Sunday, August 7, 2011


In the world of electronics, thermocouples are a widely used temperature sensors to turn the heat difference in the measured object to the change potesial temperature / voltage (voltage). Simple thermocouple can be installed, and have the same type of standard connectors, and can measure temperature within a temperature range large enough to limit the measurement error of less than 1 ° C.  In 1821, an Estonian physicist named Thomas Johann Seebeck discovered that a conductor (like metal) that were given different heat gradients will produce an electric voltage. This is called the thermoelectric effect. To measure the heat change is a combination of two kinds of conductors as well as frequently used at the end of a hot object being measured. This additional conductor will then have the temperature gradation, and change the voltage in contrast to the differences in body temperature. Using different metals to complete the circuit will produce a different voltage, leaving little difference in voltage allows us to make measurements, which increases according to temperature.
This difference is usually between 1 to 70 microvolt per degrees Celsius for the range of the resulting combination of modern metal. Some combinations are becoming popular as an industry standard, judging from the cost, ketersediaanya, convenience, melting point, chemical capability, stability, and results. It's important to remember that the thermocouples measure the temperature difference between two points, not absolute temperature. In many applications, one of the connections that cold-preserved as a reference temperature, while others are linked to the object of measurement. example, in the picture above, the relationship will be placed on a copper cold on the circuit board. Another temperature sensor will measure the temperature at this point, so the temperature at the tip of the object being examined can be calculated. Thermocouples can be connected in series with each other to make termopile, where each connection that heat is directed to higher temperatures and cold all connections to the lower temperature. Thus, the voltage at each thermocouple rises, allowing it to be used at higher voltages. With the constant temperature in a cold connection, which is useful for measurements in the lab, simply use the thermocouple is not easy for most direct indication of the connection and control instruments. They add cold artificial connection to their circuitry is other equipment that is sensitive to temperature (such as a thermistor or diode) to measure the temperature of the input connection on the equipment, with the specific aim of reducing the temperature gradation in between ujungujungnya. Here, the voltage derived from a known cold relationship can be simulated, and good correction can be applied. This is known as cold relationship compensation. Usually, the thermocouple is connected by means of an indication by a wire called an extension cord or compensation. The goal is clear. Extension cords used with the same amount kawatkawat with kondoktur used in thermocouple itself. Kabelkabel is cheaper than the thermocouple wires, although not too cheap, and usually produced in a form appropriate for transport over long distances - typically as a closed flexible wire or multi-core cables. Kabelkabel usually have a specification for a larger temperature range than the thermocouple wires. These cables are recommended for high accuracy. Cable compensation on the other hand, is less precise, but cheaper. They wear small differences, usually a mixture of cheap conductor material having the same thermoelectric coefficient with thermocouples (working on a limited temperature range), with results that are not as accurate as extension cables. This combination produces output similar to the thermocouple, but the operating temperature range on the cable compensation is limited to keep the small errors obtained. Extension or compensating cable must be selected according to the needs of the thermocouple. This selection generates a voltage proportional to the temperature difference between hot and cold connections, and the poles must be properly connected so that the additional voltage added to the thermocouple voltage, replace the temperature difference between hot and cold connections. The relationship between temperature difference voltage generated by thermocouple is not a linear function but the function of an interpolation polynomial coefficients has n between 5 and 9. In order to obtain accurate measurement results, the equation is usually implemented on a digital controller or stored in a table of observations. Some older equipment using analog filters.  There are several types of thermocouples, depending on application use Type K (Chromel (NiCr alloy) / Alumel (NiAlalloy)) thermocouple for general purpose. Cheaper. Available for the temperature range -200 ° C to +1200 ° C. Type E (Chromel / Constantan (CuNialloy)) Type E has a large output (68 μV / ° C) makes it suitable for use at low temperatures. Other properties of type E is a non-magnetic type. Type J (Iron / Constantan) The range is limited (-40 to +750 ° C) makes it less popular than type K type A has a sensitivity of ~ 52 μV / ° C Type N (Nicrosil (NiCrSi alloy) / Nisil (NiSi alloy)) stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without platinum. Can measure temperatures above 1200 ° C. Sensitivity is about 39 μV / ° C at 900 ° C, slightly below the type K. Type N is the improvement of K-type thermocouple types B, R, and S is a noble metal thermocouples that have similar characteristics. They are the most stable of all thermocouples, but due to low sensitivity (about 10 μV / ° C) they are usually only used to measure high temperatures (> 300 ° C). Type B (PlatinumRhodium / PtRh) Suitable temperature measurements up to 1800 ° C. Type B gives the same output at 0 ° C to 42 ° C so it can not be used below 50 ° C. Type R (Platinum / Platinum with 7% Rhodium) Suited for high temperature measurements up to 1600 ° C. low sensitivity (10 μV / ° C) and high cost makes them not suitable for general purpose use. Type S (Platinum / Platinum with 10% Rhodium) Suited for high temperature measurements up to 1600 ° C. low sensitivity (10 μV / ° C) and high cost makes them unsuitable for general purposes. Because of its high stability type S is used for standard measurements of the melting point of gold (1064.43 ° C).
Type T (Copper / Constantan) Suitable for measurements from -200 to 350 ° C. Positive conductor is made of copper, and a negative made of Constantan. Often used as an alternative gauge copper wire from the study. Type T has a sensitivity ~ 43 μV / ° C. Thermocouples are most suitable for measuring a wide range of temperatures, up to 1800 K. In contrast, less suitable for measurements where small temperature differences should be measured with high accuracy levels, for example 0100 ° C temperature range with accuracy of 0.1 ° C. For these applications, thermistor and RTD are more suitable. Thermocouples use a common example among others:
o Iron and Steel Industry o Safety on alatalat heater
o To termopile radiation sensors
o radioisotope thermal power plant, one of the applications termopile.

No comments:

Post a Comment