Types of Sensors Used in Food Processing

Inside a digital monitoring system, two temperature sensors can look identical on a shelf and behave very differently once installed. One is wired into a controller and reports every second for process control; another sleeps in a delivery van, wakes every ten minutes, and stores a week of readings for download on return. Classifying sensors by how they sense, how they connect, and how they deliver data helps you build a system where each device matches its job.

Parameter alone — temperature, pressure, gas — is only half the specification. Sensing principle, output type, power, communication, and logging behaviour decide whether data arrives in time, survives your environment, and can be trusted for decisions.

Sensing principles behind the readings

Resistive principles underpin RTDs and thermistors: a material changes electrical resistance with temperature, and electronics infer the value from that change. RTDs give stable, accurate performance for critical monitoring; thermistors give high sensitivity over a narrower range for refrigeration and product temperature logging.

Thermoelectric sensing in thermocouples generates a small voltage at a junction of dissimilar metals. Devices are robust and fast, well suited to ovens and processes where a logger or controller needs rapid feedback, provided extension cables and connectors match the thermocouple type.

Capacitive principles serve humidity and some level sensing, piezoelectric and piezoresistive effects serve pressure, and optical principles serve gas and product inspection. NDIR gas sensors pass infrared light through a sample and measure absorption; electrochemical cells generate current from a chemical reaction with the target gas. Knowing the principle tells you the likely interference: steam and contamination for optical and capacitive devices, fouling for electrochemical and pH electrodes, vibration and mounting stress for pressure devices.

Analogue, digital, and smart outputs

Older analogue sensors output a current or voltage, commonly an industry-standard loop signal, that a controller scales into units. Wiring errors or wrong scaling in the controller can produce plausible but wrong values, so commissioning checks against an independent instrument are essential.

Digital sensors output values in engineering units over a data bus or network protocol. Calibration data can live in the device, and the device can report its own identity and fault status. Smart sensors go further, applying filtering, limit checks, and self-diagnostics before transmitting. Each step adds capability and also a configuration that must be controlled: a changed averaging setting or unit selection can alter records just as surely as a faulty probe.

Wired versus wireless in monitoring systems

Wired devices

Wired sensors draw power from the system and communicate continuously, supporting fast sampling for control loops in pasteurisers, fillers, and cooking equipment. Cabling must survive washdown, heat, and mechanical damage, so glands, conduit, and hygienic fittings form part of the sensor choice, not an afterthought.

Wireless devices

Wireless sensors free you to monitor cold rooms, ripening areas, warehouses, and transport without new cabling. Trade-offs are battery management and signal reliability. Metal racking, insulated panels, and water-rich product absorb radio signals, so a site survey and gateway placement decide success. Battery status should itself be monitored and alarmed, with replacement planned before failure rather than discovered during an audit.

Data loggers versus continuous monitoring

Standalone loggers record at set intervals into internal memory. Strengths include independence from plant networks, easy deployment for transport and validation studies, and continued recording during power or network loss. Weakness is delay: nobody sees an excursion until data is downloaded, unless the logger also transmits.

Continuous online systems stream readings to a central platform, enabling live alarms and immediate intervention. Weakness is dependence on network, servers, and alert discipline. Many plants use both: online monitoring for fixed critical points, loggers for vehicles, trials, seasonal capacity, and backup during system maintenance.

Sampling interval selection connects the two approaches. Log a fast-changing cooker too slowly and you miss peaks; log a stable warehouse too fast and you generate storage and battery costs with no extra insight. Base intervals on how quickly a harmful change can develop and how quickly staff can respond.

Choosing by system role

• Control sensors: fast, wired, integrated with a PLC, with fail-safe behaviour defined if the signal is lost.
• Critical monitoring sensors: accurate, alarmed, timestamped, with calibration traceability and data buffering.
• Trend sensors: good repeatability and coverage, used for energy, maintenance, and quality patterns rather than release decisions.
• Verification devices: portable, higher accuracy reference probes and loggers used to check fixed installations.

Document the role for each device in your monitoring plan. During staff changes and software updates, that document prevents a trend sensor from quietly being treated as release evidence, or a control sensor from being replaced with a slower equivalent that changes how equipment behaves.

Environmental rating ties all these choices together. A wireless logger suited to a dry warehouse may fail quickly in a wet process area, and a control probe rated only for moderate temperatures cannot be moved casually into a cooking vessel during a breakdown. Record the rated range, protection level, battery type, and communication method for each device in the same register that records its role. When staff need a replacement at short notice, that register prevents a well-intentioned swap from introducing a device that senses correctly but cannot survive, communicate, or log in the way the system expects.

Key points

• Classify sensors by sensing principle, output, connection, and logging behaviour — not only by the parameter measured.
• Resistive, thermoelectric, capacitive, optical, and electrochemical principles each carry distinct interference and maintenance needs.
• Wired devices suit fast control loops; wireless devices suit flexible coverage but need signal surveys and battery monitoring.
• Loggers record independently but report late; continuous systems alarm live but depend on networks and response discipline.
• Assign every device a clear system role — control, critical monitoring, trend, or verification — and document it.