Temperature Sensors

What Are Temperature Sensors?

Temperature sensors are devices that detect temperature and convert it into a usable electrical value. Depending on the design, that value may be a changing resistance, a millivolt signal, or a conditioned analog output. Common sensor families include RTDs, thermistors, and thermocouples. In building controls, you will also see packaged room, duct, immersion, and outdoor air sensors. In industrial automation, probe sensors, threaded sensors, head-mounted assemblies, and transmitter-equipped process sensors are common. The right product depends on what is being measured, how the control system reads it, and how much accuracy and ruggedness the application needs.

Where Are Temperature Sensors Used?

Temperature sensors are used in air handlers, rooftop units, boilers, hydronic loops, chillers, refrigeration systems, make-up air units, process tanks, piping, ovens, packaging equipment, compressors, and motor or bearing monitoring systems. HVAC and BAS work often uses room, duct, immersion, and outdoor sensors for control and energy management. OEM and panel builders may use compact probe or threaded sensors tied into PLC analog inputs. Process applications may require thermocouples or RTDs with thermowells, transmitters, and higher-temperature materials. Maintenance teams also use them heavily for replacement work where the existing controller expects a specific sensor curve or signal type.

How To Choose Temperature Sensors

Start with the controller or instrument input. If the receiving device expects a 10K thermistor, 100 ohm platinum RTD, Type J thermocouple, or 4-20 mA transmitter, that requirement usually decides the sensor family. Next, confirm the temperature range, required accuracy, and response speed. Then choose the physical format: room sensor for occupied spaces, duct probe for air streams, immersion sensor for wells or liquid piping, strap-on for pipe surface monitoring, or threaded probe for machinery and process equipment. Also check cable length, lead wire type, enclosure material, insertion depth, and whether the location needs weather resistance, washdown resistance, or chemical compatibility. For retrofit jobs, always verify the existing sensor curve and wiring before ordering.

Trade Rules Of Thumb

As a typical rule of thumb, thermistors are common where HVAC controls need good sensitivity over a narrower operating band, while RTDs are often chosen where repeatability and long-term stability matter more. Thermocouples are commonly used when temperatures go beyond the practical range of many thermistors or where fast response and rugged probe construction are needed. For room sensing, placement matters as much as sensor quality - avoid direct sun, supply air wash, exterior wall bias, and heat-producing equipment. For immersion sensing, deeper insertion and good thermal contact usually improve response. For long wire runs on passive sensors, lead resistance and electrical noise can affect readings, so transmitter-based solutions may be preferred in larger industrial sites. These are practical selection habits, not code rules.

Sizing Guidelines

Temperature sensors are not sized like overcurrent devices or conductors, but there are still key selection dimensions. Choose a temperature range that comfortably covers normal operation plus upset conditions. Select insertion length so the sensing portion is properly exposed to the medium being measured. For duct sensors, longer probes may help average larger air streams, while short probes may be enough for smaller ducts or equipment cabinets. For immersion assemblies, match the sensor to the thermowell or well pocket dimensions and material. For pipe strap-on sensing, make sure the sensor body and strap arrangement fit the pipe diameter and insulation detail. If using a transmitter, confirm supply voltage, output scaling, and controller input range. Final selection and installation should be verified against the equipment design, manufacturer instructions, and applicable Canadian electrical and mechanical requirements.

Common Installation Practices

Good installation practice starts with putting the sensor where it measures the actual process, not a distorted local condition. In ducts, install where airflow is representative and away from dead spots or immediate discharge turbulence unless the design specifically calls for that location. In piping, use thermal compound where appropriate and insulate over strap-on or surface sensors when trying to track fluid temperature more closely. In process work, thermowells are often used so sensors can be serviced without opening the system. Separate low-level sensor wiring from VFD output conductors and other noise sources where possible. Label sensor type and curve during installation, especially in BAS retrofits, because many replacement problems come from undocumented sensor values rather than failed hardware.

Common Mistakes

One of the most common mistakes is assuming any temperature sensor will work as a replacement if the probe looks similar. A 10K thermistor, 1K RTD, Pt100 RTD, and thermocouple may all fit physically but will not read correctly on the wrong input. Another frequent issue is ignoring the environment - standard plastic room or duct assemblies are not suitable for washdown, corrosive, or high-vibration areas. Buyers also sometimes overlook insertion length, thread size, or enclosure style, which creates field modification work that could have been avoided. In controls work, poor sensor placement can cause hunting, nuisance alarms, or energy waste even when the sensor itself is functioning properly. For process applications, not checking sheath material compatibility can shorten service life significantly.

Brand Comparisons

DwyerOmega is widely cross-shopped for broad sensing coverage and is often a practical choice when you need industrial and HVAC-oriented temperature sensing options from a known instrumentation supplier. Greystone Energy Systems is commonly associated with building automation and HVAC sensing, making it a strong fit for room, duct, immersion, and outdoor air applications where BAS compatibility matters. ProSense is often considered for value-focused industrial sensing and control jobs where straightforward specification and replacement practicality are important. Delta Electronics is better known in automation and control than as a general temperature sensing specialist, but may still fit certain panel and OEM environments depending on the exact product family. In the broader market, Omron, Banner Engineering, Pepperl+Fuchs, SICK, ifm, Balluff, Autonics, Honeywell, Schneider Electric, and Eaton are often encountered in automation, sensing, and control ecosystems. If you are matching an installed system, staying with the existing brand or at least the exact sensor type and signal format may be the lowest-risk path. If you are building new, an available alternative can be a sensible choice when it meets the control input, environmental, and mounting requirements without adding conversion work.

Related Products

Temperature sensors are commonly purchased with thermowells, connection heads, transmitters, panel meters, PLC analog input modules, BAS controllers, relays, enclosures, cable glands, shielded control cable, and terminal blocks. HVAC jobs may also require sensor guards, mounting flanges, immersion wells, and control transformers. Industrial process installations may need RTD or thermocouple extension wire, signal isolators, and local indication. When replacing a failed assembly, it is often worth checking whether the problem is actually the sensor, the transmitter, the controller input, or damaged field wiring before ordering parts.

Frequently Asked Questions

What is the difference between an RTD, a thermistor, and a thermocouple?

An RTD typically offers good accuracy and stability, a thermistor is often very sensitive over a narrower temperature band and common in HVAC controls, and a thermocouple is usually chosen for wider temperature ranges and tougher process conditions. The correct choice depends on the controller input and the application.

Can I replace one brand of temperature sensor with another?

Yes, in many cases, but only if the replacement matches the original sensor type, resistance curve or thermocouple type, output signal, mounting style, and temperature range. Physical fit alone is not enough for a correct replacement.

When should I use a transmitter output instead of a passive sensor?

Transmitters are often preferred on longer wire runs, in electrically noisy environments, or when the control system expects a standard analog signal such as 4-20 mA or 0-10 V. They can also simplify integration with PLCs and remote monitoring systems.

Are room and duct temperature sensors interchangeable?

Not usually. Room sensors are designed for occupied-space mounting and appearance, while duct sensors are built to project into an air stream. Even if the sensing element is similar, the housing and application are different.

What should I verify before ordering a replacement temperature sensor?

Check the existing sensor type, resistance value or thermocouple designation, wiring arrangement, insertion length, mounting thread or enclosure style, temperature range, and the controller input it connects to. If possible, confirm the original part number from the equipment documentation.

Do temperature sensors need special wiring practices?

They often do. Low-level sensor signals can be affected by electrical noise, lead resistance, and poor terminations. Good practice includes using the correct cable type, keeping sensor wiring away from high-noise conductors where practical, and following the manufacturer wiring guidance.