Duct thermocouples are used for temperature measurement in air ducts and are designed for flowing air in the duct. The perforated stainless steel sleeve protects the measuring element and at the same time ensures a fast response time.
Ventilation and air conditioning systems are often subject to changing airflows, humidity, and dust, so these probes must be robust in design and clean in workmanship.
We have compiled a guide for you on what you should consider when selecting, purchasing, installing, and operating a duct thermocouple.
What is a duct thermocouple and what is it used for?
A duct thermocouple is a temperature probe for measuring the air temperature directly in the air duct. It consists of two wires made of different alloys, the connection of which generates a thermoelectric voltage and thus determines the temperature. The perforated stainless steel sleeve with exposed measuring point ensures fast response times and good coupling to the flowing air.
Typical applications are in heating, ventilation, air conditioning, and building automation, for example in supply and exhaust air, in ventilation control centers, as well as in heating and cooling technology and in mechanical engineering. Duct thermocouples are available as type J and type K and, depending on the environment, are designed with silicone or glass fibre cable.
Clamp fittings or mounting flanges are used for installation in the air duct, with mini or standard thermocouple connector available on request for quick connection to controllers and temperature controllers. This makes duct thermocouples ideal for precise air temperature measurement, control, and monitoring in air ducts.
Functionality in the air duct and special features compared to cable and contact thermocouples
A duct thermocouple measures the temperature of the air in the air duct using the Seebeck effect. Two different metal conductors are connected at the measuring point and generate a thermoelectric voltage, which is recorded by the measuring device and converted into temperature. The measuring point is located in a perforated stainless steel sleeve, directly in the air flow, which ensures good coupling and short response times.
Compared to cable probes with a closed sleeve, duct probes react faster in air because heat transfer is improved by the openings. Contact probes measure the surface temperature of pipes or housings and are highly dependent on contact quality and thermal conduction paste, while duct thermocouples measure the actual air temperature in the flow cross-section. Clamp point compensation in the measuring device is important because thermocouples measure the difference between the measuring point and the reference point, and the correct type J or K must be set.
Designs and components for duct thermocouples
We offer type J and type K duct thermocouples with two different cables.
Duct thermocouples consist of a perforated stainless steel sleeve with an exposed measuring point, which enables a fast response time in flowing air, and a selectable mounting length for the appropriate position in the air duct.
Type J and type K thermocouples are used, combined with silicone cable up to 180 degrees Celsius or glass fibre cable up to 400 degrees Celsius, depending on the environment. Insulated wire end ferrules and thermocouple connectors in mini or standard sizes are available for connection, with anti-kink springs on request. The probe head has no protection class, as the measuring element is exposed; the cables have their own resistance depending on the material.
Installation is carried out using clamp screw connections or mounting flanges, which enable secure fastening in the duct.
The exact measuring ranges of our thermocouples can be found in the operating instructions for each probe or in our download area, along with the characteristic curves.
Types and measuring ranges: Type J vs. Type K
Duct thermocouples are primarily used as type J and Type K. Type J consists of iron and copper nickel and covers a range of approximately minus 40 to 750 degrees Celsius in Class 1. Type K consists of nickel chromium and nickel aluminum and is suitable for approximately minus 40 to 1000 degrees Celsius in Class 1.
For use in air ducts, it is not usually the theoretical thermocouple limit that is decisive, but the permissible temperature of the cable. With silicone cables, applications from minus 50 to plus 180 degrees Celsius are possible, with glass fibre cables up to plus 400 degrees Celsius.
Select the Type that is suitable for the measuring device and the required temperature range, and make sure that the measuring device uses the correct Type and terminal point compensation.
Selection guide for your application
First check compatibility with the measuring device and select the appropriate thermocouple type J or K according to the operating instructions.
- Define the measuring range and the required tolerance and ensure that the permissible cable temperature of silicone up to 180 degrees Celsius or glass fibre up to 400 degrees Celsius is not exceeded.
- Select the mounting length so that the measuring point is securely located in the air flow and not in the wall area, typically in the middle of the duct or behind a mixing section.
- Determine the connection type with insulated end ferrules for terminals or thermocouple connectors in mini or standard size for quick device replacement.
- Design the cable length so that clean cable routing with a drip loop is possible and avoid bending radii below the specifications.
- Take environmental conditions such as humidity, dust, and vibration into account and, in the case of highly fluctuating flow, select a position that is as free as possible without shading from dampers or filters.
