毕业设计文献翻译热电偶温度传感器(2)

毕业设计文献翻译热电偶温度传感器Thermocouple Temperatur sensor

worth understanding:

1. A third metal may be introduced into a thermocouple circuit and have no impact, provided that both ends are at the same temperature. This means that the thermocouple measurement junction may be soldered, brazed or welded without affecting the thermocouple's calibration, as long as there is no net temperature gradient along the third metal.

Further, if the measuring circuit metal (usually copper) is different to that of the thermocouple, then provided the temperature of the two connecting terminals is the same and known, the reading will not be affected by the presence of copper.

2. The thermocouple's output is generated by the temperature gradient along the wires and not at the junctions as is commonly believed. Therefore it is important that the quality of the wire be maintained where temperature gradients exists. Wire quality can be compromised by contamination from its operating environment and the insulating material. For temperatures below 400°C, contamination of insulated wires is generally not a problem. At temperatures above 1000°C, the choice of insulation and sheath materials, as well as the wire thickness, become critical to the calibration stability of the thermocouple.

The fact that a thermocouple's output is not generated at the junction should redirect attention to other potential problem areas.

3. The voltage generated by a thermocouple is a function of the temperature difference between the measurement and reference junctions. Traditionally the reference junction was held at 0°C by an ice bath:

The ice bath is now considered impractical and is replace by a reference junction compensation arrangement. This can be accomplished by measuring the reference junction temperature with an alternate temperature sensor (typically an RTD or thermistor) and applying a correcting voltage to the measured thermocouple voltage before scaling to temperature.

The correction can be done electrically in hardware or mathematically in software. The software method is preferred as it is universal to all thermocouple types (provided the characteristics are known) and it allows for the correction of the small non-linearity over the reference temperature range.

4. The low-level output from thermocouples (typically 50mV full scale) requires that care be taken to avoid electrical interference from motors, power cable, transformers and radio signal

pickup. Twisting the thermocouple wire pair (say 1 twist per 10 cm) can greatly reduce magnetic