J-Type Thermocouple

The thermocouple type J consists of an iron and a constantan wire. It is standardised for temperatures between -40 and 750 °C. The type J is often used in the plastics industry, for example for measurement in the extruder or in the hot runner.

In addition to Type J, seven other thermocouple types are standardised in DIN EN 60584: Type B, Type E, Type K, Type N, Type R, Type S and Type T.

In the processing industry and in mechanical engineering, experience has shown that type K and type J are the most popular (up to 1,200 °C). Type N, practically a kind of successor to type K, is slowly gaining in importance among the “base metal” thermocouples.

The precious metal thermocouples type B, type R and type S are made of platinum and rhodium and are correspondingly high-priced. They are mainly used in the glass and heavy industry (up to 1,700 °C).

Yes, thermocouples age depending on their environmental conditions. Due to influences such as thermal ageing or chemical impurities, the thermoelectric voltage changes over time – a measurement error, also called drift, occurs. Thermocouples with precious metals such as type R and S are less affected by this.

The functioning of the thermocouple is based on its material pairing. It is essential that the thermoelectric properties of the lead wires correspond to those of the thermocouple so that the measurement result is not falsified. Depending on the requirements, a “true” thermocouple cable or a compensating cable can be used.

In the case of the thermocouple cable, the cable consists of the same material pairing as the thermocouple itself. This is the technically best solution, but often the more expensive.

The compensating cable is a cheaper alternative. Here, materials are used that have very similar thermoelectric properties as the thermocouple itself within a certain temperature range.

Of course, the same applies to connectors: we only use compensated connectors for our thermocouples.

Thermocouples and their limits of error are standardised in DIN EN 60584. There are tolerance classes 1 to 3. Class 1 is the one with the highest accuracy.

Generally speaking, thermocouples are less accurate than e.g. Pt100 and Pt1000. So if you need maximum accuracy, you are better off with a resistance thermometer (provided the temperature range is suitable).

Thermocouples have other advantages: some types can be used in very wide temperature ranges and can tolerate much higher temperatures. In addition, thermocouples have a lower thermal mass and can be realised in smaller diameters. This contributes to extremely short response times.

We manufacture sheathed thermocouples from a mineral-insulated metal sheathed cable, widely known as MIMS cable. Its outer sheath is made of stainless steel, on the inside the two wires run in densely pressed magnesium oxide powder. Compared to open thermocouples or those protected only by a sleeve, sheathed thermocouples are more durable and robust.

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Regarding the response time of a sensor, the following rule of thumb applies: If you double the diameter, you quadruple the response time. So choose the diameter wisely!

As with all temperature sensors, the immersion length should be at least 5x the diameter of the sensor to avoid thermal bridges.