Thermocouples

Many combinations of materials have been used to produce acceptable thermocouples, each with its own particular application spectrum. However, very few specific types are now easily available, and covering by far the majority of the temperature and environmental applications.

The standard covers the eight specified and most commonly used thermocouples. These thermocouple types can be subdivided in 3 groups, base metal, Noble (rare) metal and Refractory metal thermocouple.Base Metal Thermocouples

Base metal thermocouple types are composed of common, inexpensive metals such as nickel, iron and copper. The thermocouple types E, J, K, N and T are among this group and are the most commonly used type of thermocouple.

Noble Metal Thermocouples

Noble metal thermocouples are manufactured with wire that is made with precious or “noble” metals like Platinum and Rhodium. The main types are R, S and B.

Refractory Metal Thermocouples

Refractory metal thermocouples are manufactured with the exotic metals tungsten and Rhenium. These metals are expensive, difficult to manufacture and wire made with these metals are very brittle.

Refractory metal thermocouples are manufactured with wire that is made from the exotic metals Tungsten and Rhenium. These metals are expensive, difficult to manufacture and wire made with these metals are very brittle. These thermocouples are intended to be used in vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at temperatures above 300°C. There are several different combinations of alloys that have been used in the past but only one (C Type) generally used at this time.

• Type C – Type C – The type C thermocouple is made with a tungsten/5% rhenium positive leg and tungsten 26% Rhenium negative leg and has a temperature range of 0 – 2320°C (32 – 4208 °F).
• Type G- Type G thermocouple technically also known as WM26Re. The type G thermocouple has alloy combination of tungsten (W) as positive lead and tungsten + 26% Rhenium (W-26% Re) as negative lead. Maximum useful temperature range of this thermocouple is 0 to 2320°C.
• Type D- Type D- Type D thermocouple technically also known as W3ReM25Re. Type D thermocouple has alloy combination of tungsten + 3% rhenium (W-3%Re) as positive lead and tungsten + 25 % Rhenium (W-56% Re) as negative lead. Maximum useful temperature range of this thermocouple is 0 to 2320°C./li>

• ASTM E 235: Standard Specification for Thermocouples, Sheathed, Type K and Type N for Nuclear or for other High-Reliability Applications.
• ASTM E 839: Standard Test Methods for Sheathed Thermocouples and Sheathed Thermocouple Cable.
• ASTM E 220: Test Methods for Calibration of Thermocouples by Comparison Techniques
• ASTM E 230: Specification and Temperature-EMF Tables for Standardized Thermocouples.
• ASTM E 585: Standard specification for compacted MI, MS, base metal thermocouple cables.
• ASTM E 608: Standard specification for compacted MI, MS, base metal thermocouples.
• ASTM E 696: Standard specifications for tungsten – rhenium alloy thermocouple wire.
• ASTM E 1652: ASTM E 1652: Standard specification for Magnesium Oxide & Aluminum Oxide powder & crushable insulators used in metal sheathed PRT’s, noble metal thermocouples, base metal thermocouples, and their respective cables.
• IS 12579: Specification for Base Metal Mineral Insulated Thermocouple Cables and Thermocouples.
• GB/T 1598- 2010: Chinese standard for platinum thermocouples.
• IEC 584: International standard for thermocouples.

To form the hot junction, a suitable method has to be adopted to obtain a good electrical contact between the thermocouple wires.
For Chromal/Alumal and other combinations, for use in high temperature measurements, welding is the only method to obtain a suitable joint. For this purpose Tig welding & Laser beam welding is mostly used.

Tig welding

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas.

Laser beam welding

Laser beam welding (LBW) is a welding technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum and titanium. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces.

• Because Thermistors are semiconductors, they are more susceptible to permanent de-calibration at high temperatures than are RTD’s or thermocouple.
• The use of thermistors is generally limited to a few hundred degree Celsius and manufacturers warn that extended exposures will cause the thermistor to drift out of its specified tolerance.
• Thermistors can be made very small which means they will respond quickly to temperature changes. It also means that their small thermal mass makes them especially susceptible to self-heating errors.
• Thermistors are good deal more fragile than RTD’s or thermocouple and they must be carefully mounted to avoid crushing or bond separation.

