Thermocouples

Can I replace U and L thermocouples per DIN 43710 with Type T and J thermocouples per DIN IEC 60584?

No. Type T and J themocouples have a different thermal voltage characteristic, which would lead to a measuring error. Type U and Type L thermocouples should only be delivered as replacement parts in old installations. With the construction of new plant, these are no longer permitted.

How thick is the wall thickness of an MI cable?

Most manufacturers give a minimum wall thickness which corresponds to 10% of the external diameter of the MI cable.

What are mineral-insulated (MI) cables?

Mineral-insulated cables for resistance thermometers consist of one or more copper wires that are embedded in highly-compacted magnesium oxide and sheathed in casing tube made from, for example, 1.4571 stainless steel.  For thermocouples, instead of copper wires, thermocouple cables suitable for the thermocouple type are used.  The most common standard sheath material for thermocouples is Inconel 2.4816.

What are the response times of the various measuring inserts?

The measurement of the response time is carried out in flowing water in accordance with DIN EN 60751 Section 4.3.3 and VDI / VDE 3522.

v=0,4m/s T start = approx. 20°C  T end = approx. 30°C
 
6mm 1xTyp K, isolated: T50= 4,0sec. T90=11,0sec.
6mm 1xTyp K, non-isolated: T50= 1,0sec.  T90=2,7sec.
6mm 1xPt100, thin-film sensor: T50= 8,5sec. T90=20,5sec.
6mm 1xPt100, ceramic sensor: T50= 7,0sec. T90=19,0sec.

What do the designations of Temperature Class mean?

The ignition temperature is the lowest temperature at which an inflammable mixture of gases can ignite at a flame, a hot surface or otherwise generated spark. Gases and vapours are divided into Classes in which the temperature of the surface must always be lower than that of the mixture. (T1 > 450 °C, T2 > 300 °C, T3 > 200 °C, T4 > 135 °C, T5 > 100 °C, T6 > 85 °C).

What do the Zones in explosion protection mean?

Gases:
Zone 0 (Category 1): permanent or long-term danger of explosions
Zone 1 (Category 2): hazardous atmosphere occasionally exists
Zone 2 (Category 3): explosive atmosphere exists only rarely, and then only for a short time

Dusts:
Zones 20, 21, 22 with the same meanings

What does „Intercrystalline corrosion" mean?

IC (Intercrystalline Corrosion) is a form of corrosion that can occur in most alloys at the appropriate conditions. It is also known as "grain disintegration" or „chromium depletion" The corrosion takes place along the grain boundaries. In steels alloyed with chromium, the chromium contained in the material combines on heating (often while welding) with the carbon to form chromium carbide.  Thus the chromium is no longer available for corrosion protection (formation of a passive layer) in the heated area. This occurs particularly in high-carbon steels. With corrosion-resistant steels, such as 1.4571 (AISI 316Ti), the binding of carbon with titanium or niobium to niobium or titanium carbide (stabilised steels) or lowering the carbon content, e.g. 1.4404 (AISI 316L) acts against IC. These measures prevent the harmful reduction of chromium content along the grain boundaries.

What is green rot?

Thermocouples are subject to ageing and change their temperature/thermal voltage characteristic.
In Type K thermocouples high temperatures can result in substantial changes to the thermal voltage due to chrome depletion in the NiCr leg, leading to a lower thermal voltage.
This effect is accelerated if there is a shortage of oxygen, since a complete oxide layer, which would protect it from further oxidation, cannot be formed on the surface of the thermocouple. Chromium is oxidised, while nickel isn't. This results in the so-called "green rot", destroying the thermocouple.When NiCr-Ni thermocouples that have been operating above 700 °C are cooled quickly, this cooling causes certain states in the crystalline structure (short-range order) to freeze, which in Type K thermocouples can result in a change of the thermal voltage of up to 0.8 mV (K effect).

What is the permissible minimum bending radius for an MI cable?

VDI/VDE 3511 Sheet 2 recommends a radius of curvature R of ≥ 5 x D (D=external diameter of the MI cable), some manufacturers of MI cable even give ≥ 3 x D as the minimum bending radius.

What is thermal voltage (or the Seebeck effect)?

The effect, named after Thomas Johann Seebeck, describes the fact that an electric voltage exists when two different metallic conductors are connected at two different points, if there is a temperature difference between the connected and the open end of the „thermocouple“.

What measures should be taken with the installation of high-temperature probes with ceramic thermowells?

Installation and removal should be carried out slowly and gradually, in order to prevent the ceramic thermowell being destroyed by internal thermal stresses.  This must be either preheated or slowly inserted, e.g. 1 ... 2 cm / min for temperatures to 1,600 ° C and 10 .. 20 cm / min at 1,200 ° C.