Thermocouples, as a type of temperature sensor, are usually used in conjunction with display instruments, recording instruments and electronic regulators. They can directly measure the temperature of various liquid, vapor and gas media as well as the surface temperature of solids in the range from 0°C to 1600°C in production processes.
Thermocouples are specifically developed for measuring extremely high temperatures. Thethermocouple wires are installed in protective tubes, and their connecting wires are contained in the capillary holes of ceramic insulating tubes or insulating rods. Usually, the protective tubes are made ofhigh-temperature alloy steel or high-temperature ceramics, with or without additional inner tubes, toprotect the thermocouples from damage caused by process media, mechanical and chemical factors
This type of thermocouple is equipped with a variety of optional process connection parts, such asflange with mating ring, threaded bushing and integral welded flange, etc., which can be directly installed in the measurement process. A thermocouple is generally composed of main components suchas temperature sensing element, installation and fixing device and junction box. The diameter of thermocouple wire is usually 1.2 to 4.0 mm.
Thermocouples have the advantages of good linearity, large thermoelectric emf, high sensitivity,good stability and uniformity, strong oxidation resistance and low cost. They are widely used in
oxidizing and inert atmospheres. However, they cannot be directly used at high temperatures in sulfur, reducing oralternating reducing and oxidizing atmospheres, nor in vacuum. They are not recommended for use inweakly oxidizing atmospheres either.
blast furace, hot blast stove
Annealing and heat treatment processes
Incineration of garbage, biological waste and hazardous waste
Industrial heating equipment, heating devices,power engineering, and reaction glass, porcelain, and ceramicindustries, cement and brick production.
① High measurement accuracy. As the thermocouple is in direct contact with the object beingmeasured, it is not affected by intermediate media.
②The measurement range is wide. Commonly used themmocouples can continuously measure from -50 to +1600°C. Some special thermocouples can measure as low as -269°C(such as gold-iron-nickel-chromium) and as high as +2800°C(such as tungsten-rhenium).
③ Simple in structure and convenient to use. Thermocouples are usually composed of two different metalwires and are not restricted by size or shape. They are encased in protective tubes and are very convenientto use.
Measurement range and allowable error range
| Model number | Indexing Number | Level I | Level II | ||
| Allowable deviation value | Temperature measurement range (℃ ) | Allowable deviation value | Temperature measurement range (℃ ) | ||
| WTK | K | ±1.5℃ | -40~+375 | ±2.5℃ | -40~+333 |
| ±0.004(t) | 375~1000 | ±0.0075(t) | 333~1200 | ||
| WTN | N | ±1.5℃ | -40~+375 | ±2.5℃ | -40~+333 |
| ±0.004(t) | 375~1000 | ±0.0075(t) | 333~1200 | ||
| WTE | E | ±1.5℃ | -40~+375 | ±2.5℃ | -40~+375 |
| ±0.004(t) | 375~800 | ±0.004(t) | 375~900 | ||
| WTJ | J | ±1.5℃ | -40~+375 | ±2.5℃ | 0~+333 |
| ±0.004(t) | 375~750 | ±0.004(t) | 333~750 | ||
| WTT | T | ±0.5℃ | -40~+125 | ±1℃ | 0~+133 |
| ±0.004(t) | 125~350 | ±0.0075(t) | 133~350 | ||
| WTS | S | ---- | ±2.5℃ | 0~+600 | |
| ±0.0025(t) | 600~1300 | ||||
| WTR | R | ---- | ±2.5℃ | 0~+600 | |
| ±0.0025(t) ±2.5℃ | 600~1300 0~+600 | ||||
| WTB | B | ---- | |||
| ±0.0025(t) | 600~1600 | ||||
Note: t represents the actual measured temperature value(C) of the temperature-sensing element.When calculating the error of a thermocouple, the compensation temperature of O C atthe cold end is used as the reference.
Thermocouple time constant
| Thermal inertia levelT | Time constant (seconds) | Thermal inertia level | Time constant (seconds) |
| Ⅰ | 90—180 | Ⅲ | 10—30 |
| Ⅱ | 30—90 | Ⅳ | ＜10 |
Protective tube
99 alumina tube, custom-made stainless steel 3039 outer tube available.
Default diameter is 16mm; Other sizes are available upon request.
Direct output type
| Measuring element | K/N/E/J/T/S/B/R type thermocouples |
| Wiring method | ■Two-wire system, ■3-wire system, ■4-wire system |
| Tolerance values of measuring elements | ± 1.5℃ ; ± 2.5℃ ± 1.5 degrees Celsius; ± 2.5 degrees Celsius |
| Output format | Electrode potential difference signal |
| Electrical connection | ■Ordinary junction box ■Explosion-proof junction box ■Display junction box |
| Explosion-proof protection (optional) | ExdIICT |
Technical parameters of 4-20mA output typ
| easuring element | thermocouple |
| Calibration number | K/N/E/J/T/S/B/R type |
| Tolerance values of measuring elements | ±1.5℃；±2.5℃ ±1.5 degrees Celsius；±2.5 degrees Celsius |
| Basic configuration | 0 - 150℃ (default) |
| Output format | 4-20mA, 2-wire system (optional) |
| Load RA | RA ≤ (UB - 9V) / 0.023A |
| Load impact | ±0.05%/100 Ω |
| power supply | DC 12 - 32V |
| Power protection | Reverse protection |
| Impact of power supply variations | ±0.025%/V |
| Electromagnetic Compatibility (EMC) | Immunity and burst pulses |
| Electrical connection | Common junction box Explosion-proof junction box Display junction box |
| Explosion-proof protection (optional) | ExdIICT4 |
Insulation resistance at high temperatures
At high temperatures, the insulation resistance of the thermoelectric electrodes (including dualelement types) to the protective tube and between the dual-element thermoelectric electrodes (per meter) should be greater than the values specified in the following table.