• To select an ideal thermocouple, first we need to understand the need of measurement application.
  • Factors affecting temperature change
  • Accuracy required- impact of sensor accuracy on overall measurement accuracy.
  • Length of deployment
  • Thermocouple material selection
  • Selection of the Measuring Junction
  • Durability
• Range of temperature which is to be measured.
  • Determine the maximum and minimum range in which you want to measure the temperature and select the thermocouple with higher Maximum temperature range.
  • Check whether the linearity of thermocouple meets the range requirement.
• Environmental Consideration
  • Select the correct sheath material to resist chemical reaction.
  • Perfect isolation to resist noise protection.
  • Thermocouple should withstand vibration and abrasion.
• Appropriate connectors and cables to be used between thermocouple and measuring instruments.
• Appropriate measuring instrument should be used to give an accurate result.

Thermocouple is a pair of dissimilar metal wires joined at one end, which generate a net thermoelectric voltage between open pair according to the temperature difference between the ends.

Working:

• Thermocouple works on Seebeck Effect, which forms the basis of modern thermocouple technology.
• When two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.
• The junction that is put into the process in which temperature is being measured is called the HOT JUNCTION. The other junction which is at the last point of thermocouple material and which is almost always at some kind of measuring instrument is called the COLD JUNCTION.

Thermocouples are suitable for measuring over a large temperature range, up to 2300ºC. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0-100ºC with 0.1 ºC accuracy. For such applications thermistors and resistance temperature detectors are more suitable. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. Some other applications are as follows:

• Steel Industry
• Cement Industry
• Pharmaceutical Industry
• Petrochemical Industry
• Nuclear Industry
• Power Industry
• Laboratories
• Furnace Industry

There are two types of thermocouple construction used most commonly. These are MI (Mineral Insulated) Thermocouples & Non-MI Thermocouples.

Mineral Insulated Thermocouples:

Mineral (Mostly Magnesium Oxide) insulated thermocouples, are used in many process and laboratory applications. They are rugged in nature and bendable, and their fairly high temperature ratings make MgO thermocouples a popular choice for a multitude of temperature measuring applications.

MgO sensors are constructed by placing an element or elements into a sheath of a suitable material and size, insulating the elements from themselves and the sheath with loose filled or crushable Magnesium Oxide powder or insulators, and then swaging or drawing the filled sheath down to its final reduced size. The swaging process produces an element with highly compacted MgO insulation and provides high dielectric strength insulation between the elements themselves and their sheath.

Mineral insulated Thermocouples consist of thermocouple wire embedded in a densely packed refractory oxide powder insulate all enclosed in a seamless, drawn metal sheath (usually stainless steel).

At one end cores and sheath are welded from a “hot” junction. At the other end, the thermocouple is connected to a “transition” of extension wires, connecting head or connector.

Non M.I. Thermocouples

In Non-M.I. thermocouples, thermocouple wires are either insulated with ceramic beads or ceramic tubes, after insulation of ceramic, covered by a metal sheath (usually stainless steel) and some form of termination (extension lead, connecting head or connector for example) is provided. In this type of construction thermocouple wires are protected from the measuring environment when a sheath protection is provided. The sheath material is dependent on the measuring environment usually stainless steel is used. According to the corrosive environment sheath selection is changed.

This construction does not provide flexibility & not found in small sizes. Not too good mechanical strength.

In Non M.I. construction sheath may be of ceramic or metal as per suitability.

Exposed, Grounded and Ungrounded all types of junctions are formed in both the M.I, & Non M.I. construction.

• Thermocouple are used in for different applications in Petrochemical Industry :
• Multi point thermocouples for Reactors, Crackers and Liquefied gas tanks.
• MI thermocouples for Tube Surface
• Sulphur Recovery Unit
• Cracker and Liquefied Gas in Tanks
• Fluid Catalytic Cracking Unit(FCCU)

In pharmaceutical industries, thermocouples are used for Validation and Process. Various areas of applications are:

FOR VALIDATION

• Autoclave
• Bung Processor
• Dry Heat Sterilizer (tunnel)
• Lypholizer (FD)

FOR PROCESS

• Tank
• Vessels
• Boiler
• Reactor
• Distillation Column
• Dryer
• Granulation

Thermocouple are widely used in different units of Cement plant:

• Kiln
• Clinker Temp.
• Pre-Heater.
• ESP (Electro-Static Precipitators) system.
• Coal Silo.

Thermocouples are having significant role in Steel industries. Area wise, various types of thermocouples are used in different units of steel industry:

STOVE DOME THERMOCOUPLE

Stove dome/hot blast stove is one of the most critical sections in the steel industry. The hot blast has 3 stage operations, either on gas, on blast or bottled up (ready and waiting to be put on blast).

Temperature is one of the major parameter for controlling this critical process. The temperature inside the blast furnace is about 1100°C and high pressure. The high temperature and high pressure makes the assembly very critical.

