Specifications – WIKA TR10-A Benutzerhandbuch
Seite 12
3345267.12 01/2014 GB/D
12
WIKA operating instructions RTD and TC, intrinsically safe designs (Ex i)
GB
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 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 %
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 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 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.
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. Specifications