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Thermocouple

About D.E.V.I.C.E.
About D.E.V.I.C.E.

D.E.V.I.C.E. is an encyclopedia of terms used by manufacturers of test and measurement equipment. T&M Atlantic created this service to better explain the functionality of instruments it offers, and to highlight the latest developments in the world of T&M Atlantic. We are using such tools as flash and animation to bring words and pictures to life and to create not just an understanding but also an appreciation for technology that goes into the design of every instrument.

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A thermocouple consists of two conductors of different materials (usually metal alloys) that produce a voltage in the vicinity of the point where the two conductors are in contact. The voltage produced is dependent on, but not necessarily proportional to, the difference of temperature of the junction to other parts of those conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a temperature gradient into electricity. Commercial themocouples are inexpensive, 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.

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouple types

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 based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether it is magnetic or not. Standard thermocouple types are listed below with the positive electrode first, followed by the negative electrode.

Type K (chromel {90% nickel and 10% chromium}—alumel {95% nickel, 2% manganese, 2% aluminium and 1% silicon}) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel. It is inexpensive, and a wide variety of probes are available in its −200C to +1250C / -328F to + 2282F range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics may vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 350C / 662F.

Type E (chromel–constantan) has a high output (68 V/C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic. Wide range is -50 to 740C / -58 to 1364Fand Narrow range is -110 to 140C / -166 to 284F.

Type J (ironconstantan) has a more restricted range than type K (-40 to +750C / -40 to 1382F), but higher sensitivity of about 55 V/C. The Curie point of the iron (770C / 1418F) causes an abrupt change in the characteristic, which determines the upper temperature limit.

Type N (NicrosilNisil) (nickel-chromium-silicon/nickel-silicon) thermocouples are suitable for use between -270C and 1300C / -454 and2372Fowing to its stability and oxidation resistance. Sensitivity is about 39 V/C at 900C / 1652F, slightly lower compared to type K.

Type B thermocouples use a platinumrhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800C / 3272F. Type B thermocouples produce the same output at 0C and 42C /32F and 107.6F, limiting their use below about 50C / 122F.

Type R thermocouples use a platinumrhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600C /2912F.

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600C /2912F. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43C / 1947.98F).

Type T (copper constantan) thermocouples are suited for measurements in the −200 to 350C range / -328 to 662F. Often used as a differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 V/C.

Type C (tungsten 5% rhenium tungsten 26% rhenium) thermocouples are suited for measurements in the 0C to 2320C /32F to 4208Frange. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures. It must never be used in the presence of oxygen at temperatures above 260C / 500F.

Type M thermocouples use a nickel alloy for each wire. The positive wire (20 Alloy) contains 18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. These thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400C / 2552F. It is less commonly used than other types.

Type Temperature range (continuous) Temperature range (short term) Tolerance class one Tolerance class two IEC Color code BS Color code ANSI Color code
K 0 to 1100C /32 to 2012F -180 to 1300C / -292 to 2372F ±1.5 between -40 and 375C / -40 and 707F
0.004×T between 375 and 1000C /707 and 1832F
2.5 between -40 and 333C / -40 and 631.4F
0.0075?T between 333 and 1200C /631.4 and 2192F
J 0 to 750C /32 to 1382F -180 to 800C / -292 to 1472F 1.5 between -40 and 375? / -40 and 707F
0.004?T between 375 and 750? /707 and 1382F
2.5 between -40 and 333C / -40 and 631.4F
0.0075?T between 333 and 750C /631.4 and 1382F
N 0 to 1100C /32 to 2012F -270 to 1300C /-454 to 2372F 1.5 between -40 and 375? / -40 and 707F
0.004?T between 375 and 1000? /707 and 1832F
2.5 between -40 and 333C / -40 and 631.4F
0.0075?T between 333 and 1200C /631.4 and 2192F
R 0 to 1600C /32 to 2912F -50 to 1700C / -58 to 3092F 1.0 between 0 and 1100? / 32 and 2012F
[1 + 0.003?(T − 1100)] between 1100 and 1600C /2012 and 2912F
1.5 between 0 and 600C /32 and 1112F
0.0025?T between 600 and 1600C /1112 and 2912F
Not defined.
S 0 to 1600C / 32 to 2912F -50 to 1750C / -58 to 3182F 1.0 between 0 and 1100? /32 and 2012F
[1 + 0.003?(T − 1100)] between 1100 and 1600? /2012 and 2912F
1.5 between 0 and 600C /32 and 1112F
0.0025?T between 600 and 1600C /1112 and 2912F
Not defined.
B 200 to 1700C /392 to 3092F 0 to 1820C /32 to 3308F Not Available 0.0025?T between 600 and 1700C / 1112 and 3092F No standard use copper wire No standard use copper wire Not defined.
T -185 to 300C / -301 to 572F -250 to 400C / -418 to 752F 0.5 between -40 and 125C / -40 and 257F
0.004?T between 125 and 350C /257 and 662F
1.0 between -40 and 133C / -40 and 271.4F
0.0075?T between 133 and 350C /271.4 and 662F
E 0 to 800C /32 to 1472F -40 to 900C / -40 to 1652F 1.5 between -40 and 375C / -40 and 707F
0.004?T between 375 and 800C /707 and 1472F
2.5 between -40 and 333C / -40 and 631.4F
0.0075?T between 333 and 900C /631.4 and 1652F
Chromel/AuFe -272 to 300C /-457.6 to 572F n/a Reproducibility 0.2% of the voltage; each sensor needs individual calibration.

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