ranges. Differential pressure transmitters remain essential for flow measurement in conjunction with orifice plates and other primary elements.

Flow measurement technologies include electromagnetic, ultrasonic, Coriolis, vortex, and turbine meters. Selection depends on fluid properties, accuracy requirements, and installation constraints. Electromagnetic meters suit conductive liquids with no moving parts to wear, while Coriolis meters provide direct mass flow measurement with accuracy of ±0.1% for custody transfer applications.

Communication Protocols and Integration Requirements

Modern industrial sensors increasingly incorporate digital communication capabilities, enabling integration with plant networks and enterprise systems. Selecting appropriate communication protocols ensures seamless data exchange and simplifies commissioning and maintenance.

Traditional Analogue Signals

The 4-20mA current loop remains the most common analogue signal standard in process industries. This robust technology offers excellent noise immunity over cable runs exceeding 1,000 metres and provides inherent fault detection through live-zero signalling. A reading below 4mA indicates a wiring fault or sensor failure rather than zero process value.

Voltage signals (0-10V or ±10V) suit applications with short cable runs and low electrical noise. They offer simpler interfacing with data acquisition systems but lack the noise immunity and fault detection capabilities of current loops.

Industrial Network Protocols

Digital fieldbus networks offer significant advantages over analogue signals, including bidirectional communication, remote configuration, and advanced diagnostics. Common protocols in Atlantic Canadian industries include:

• HART (Highway Addressable Remote Transducer): Overlays digital communication on standard 4-20mA signals, providing backward compatibility with existing infrastructure

• PROFIBUS and PROFINET: Widely adopted in manufacturing applications, particularly with Siemens automation systems

• EtherNet/IP: Industrial Ethernet protocol common in Allen-Bradley/Rockwell installations

• Modbus TCP/RTU: Open protocol with broad device support and simple implementation

• IO-Link: Point-to-point communication standard for simple sensors, enabling standardized integration and enhanced diagnostics

Protocol selection should align with existing plant infrastructure and automation vendor preferences. Facilities with established Rockwell Automation systems typically benefit from EtherNet/IP-compatible sensors, while Siemens-centric plants may prefer PROFINET devices.

Accuracy, Resolution, and Performance Specifications

Understanding sensor performance specifications prevents both over-specification (unnecessary cost) and under-specification (inadequate performance). Key parameters require careful interpretation to ensure sensors meet application requirements.

Accuracy and Precision

Accuracy describes how closely a sensor's output matches the true value of the measured parameter. Manufacturers express accuracy in various forms: percentage of reading, percentage of full scale, or absolute units. A pressure transmitter with ±0.1% of span accuracy on a 0-100 bar range offers ±0.1 bar uncertainty across the entire range, while ±0.1% of reading accuracy provides ±0.01 bar uncertainty at 10 bar but ±0.1 bar at 100 bar.

Precision (repeatability) indicates how consistently a sensor reproduces the same output for identical inputs. High-precision sensors produce tightly grouped readings even if they exhibit consistent offset from true values. For closed-loop control applications, repeatability often matters more than absolute accuracy.

Resolution and Response Time

Resolution represents the smallest change a sensor can detect and report. Digital sensors specify resolution in bits or decimal places. A 16-bit analogue-to-digital converter provides 65,536 discrete levels across the measurement range, while a 12-bit converter offers only 4,096 levels.

Response time specifications indicate how quickly sensors react to process changes. The time constant (τ) represents the time required to reach 63.2% of a step change, while T90 indicates time to reach 90% of the final value. Fast processes such as injection moulding or high-speed packaging require sensors with response times measured in milliseconds.

Safety, Certification, and Regulatory Compliance

Industrial installations across Nova Scotia and the Maritime provinces must comply with Canadian electrical codes and industry-specific regulations. Sensor selection must account for these requirements to ensure safe operation and regulatory compliance.

Hazardous Location Classifications

Facilities processing flammable materials—including petroleum refineries, grain elevators, and paint manufacturing plants—require sensors certified for hazardous locations. The Canadian Electrical Code (CEC) adopts classifications similar to the National Electrical Code (NEC):

• Class I: Locations with flammable gases or vapours (Division 1 for normally present, Division 2 for abnormal conditions)

• Class II: Locations with combustible dust

• Class III: Locations with ignitable fibres or flyings

Intrinsically safe (IS) sensors limit electrical energy below ignition thresholds, enabling use in Division 1 locations without explosion-proof enclosures. IS barriers installed in safe areas limit voltage, current, and stored energy to the hazardous location. Explosion-proof (XP) sensors contain any internal explosion and prevent flame propagation to the surrounding atmosphere.

Functional Safety Standards

Safety instrumented systems (SIS) protecting personnel and equipment require sensors meeting IEC 61508 and IEC 61511 functional safety standards. Safety Integrity Levels (SIL) from 1 to 4 specify required risk reduction, with most industrial applications requiring SIL 2 or SIL 3 performance. SIL-rated sensors provide documented failure rates and diagnostic coverage enabling safety system designers to achieve target integrity levels.

Total Cost of Ownership and Lifecycle Considerations

Sensor selection decisions should consider total cost of ownership rather than initial purchase price alone. A sensor costing twice as much may prove more economical over its service life through reduced maintenance, longer replacement intervals, and improved process performance.

Installation and Commissioning Costs

Installation costs vary significantly between sensor types. A simple photoelectric sensor may require only minutes to install, while a Coriolis flow meter installation involves process shutdown, pipe cutting, welding, and extensive commissioning. Sensors with IO-Link or similar smart communication protocols reduce commissioning time through automated parameter download and simplified calibration verification.

Maintenance and Replacement Planning

Preventive maintenance requirements differ substantially across sensor technologies. Sensors with moving parts—such as turbine flow meters and mechanical switches—require periodic inspection and eventual replacement of wear components. Solid-state sensors generally offer longer service life but may still require periodic calibration verification.

Availability of replacement parts and local technical support influences long-term ownership costs. Selecting sensors from manufacturers with Canadian distribution and service centres—ideally with presence in Atlantic Canada—ensures timely support when problems arise.

Partnering with Experienced Engineering Professionals

Successful sensor selection requires balancing numerous technical, economic, and operational factors. While this guide provides foundational knowledge, each application presents unique challenges that benefit from experienced engineering analysis.

At Sangster Engineering Ltd., our team brings decades of industrial automation experience to clients throughout Nova Scotia and Atlantic Canada. We understand the specific challenges facing Maritime industries—from the corrosive coastal environment to the seasonal temperature extremes that test equipment reliability. Our engineers work directly with clients to analyse process requirements, evaluate sensor options, and develop specifications that optimise performance while controlling costs.

Whether you're designing a new automated system, upgrading aging instrumentation, or troubleshooting persistent sensor failures, we provide the technical expertise and local knowledge to ensure success. Contact Sangster Engineering Ltd. today to discuss your sensor selection challenges and discover how our professional engineering services can enhance your industrial automation systems.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.