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3345267.12 01/2014 GB/D

12 WIKA operating instructions RTD and TC, intrinsically safe designs (Ex i)

Pt13%Rh-Pt

Pt10%Rh-Pt

IEC 60584 part 2

1 0 ... +1,600 °C

±1.0 °C or

±[1 + 0.003 (

t - 1100)] °C

2 0 ... +1,600 °C

±1.5 °C or ±0.0025

∙ | t |

ASTM 14.03 E230

Special 0 ... +1,480 °C ±0.6 °C or ±0.1 %

Standard 0 ... +1,480 °C ±1.5 °C or ±0.25 %

Pt30%Rh-

Pt6%Rh

IEC 60584 part 2

2 +600 ... +1,700 °C

±0.0025

∙ | t |

3 +600 ... +1,700 °C

±4.0 °C or ±0.005

∙ | t |

ASTM 14.03 E230

Special - -

Standard +870 ... +1,700 °C ±0.5 %

1) I t I is the value of the temperature in °C without consideration of the sign

2) The greater value applies

There are dierent notations of type K thermocouples in Europe and North America:

Europe: NiCr-NiAl or NiCr-Ni

North America: Ni-Cr / Ni-Al

There is no physical dierence, it is just the naming caused by historical reasons.

3.2.2 Potential measurement uncertainties

Important factors which counteract the long-term stability of thermocouples.

Ageing eects/poisoning

■

Oxidation processes in thermocouples which are not appropriately protected (“bare”

thermocouple wires) result in falsications of the characteristic curves.

■

Foreign atoms (poisoning) that diuse into the original alloys lead to changes of these original

alloys and thus falsify the characteristic curve.

■

The inuence of hydrogen leads to the embrittlement of the thermocouples.

The Ni leg of the type K thermocouple is often damaged by sulphur which is contained in exhaust

gases, for example. Thermocouple types J and T age slightly, as the pure metal leg oxidises rst.

In general, rising temperatures cause accelerated ageing eects.

Green rot

If type K thermocouples are used at temperatures from approx. 800 °C to 1,050 °C, considerable

changes of the thermoelectric voltage can occur. The cause of this is a chromium depletion or

the chrome oxidation in the NiCr leg (+ leg). The precondition for this is a low concentration of

oxygen or steam in the immediate environment of the thermocouple. The nickel leg is not aected

by it. The consequence of this eect is a drift of the measured value caused by decreasing

thermoelectric voltage. This eect is accelerated if there is a shortage of oxygen (reducing

atmosphere), since a complete oxide layer, which would protect it from further oxidation of the

chromium, cannot be formed on the surface of the thermocouple.

The thermocouple is permanently destroyed by this process. The name green rot is derived from

the greenish shimmering colouration on the breaking point of the wire.

3. Specications

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 ... 75 76

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Wika TR10-A Manual de usuario Pagina 12

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