| Specified operating temperature (℃ ) | Test temperature (℃ ) | Insulation resistance (Ω ) |
| ≥600 | 600 | 72000 |
| ≥800 | 800 | 25000 |
| ≥1000 | 1000 | 5000 |
According to the different joint forms and sheath material groups, they are classified in accordance with the DIN EN 50446 standard.
Thermocouple design is divided into:
Metal sheathed high-temperature thermocouple
High-temperature thermocouple with ceramic sheath and metal support tube
Nominal lengths: 500,710, 1000,1400,2000mm
10,22,16 mmOuter diameter of protective sleeve d
The length of N support tubes is 150mm or 200mm.
D Support pipe diameters:15mm,22mm,32mm
Nominal lengths: 500,700,1000,1400,2000(unit: L)
Outer diameter of protecti16,24,26ve sleeve d
Metal protective sleeve
The maximum operating temperature of the metal protective sleeve can reach 1200C, depending on the materialused. Generally,base metal thermocouples are used for this sensor (Type K, J or N).
Ceramic protective sleeve
The maximum operating temperature of the ceramic protective sleeve can reach 1600 ° C, depending on the type of ceramic used. We can also provide models with higher operating temperatures according to customer requirements.
Usually, precious metal thermocouples are used as this sensor (Type R, S or B). For measuring temperatures above 1200° C, only precious metal thermocouples can be used as sensors. For precious metal thermocouples, there is a risk of "poisoning" due to contamination. The higher the temperature, the greater the risk of this problem. Therefore, when measuring temperatures above 1200° C, it is advisable to use gas-tight ceramics, preferably high-purity C 799.
The process connection remains gas-tight even at a maximum pressure of 0.1 MPa. For toxic or safety-critical gases and special installation positions, it is recommended to take additional structural measures beyond the standard features to prevent leakage of the process medium from the joint in case of cracking of the protective sleeve (pressure-sealing medium runs directly through the joint).
The protective casing is made of (concave) or (flat) tubing. The two types are exactly the same in technical terms. The basic form selection is the responsibility of the manufacturer.
For the enamel metal protective sleeve, the bottom is usually always concave. The protective sleeve is pressed into the junction box and clamped.
In addition, we can also provide junction boxes that are fixed to the protective conduit by means of threads.
This can achieve an IP65 protection rating. By clamping an adjustable process connection onto the protective sleeve, a variable insertion length is achieved.
| Metal protective sleeve | Ceramic inner tube | |
| Outer diameter（φd） | Wall thickness（S） | Outer diameter（d1） |
| 22 | 2 | 15 |
| 16 | 2 | 10 |
Standard nominal length L =500、700、1000、1400 and 2000mm Other lengths can be provided as per customer requirements.
Non-airtight porous ceramics
It has a strong tolerance to temperature changes, with the maximum operating temperature reaching up to 1600℃ , and is not affected by gases.
Corrosion of the body
Used as an external protective tube combined with an airtight inner protective tube, the airtight C 610 ceramic has the highest working temperature.
in the steam
The temperature can reach up to 1500℃ . It cannot be used in an alkaline vapor environment. The high-purity gas-tight C 799 ceramic is the best.
The working temperature can reach up to 1600℃ , and it has a certain tolerance to temperature changes. It cannot be used in an alkaline environment.
The ceramic protective sleeve is made of high-temperature sintered alumina ceramic, with a closed hemispherical end. Due to its relatively low mechanical strength, a metal support tube is required to fix the process connection to the thermocouple.
The ceramic protective sleeve is bonded to the inner surface of the support tube with fire-resistant ceramic compound. The support tube is inserted into the junction box and clamped.
| Ceramic protective sleeve | Ceramic inner tube | Support pipe | ||
| Outer diameter（φd） | Wall thickness （S） | Outer diameter（d1） | Outer diameter（D） | length（N） |
| 22、26 | 2-4 | 15、16 | 32、33 | 200、150 |
| 15、16 | 2 | 10 | 15 | 150 |
Standard nominal length L = 350, 500, 700, 1000, 1400 and 2000 mm. Other lengths can be provided as per customer requirements.
Enamel protective casing
When using an enamel protective sleeve, a threaded liner should be used to prevent damage to the enamel layer.
Non-hermetic applications
(0.1 Mpa or less for airtight applications 1) A single raised face flange is sufficient; there is no need for a butt weld flange. The raised face flange can be adjusted on the support pipe and then fixed with a clamp. Therefore, the insertion length can vary within the length of the support pipe and can be easily adjusted at the installation point. Example of installing a thermocouple with a metallic protective sleeve.
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