VI. How many types of RTD lead wire connections are there? What are their effects?
There are 3 types of lead wire connections for RTDs:2-wire, 3-wire, and 4-wire systems.
2-wire RTD: Simple wiring, but includes additional error from lead wire resistance.Therefore, it is not suitable for Class A precision RTDs, and lead wires should not be excessively long.
3-wire RTD: Can eliminate the influence of lead wire resistance, with higher accuracy than 2-wire.It is the most widely used for process measurement.
4-wire RTD: Not only eliminates lead wire resistance, but also compensates for the resistance of connecting wires when their resistances are equal.It is used for high-precision measurement applications.
VII. What are the advantages and disadvantages of Type N thermocouple compared with Type K?
Advantages of Type N thermocouple:
Strong oxidation resistance at high temperatures and excellent long-term stability.In Type K thermocouples, preferential oxidation of Cr and Si in the NiCr positive electrode causes uneven alloy composition and drift of thermoelectric EMF.Type N thermocouple uses higher Cr and Si content, changing the oxidation mode from internal oxidation to external oxidation, so oxidation occurs only on the surface.
Good thermal cycle stability at low temperatures, and magnetic transformation is suppressed.
Strong nuclear radiation resistance.Type N eliminates easily degradable elements Mn and Co found in Type K, further improving neutron radiation resistance.
Better linearity of thermoelectric characteristics than Type K in the range of 400–1300 °C.
Disadvantages of Type N thermocouple:
The material of Type N is harder than Type K and more difficult to process.
Relatively higher cost.The thermal expansion coefficient of Type N is about 15% lower than stainless steel, so the sheath of Type N mineral-insulated thermocouples should be made of NiCrSi/NiSi alloy.
Large non-linear error in the range of -200–400 °C.
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