毕业设计文献翻译热电偶温度传感器(5)

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

German and Japanese standards it is always the positive lead. The British, French and International standards avoid the use of red entirely!

Thermocouple mounting

There are four common ways in which thermocouples are mounted with in a stainless steel or Inconel sheath and electrically insulated with mineral oxides. Each of the methods has its advantages and disadvantages.

Sealed and Isolated from Sheath: Good relatively trouble-free arrangement. The principal reason for not using this arrangement for all applications is its sluggish response time - the typical time constant is 75 seconds

Sealed and Grounded to Sheath: Can cause ground loops and other noise injection, but provides a reasonable time constant (40 seconds) and a sealed enclosure.

Exposed Bead: Faster response time constant (typically 15 seconds), but lacks mechanical and chemical protection, and electrical isolation from material being measured. The porous insulating mineral oxides must be sealed

Exposed Fast Response: Fastest response time constant, typically 2 seconds but with fine gauge of junction wire the time constant can be 10-100 ms. In addition to problems of the exposed bead type, the protruding and light construction makes the thermocouple more prone to physical damage.

Thermocouple compensation and linearization

As mentioned above, it is possible to provide reference junction compensation in hardware or in software. The principal is the same in both cases: adding a correction voltage to the thermocouple output voltage, proportional to the reference junction temperature. To this end, the connection point of the thermocouple wires to the measuring device (i.e. where the thermocouple materials change to the copper of the circuit electronics) must be monitored by a sensor. This area must be design to be isothermal, so that the sensor accurately tracks both reference junction temperatures.

The hardware solution is simple but not always as easy to implement as one might expect.

The circuit needs to be designed for a specific thermocouple type and hence lacks the flexibility of the software approach.

The software compensation technique simplifies the hardware requirement, by eliminating the reference sensor amplifier and summing circuit (although a multiplexer may be required).

The software algorithm to process the signals needs to be carefully written. A sample algorithm details the process.

A good resource for thermocouple emf tables and coefficients is at the US Commerce Dept's NIST web site. It covers the B, E, J, K, N, R, S and T types.

The thermocouple as a heat pump

The thermocouple can function in reverse. If a current is passed through a thermocouple circuit, one junction will cool and the other warm. This is known as the Peltier Effect and is used in small cooling systems. The effect can be demonstrated by alternately passing a current through a thermocouple circuit and then quickly measuring the circuit's Seebeck voltage. This process has been used, with very fine thermocouple wire (0.025 mm with about a 10 mA current), to measure humidity by ensuring the cooled junction drops below the air's dew point. This causes condensation to form on the cooled junction. The junction is allowed to return to ambient, with the temperature curve showing an inflection at the dew point caused by the latent heat of