毕业设计文献翻译热电偶温度传感器(4)

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

main thermocouple types - B, E, J, K, N, R, S and T - were re-characterised in 1993 to reflect the scale changes. (See: NIST Monograph 175 for details). The remaining types: C, D, G, L, M, P and U appear to have been informally re-characterised.

Try the thermocouple calculator. It allows you the determine the temperature by knowing the measured voltage and the reference junction temperature.

Thermocouple wire grades

There are different grades of thermocouple wire. The principal divisions are between measurement grades and extension grades. The measurement grade has the highest purity and should be used where the temperature gradient is significant. The standard measurement grade (Class 2) is most commonly used. Special measurement grades (Class 1) are available with accuracy about twice the standard measurement grades.

The extension thermocouple wire grades are designed for connecting the thermocouple to the measuring device. The extension wire may be of different metals to the measurement grade, but are chosen to have a matching response over a much reduced temperature range - typically -40°C to 120°C. The reason for using extension wire is reduced cost - they can be 20% to 30% of the cost of equivalent measurement grades. Further cost savings are possible by using thinnergauge extension wire and a lower temperature rated insulation.

Note: When temperatures within the extension wire's rating are being measured, it is OK to use the extension wire for the entire circuit. This is frequently done with T type extension wire, which is accurate over the -60 to 100°C range.

Thermocouple wire gauge

At high temperatures, thermocouple wire can under go irreversible changes in the form of modified crystal structure, selective migration of alloy components and chemical changes originating from the surface metal reacting to the surrounding environment. With some types, mechanical stress and cycling can also induce changes.

Increasing the diameter of the wire where it is exposed to the high temperatures can reduce the impact of these effects.

The following table can be used as a very approximate guide to wire gauge:

At these higher temperatures, the thermocouple wire should be protected as much as possible from hostile gases. Reducing or oxidizing gases can corrode some thermocouple wire very quickly.

Remember, the purity of the thermocouple wire is most important where the temperature gradients are greatest. It is with this part of the thermocouple wiring where the most care must be taken.

Other sources of wire contamination include the mineral packing material and the protective metal sheath. Metallic vapour diffusion can be significant problem at high temperatures. Platinum wires should only be used inside a nonmetallic sheath, such as high-purity alumna.

Neutron radiation (as in a nuclear reactor) can have significant permanent impact on the thermocouple calibration. This is due to the transformation of metals to different elements.

High temperature measurement is very difficult in some situations. In preference, use non-contact methods. However this is not always possible, as the site of temperature measurement is not always visible to these types of sensors.

Colour coding of thermocouple wire

The colour coding of thermocouple wire is something of a nightmare! There are at least seven different standards. There are some inconsistencies between standards, which seem to have been designed to confuse. For example the colour red in the USA standard is always used for the negative lead, while in