WIKA TR10-C - Handbuch (Seite 12)

14106886.01 06/2014 GB/D
12 WIKA operating instructions resistance thermometers and thermocouples, Ex n
GB
T Cu-CuNi
IEC 60584 part 2
1 -40 ... +350 °C ±0.5 °C or 0.0040 ∙ | t |
2 -40 ... +350 °C ±1.0 °C or 0.0075 ∙ | t |
3 -200 ... +40 °C ±1.0 °C or 0.015 ∙ | t |
ASTM E230
Special 0 ... +370 °C ±0.5 °C or ±0.4 %
Standard -200 ... 0 °C ±1.0 °C or ±1.5 %
Standard 0 ... +370 °C ±1.0 °C or ±0.75 %
R
S
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 E230
Special 0 ... +1,480 °C ±0.6 °C or ±0.1 %
Standard 0 ... +1,480 °C ±1.5 °C or ±0.25 %
B
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 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 different 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 difference, 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 effects/poisoning
■ Oxidation processes in thermocouples which are not appropriately protected (“bare”
thermocouple wires) result in falsifications of the characteristic curves.
■ Foreign atoms (poisoning) that diffuse into the original alloys lead to changes of these original
alloys and thus falsify the characteristic curve.
■ The influence 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 first.
In general, rising temperatures cause accelerated ageing effects.
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 affected
by it. The consequence of this effect is a drift of the measured value caused by decreasing
thermoelectric voltage. This effect 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.
3. Specifications