How do I measure temperature in an industrial process?

Industrial temperature measurement uses sensors inserted into or mounted on the process to convert thermal energy into a readable signal. The most common sensor types are thermocouples (two dissimilar metal wires joined at a tip that generate a small voltage proportional to temperature — rugged, inexpensive, and covering the widest range from cryogenic to over 3,000°F), RTDs (Resistance Temperature Detectors — a platinum element whose resistance changes predictably with temperature, offering higher accuracy and stability than thermocouples but at a higher cost and a narrower range up to ~1,200°F), and bimetal thermometers (a mechanical dial indicator using a coiled bimetallic strip — no power required, used for local visual indication). For non-contact measurement, infrared pyrometers measure emitted thermal radiation and are used where sensors cannot physically touch the target. The sensor output is typically sent through a temperature transmitter that converts it to a standard 4–20 mA signal for the PLC or DCS.

What is the difference between a thermocouple and an RTD?

Thermocouples and RTDs are both sensors that measure temperature, but they work differently and have distinct strengths. A thermocouple generates a millivolt signal based on the temperature difference between its measuring junction and the reference junction — it is self-powered, very rugged, responds quickly, and covers an extremely wide temperature range (some types reach 3,200°F+). However, thermocouple accuracy is typically ±1–2°C and the signal can drift over time. An RTD uses a precision platinum element whose electrical resistance increases linearly with temperature — it provides much better accuracy (±0.1–0.5°C), excellent long-term stability, and a more linear signal, but it costs more, is more fragile, requires an excitation current, and is limited to about 1,200°F. Choose a thermocouple when you need wide range, fast response, or low cost in harsh environments. Choose an RTD when accuracy and stability are the priority, especially in the –200°F to 1,000°F range.

How do I choose the right thermocouple type for my application?

Thermocouple types are designated by letter (J, K, T, E, N, R, S, B) and each has a different temperature range, accuracy, and chemical compatibility. Type K (chromel-alumel) is the most widely used general-purpose thermocouple — it covers –328°F to 2,282°F and works well in oxidizing atmospheres. Type J (iron-constantan) is popular for lower-temperature applications up to about 1,400°F and is common in plastics, rubber, and older equipment, but it is not suitable for oxidizing environments above 1,000°F because the iron wire oxidizes. Type T (copper-constantan) offers excellent accuracy at low temperatures down to –328°F and is used in food, pharmaceutical, and cryogenic applications. For very high temperatures (up to 3,200°F), noble metal types R, S, and B use platinum-rhodium wires but are expensive. Select the type based on your temperature range, the atmosphere or media the thermocouple will be exposed to, the required accuracy, and compatibility with your existing instrumentation.

What is a temperature transmitter and do I need one?

A temperature transmitter is a device that converts the low-level signal from a thermocouple or RTD into a standard, noise-immune output signal — typically 4–20 mA analog or a digital protocol like HART — that can be transmitted over long cable runs to a PLC, DCS, or recorder. Without a transmitter, the raw millivolt signal from a thermocouple is very susceptible to electrical noise, especially in industrial environments with VFDs, motors, and welding equipment nearby. A transmitter amplifies and conditions the signal close to the sensor, dramatically improving signal integrity and measurement accuracy. Transmitters also provide linearization (correcting the non-linear thermocouple signal), cold-junction compensation, sensor burnout detection, and diagnostic information. You need a transmitter whenever the distance between the sensor and the control system is more than a few feet, when accuracy matters, or when you need to integrate the temperature signal into a 4–20 mA loop or digital fieldbus network.

What is a temperature switch and when would I use one instead of a transmitter?

A temperature switch is a device that opens or closes an electrical contact when the measured temperature reaches a preset value — it provides a simple on/off signal rather than a continuous measurement. Temperature switches are used for protection and alarm functions where you need to trigger an action at a specific temperature threshold: shutting down a motor if bearing temperature exceeds a limit, activating a cooling fan when a cabinet gets too hot, triggering an alarm if a tank heater overheats, or interlocking a process to prevent operation outside safe temperature limits. Use a temperature switch when you only need to know whether temperature is above or below a set point and need a reliable, independent safety device. Use a transmitter when you need to continuously monitor the actual temperature value, trend it over time, or feed it into a control loop. In many critical applications, both are used together — a transmitter for monitoring and control, and an independent temperature switch as a safety backup.

Temperature Measurement, Instrumentation and Control - Ashcroft

We carry a complete selection of temperature measurement, instrumentation, and control devices to suit any industrial or commercial application. For various reasons, temperature measurement, instrumentation, and Co...

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Bi-Metal Thermometers

Bimetal thermometers are widely used temperature measurement devices that employ a bimetallic strip to indicate temperature changes. These thermometers consist of two metal strips, typically made of different metals with different coefficients of thermal expansion, bonded together. The metals expand or contract at different rates as the temperature changes, causing the bimetallic strip to bend. This bending motion is then translated into a rotational movement displayed on a calibrated dial to indicate the temperature. Bimetal thermometers are known for their simplicity, durability, and cost-effectiveness. They are commonly used in various industries and applications requiring reliable temperature monitoring, such as HVAC systems, automotive, food processing, and industrial processes. Bimetal thermometers offer advantages such as wide temperature ranges, resistance to shock and vibration, and ease of installation. They are available in different sizes and configurations to suit specific application requirements. With their robust construction and accurate temperature indication, bimetal thermometers provide a reliable and cost-effective temperature measurement solution in various industrial and commercial settings.

Related categories: Thermometers | Industrial Thermometers

Temperature Switches

Temperature switches open or close a contact at a set temperature to start fans, stop heaters, trigger alarms or shut down equipment. They're simple, dependable temperature protection.

Switch Types

Mechanical switches – bulb and capillary or bimetal
Electronic switches – adjustable set points and displays
Limit switches – high-limit safety shutoff
Explosion-proof switches – hazardous areas

Choosing a Switch

Select range, deadband, contact rating and sensor style. Use thermowells for pressurized systems. Manual-reset limits add safety.

Centro supplies temperature switches for equipment protection and control.

Thermowells

Thermowells protect temperature sensors from pressure, flow, corrosion and abrasion. They also let you remove sensors for replacement or calibration without draining or shutting down the process.

Thermowell Types

Threaded thermowells – common for general service
Socket weld and weld-in – high-pressure installations
Flanged thermowells – easy removal and corrosive service
Sanitary thermowells – tri-clamp for food and pharma
Tapered and stepped stems – strength and fast response

Choosing a Thermowell

Select material, insertion length, bore size and connection. For high-velocity flows, perform a wake frequency calculation to avoid stem failure.

Centro supplies thermowells for temperature sensors and thermometers.

Temperature Accessories

Temperature accessories help your sensors and instruments read accurately and last longer. Thermowells, fittings, connection heads and extension wire protect sensors from the process and make installation and replacement simpler.

Accessories Available

Thermowells – threaded, socket weld and flanged wells protect sensors from pressure and flow
Compression fittings – adjustable sensor insertion depth
Connection heads – house terminal blocks and head-mount transmitters
Thermocouple and RTD extension wire – carry signals back to the controller
Connectors and plugs – quick disconnects for sensors

Choosing Accessories

Select thermowell material and length for the process, with the tip in the flow, and check the wake frequency calculation for high-velocity lines. Use extension wire that matches the thermocouple type to avoid reading errors. Head and enclosure ratings should match the environment.

Centro carries temperature accessories to complete your sensor installations.

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Temperature Measurement, Instrumentation and Control - Ashcroft

Switch Types

Choosing a Switch

Thermowell Types

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