The Type B thermocouple is used in extremely high temperature applications. It has the highest temperature limit of all of the thermocouples listed above. It maintains a high level of accuracy and stability at very high temperatures. The type B has a lower output than the other noble metals (type R & type S) at temperatures below 1,112F (600C).
Types Of Thermocouples Pdf Download
DI-2008 USB Thermocouple Data Acquisition system with Windaq software. Features 4 or 8 analog input channels programmable as a thermocouple or voltage input, 16-bit measurement resolution, and a full scale range of 10mV to 50V for voltage measurements or Thermocouple types J, K, T, B, R, S, E, or N. Includes a DI-2008 instrument, a 6-foot USB cable, a screwdriver for signal connections, an NIST-traceable calibration certificate, and downloadable WinDaq software
Commercial thermocouples are inexpensive,[2] interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self-powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (C) can be difficult to achieve.[3]
In 1821, the German physicist Thomas Johann Seebeck discovered that a magnetic needle held near a circuit made up of two dissimilar metals got deflected when one of the dissimilar metal junctions was heated. At the time, Seebeck referred to this consequence as thermo-magnetism. The magnetic field he observed was later shown to be due to thermo-electric current. In practical use, the voltage generated at a single junction of two different types of wire is what is of interest as this can be used to measure temperature at very high and low temperatures. The magnitude of the voltage depends on the types of wire being used. Generally, the voltage is in the microvolt range and care must be taken to obtain a usable measurement. Although very little current flows, power can be generated by a single thermocouple junction. Power generation using multiple thermocouples, as in a thermopile, is common.
Thermocouple manufacturers and metrology standards organizations such as NIST provide tables of the function E ( T ) \displaystyle \scriptstyle E(T) that have been measured and interpolated over a range of temperatures, for particular thermocouple types (see External links section for access to these tables).
Thermocouples ideally should be very simple measurement devices, with each type being characterized by a precise E ( T ) \displaystyle \scriptstyle E(T) curve, independent of any other details.In reality, thermocouples are affected by issues such as alloy manufacturing uncertainties, aging effects, and circuit design mistakes/misunderstandings.
A common error in thermocouple construction is related to cold junction compensation. If an error is made on the estimation of T r e f \displaystyle T_\mathrm ref , an error will appear in the temperature measurement. For the simplest measurements, thermocouple wires are connected to copper far away from the hot or cold point whose temperature is measured; this reference junction is then assumed to be at room temperature, but that temperature can vary.[4] Because of the nonlinearity in the thermocouple voltage curve, the errors in T r e f \displaystyle T_\mathrm ref and T s e n s e \displaystyle T_\mathrm sense are generally unequal values. Some thermocouples, such as Type B, have a relatively flat voltage curve near room temperature, meaning that a large uncertainty in a room-temperature T r e f \displaystyle T_\mathrm ref translates to only a small error in T s e n s e \displaystyle T_\mathrm sense .
One common myth regarding thermocouples is that junctions must be made cleanly without involving a third metal, to avoid unwanted added EMFs.[6]This may result from another common misunderstanding that the voltage is generated at the junction.[7] In fact, the junctions should in principle have uniform internal temperature; therefore, no voltage is generated at the junction. The voltage is generated in the thermal gradient, along the wire.
A special case of thermocouple wire is known as "extension grade", designed to carry the thermoelectric circuit over a longer distance.Extension wires follow the stated E ( T ) \displaystyle \scriptstyle E(T) curve but for various reasons they are not designed to be used in extreme environments and so they cannot be used at the sensing junction in some applications.For example, an extension wire may be in a different form, such as highly flexible with stranded construction and plastic insulation, or be part of a multi-wire cable for carrying many thermocouple circuits.With expensive noble metal thermocouples, the extension wires may even be made of a completely different, cheaper material that mimics the standard type over a reduced temperature range.[5]
Thermocouples are often used at high temperatures and in reactive furnace atmospheres. In this case, the practical lifetime is limited by thermocouple aging. The thermoelectric coefficients of the wires in a thermocouple that is used to measure very high temperatures may change with time, and the measurement voltage accordingly drops. The simple relationship between the temperature difference of the junctions and the measurement voltage is only correct if each wire is homogeneous (uniform in composition). As thermocouples age in a process, their conductors can lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures. If the aged section of the thermocouple circuit is exposed to a temperature gradient, the measured voltage will differ, resulting in error.
Certain combinations of alloys have become popular as industry standards. Selection of the combination is driven by cost, availability, convenience, melting point, chemical properties, stability, and output. Different types are best suited for different applications. They are usually selected on the basis of the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the chemical inertness of the thermocouple material and whether it is magnetic or not. Standard thermocouple types are listed below with the positive electrode (assuming T sense > T ref \displaystyle T_\textsense>T_\textref ) first, followed by the negative electrode.
Designed at the Defence Science and Technology Organisation (DSTO) of Australia, by Noel A. Burley, type-N thermocouples overcome the three principal characteristic types and causes of thermoelectric instability in the standard base-metal thermoelement materials:[14]
Type N thermocouples are suitable alternative to type K for low-oxygen conditions where type K is prone to green rot. They are suitable for use in vacuum, inert atmospheres, oxidizing atmospheres, or dry reducing atmospheres. They do not tolerate the presence of sulfur.[16]
Types B, R, and S thermocouples use platinum or a platinum/rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/C. Type B, R, and S thermocouples are usually used only for high-temperature measurements due to their high cost and low sensitivity. For type R and S thermocouples, HTX platinum wire can be used in place of the pure platinum leg to strengthen the thermocouple and prevent failures from grain growth that can occur in high temperature and harsh conditions.
These thermocouples are well-suited for measuring extremely high temperatures. Typical uses are hydrogen and inert atmospheres, as well as vacuum furnaces. They are not used in oxidizing environments at high temperatures because of embrittlement.[19] A typical range is 0 to 2315 C, which can be extended to 2760 C in inert atmosphere and to 3000 C for brief measurements.[20]
The table below describes properties of several different thermocouple types. Within the tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. For example, a thermocouple with a tolerance of 0.0025T would have a tolerance of 2.5 C at 1000 C. Each cell in the Color Code columns depicts the end of a thermocouple cable, showing the jacket color and the color of the individual leads. The background color represents the color of the connector body.
When wire insulation disintegrates, it can result in an unintended electrical contact at a different location from the desired sensing point. If such a damaged thermocouple is used in the closed loop control of a thermostat or other temperature controller, this can lead to a runaway overheating event and possibly severe damage, as the false temperature reading will typically be lower than the sensing junction temperature. Failed insulation will also typically outgas, which can lead to process contamination. For parts of thermocouples used at very high temperatures or in contamination-sensitive applications, the only suitable insulation may be vacuum or inert gas; the mechanical rigidity of the thermocouple wires is used to keep them separated.
Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, type S thermocouples are regularly used in the electric arc furnace process to accurately measure the temperature of steel before tapping. The cooling curve of a small steel sample can be analyzed and used to estimate the carbon content of molten steel.
Thermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. For example, switchgear under test for its current carrying capacity may have thermocouples installed and monitored during a heat run test, to confirm that the temperature rise at rated current does not exceed designed limits. 2ff7e9595c