Accessories for duct thermocouples
Clamp fittings and mounting flanges in suitable sizes are available for mechanical fastening, ensuring a tight and secure fit in the air duct. For the electrical connection, choose insulated end ferrules or thermocouple connectors in mini or standard sizes, depending on the device, with optional couplings for extensions. Suitable type J or type K thermocouple cables connect the probe to controllers or data acquisition systems; the cable type and color code should match the thermocouple. Thermal conduction paste is not usually required for duct probes, as the measuring point is in the air. In addition, kink protection springs and cable glands are useful if there is frequent movement or tight installation situations.
We offer the following accessories:
Replacement and troubleshooting
When replacing the probe, first check the type of thermocouple installed (J or K) and the cable configuration to ensure that the replacement probe is electrically and mechanically compatible. Note the installation position, mounting length, connection type, and cable routing, loosen the fastening, and pull the probe straight out of the channel. Insert the new probe in the same position, tighten the clamp connection or flange, and connect the cable correctly.
If the probe displays incorrect values, check the Type setting and terminal compensation on the measuring device, check the plug contacts and cable integrity, and inspect the sleeve tip for mechanical damage.
If there are jumps or dropouts, this indicates a cable break or loose connection. If the display is permanently sluggish, check the flow situation and whether the perforation is completely in the air stream. If the error remains unclear, replace it with a reference sensor for testing purposes and compare the display.
Frequently asked questions about temperature sensors
Can I get a calibrated duct thermocouple?
Yes, you can order our duct thermocouples directly with calibration. We offer both ISO calibration and DAkkS accredited calibration. You can select the required option directly on the product page in the configurator. There you can also specify the specific measuring points that are available for the respective type.
What is the typical temperature range?
The cable material, thermocouple type, and sleeve are decisive factors. With silicone cables, applications from minus 50 to plus 180 degrees Celsius are possible, with glass fibre cables up to plus 400 degrees Celsius. Type J can be used in class 1 up to approximately 750 degrees Celsius, type K up to approximately 1000 degrees Celsius. In air ducts, the cable usually limits the temperature range.
How quickly does a duct thermocouple respond?
The perforated stainless steel sleeve and the flowing air enable short response times; the specific time depends on the air velocity and the installation position. If the perforation is completely in the air stream, the probe reacts much faster than a closed cable probe.
Where is the best position in the air duct?
The ideal position is in the middle of the duct cross-section or in a well-ventilated zone behind a mixing section. Avoid proximity to walls and direct proximity to heating registers, coolers, or humidifiers without sufficient mixing.
What is the correct mounting length?
Select the mounting length so that the measuring point with the perforation is securely located in the main air flow and is not shaded. For large cross-sections, a longer mounting length may be advisable.
Which thermocouple type should I choose?
Base your choice on the measuring device and the required temperature range. Type J covers many standard tasks, while type K is suitable for higher temperatures. The measuring device must be set to the correct type.
Do I need to consider terminal point compensation?
Yes, thermocouples measure the temperature difference between the measuring point and the reference point.
The measuring device compensates for the reference junction, so the function must be active and the Type must be set correctly.
Which cable is suitable?
Silicone cables are flexible and suitable for humid environments up to 180 degrees Celsius, glass fibre cables are designed for high temperatures up to 400 degrees Celsius, but are less resistant to moisture. Choose the cable that is suitable for the environment and the installation route.
How is it attached in the duct?
Clamp fittings or mounting flanges are commonly used. Tighten the screw connection without putting strain on the tip of the sleeve and route the cable with a drip loop and the minimum bending radius.
Do I need thermal conduction paste?
Not in air ducts, as the measuring point measures in air. Thermal conduction paste is only relevant for contact and immersion probes.
What are typical causes of errors?
Incorrect Type on the measuring device, missing clamp point compensation, loose plug connection, cable break, mechanically damaged sleeve tip, or installation position outside the air flow. Shading from flaps or filters can also lead to measurement errors.
How do I check a duct probe for proper functioning?
Check the Type and settings on the device, check the connector and cable, compare the display with a reference probe, and inspect the sleeve. If necessary, replace it with a second probe for testing purposes.
Can duct thermocouples be used outdoors?
This is possible if the cable selection and sealing are suitable. The open measuring point should not be directly exposed to splashing water and dirt. Weather protection on the duct is recommended.
What accuracy can be expected?
In class 1, the tolerances according to IEC 60584 apply. In addition, the flow, installation position, and measuring device influence the total deviation. Central positioning and a sufficient mixing section improve the measurement quality.
How long can the cable be?
Select the length so that the cable can be laid without tension and with a reserve loop. If necessary, use couplings and suitable thermocouple cables of the same Type so that the measurement is not distorted.
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