COKE OVEN THERMOCOUPLE

Coke is the most important raw material fed into the blast furnace in terms of its effect on blast furnace operation and hot metal quality. The coke making process involves carbonization of coal to high temperatures (1100°C) in an oxygen deficient atmosphere in order to concentrate the carbon.

HEARTH REFRACTORY THERMOCOUPLE

Refractory thermocouples are very important for the startup phase and running phase of the refractory in the steel plants. These are normally very long length thermocouples (up to 40 meters) in mineral insulated construction. Thermocouples of various lengths can be grouped together in a single flange, for a particular area of the furnace.

DRI KILN THERMOCOUPLES

Fast response thermocouple normally K type with miniature or Standard connector for DRI kiln application and normally used with a hand held indicator for checking immediate temperature.

The junction tip of Mineral insulated thermocouple can be of three types as described previously. The tip can be insulated, grounded and reduced type.

• Insulated Tip: Insulated hot end junctions are suitable for most applications, especially where low EMF pick-up is essential. High insulation resistance is enhanced due to extreme compaction of the high purity MgO powder insulation.
• Grounded Tip: Bonded or grounded junctions offer a slightly faster temperature response than the insulated junction type. Not recommended for multi-point instrumentation.
• Reduced Tip: These junctions are ideal for applications where low mass and extremely fast response times are required, together with good mechanical strength. Reduced tip can be provided on 1.0 to 6.0 mm diameter thermocouples.
  

Three alternative tip configurations are usually offered:

• An exposed (measuring) junction is recommended for themeasurement of flowing or static non-corrosive gas temperature whenthe greatest sensitivity and quickest response is required.
  
• An insulated junction is more suitable for corrosive media althoughthe thermal response is slower. In some applications where more thanone thermocouple connects to the associated instrumentation,insulation may be essential to avoid spurious signals occurring in themeasuring circuits. If not specified, this is the standard.
  
• An earthed (grounded) junction is also suitable for corrosive mediaand for high pressure applications. It provides faster response thanthe insulated junction and protection not offered by the exposedjunction.
  

• Small over all dimension and high flexibility, which enable temperature measurement in location with poor accessibility.
• Good mechanical strength.
• Protection of the thermo element wires against oxidation, corrosion and contamination.
• Fast thermal response.

The mineral oxides used for insulation are highly hygroscopic and open-ended cables must be effectively sealed (usually with epoxy resins) to prevent moisture take-up. A carefully prepared mineral insulated thermocouple will normally have a high value of Insulation Resistance (many hundreds of Mega Ohms).

Tolerances on Temperature Reading

Tolerance denotes the maximum allowable value obtained by subtracting the temperature reading or the temperature at the hot junction from the standard temperature converted from the applicable temperature EMF table.

Maximum Operating Temperature

Operating temperature limit means the upper temperature where thermocouple can be used continuously. Maximum limit means the upper temperature where thermocouple can be used temporarily for short period of time owing to unavoidable circumstances.

Principal factors that affect the life of a thermocouple are:

• Temperature: Thermocouple life decreases by about 50% when an increase of 500 °C occurs.
• Diameter: By doubling the diameter of the wire, the life increases by 2-3 times.
• Thermic cycling: When thermocouples are exposed to thermic cycling from room temperature to above 500ºC, their life decreases by about 50% compared to a thermocouple used continuously at the same temperature.
• Protection: When thermocouples are covered by a protective sheath and placed into ceramic insulators, their life is considerably extended.

Thermocouple Response Times

The response time for a thermocouple is usually defined as the time taken for the thermal voltage (output) to reach 63.2% of maximum for the step change temperature. It is dependent on several parameters including the thermocouple dimension, construction, tip configuration and the nature of the medium in which the sensor is located.

Immersion Length

Thermocouple assemblies are” tip” sensing devices which lend them to both surface and immersion applications depending on their construction. However immersion type must be used carefully to avoid error due to stem conduction from the process which can result in a high or low reading respectively. A general rule is to immerse into the medium to a minimum of 4 times the outside diameter of the sheath; no quantitative data applies but care must be exercised in order to obtain meaningful results.

Surface Temperature Measurement

Although thermocouple assemblies are primarily tip sensing devices, the use of protection tubes renders surface sensing impractical. Physically, the probe does not lend itself to surface presentation and stem conduction would cause reading errors. If thermocouple is to be used reliably for surface sensing, it must be either exposed, welded junction from with very small thermal mass or be housed in a construction, which permits true surface contact when attaching to the surface.