Interview Questions.

Question 1. What Is A Thermocouple?

Answer :

A Thermocouple is a sensor used to degree temperature. Thermocouples consist of  twine legs made from one-of-a-kind metals. The wires legs are welded collectively at one give up, creating a junction. This junction is wherein the temperature is measured. When the junction studies a change in temperature, a voltage is created. The voltage can then be interpreted using thermocouple reference tables (connected) to calculate the temperature.
There are many kinds of thermocouples, every with its very own particular traits in phrases of temperature variety, sturdiness, vibration resistance, chemical resistance, and application compatibility. Type J, K, T, & E are “Base Metal” thermocouples, the most not unusual types of thermocouples. Type R, S, and B thermocouples are “Noble Metal” thermocouples, that are utilized in excessive temperature programs (see thermocouple temperature ranges (linked) for information).
Thermocouples are used in lots of industrial, clinical, and OEM packages. They may be observed in almost all industrial markets: Power Generation, Oil/Gas, Pharmaceutical, BioTech, Cement, Paper & Pulp, and many others. Thermocouples are also used in normal appliances like stoves, furnaces, and toasters.
Thermocouples are generally selected due to their low fee, high temperature limits, extensive temperature tiers, and sturdy nature.
Question 2. What Is A Cold (reference) Junction For Thermocouples?

Answer :

Cold, or reference junction, is the end of a thermocouple that gives a reference point.

Thermocouples measure the distinction in temperature among two junctions. They do NOT degree actual temperature. The sensing junction is wherein the thermocouple wires are welded (or in any other case related) together, and placed at a point wherein the temperature is favored. The different junction is normally located in which it is connected to instrumentation (measuring device or transmitter). This is known as the cold or reference junction. Thermocouple millivolt tables and mathematical formulation are based totally on a chilly junction temperature of 0°C. To determine real temperature, the instrumentation ought to “modify” for the difference among ambient temperature and 0°C. This adjustment is referred to as cold junction reimbursement.

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Question three. What Is The Difference Between A Thermocouple And Rtd?

Answer :

Temperature Range: First, don't forget the difference in temperature tiers. Noble Metal Thermocouples can attain three,a hundred F, at the same time as standard RTDs have a restriction of six hundred F and extended range RTDs have a limit of 1,a hundred F.

Cost: A undeniable stem thermocouple is two to three instances much less expensive than a simple stem RTD. A thermocouple head meeting is roughly 50% less high priced than an equal RTD head meeting.

Accuracy, Linearity, & Stability: As a fashionable rule, RTDs are more accurate than thermocouples. This is particularly authentic at decrease temperature stages. RTDs are also more strong and feature higher linearity than thermocouples. If accuracy, linearity, and stability are your primary issues and your utility is within an RTD’s temperature limits, go with the RTD.

Durability: In the sensors enterprise, RTDs are extensively appeared as a less durable sensor while in comparison to thermocouples. However, REOTEMP has advanced manufacturing techniques which have significantly improved the sturdiness of our RTD sensors. These strategies make REOTEMP’s RTDs nearly equal to thermocouples in phrases of sturdiness.

Response Time: RTDs can not be grounded. For this reason, they have a slower response time than grounded thermocouples. Also, thermocouples can be located interior a smaller diameter sheath than RTDs. A smaller sheath diameter will increase reaction time. For example, a grounded thermocouple internal a 1/sixteen” dia. Sheath can have a quicker reaction time than a RTD inner a ¼” dia. Sheath.

Question 4. What Are The Thermocouple Junction Types?

Answer :

Grounded Thermocouples: This is the maximum not unusual junction fashion. A thermocouple is grounded while both thermocouple wires and the sheath are all welded together to form one junction at the probe tip. Grounded thermocouples have a superb response time because the thermocouple is making direct contact with the sheath, permitting heat to transfer effortlessly. A disadvantage of the grounded thermocouple is that the thermocouple is greater liable to electric interference. This is because the sheath regularly comes into touch with the encompassing area, supplying a path for interference.

Ungrounded Thermocouples (Or Ungrounded Common Thermocouples): A thermocouple is ungrounded whilst the thermocouple wires are welded collectively but they may be insulated from the sheath. The wires are regularly separated through mineral insulation.

Exposed Thermocouples (or “naked cord thermocouples”): A thermocouple is exposed while the thermocouple wires are welded together and immediately inserted into the procedure. The reaction time is very short, however uncovered thermocouple wires are more susceptible to corrosion and degradation. Unless your application requires exposed junctions, this style isn't endorsed.

Ungrounded Uncommon: An ungrounded unusual thermocouple consists of a dual thermocouple this is insulated from the sheath and each of the elements are insulated from a different.

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Question five. What Is M.I. Cable?

Answer :

M.I. (Mineral Insulated) cable is used to insulate thermocouple wires from one another and from the steel sheath that surrounds them. MI Cable has  (or four whilst duplex) thermocouple wires running down the center of the tube. The tube is then full of magnesium oxide powder and compacted to make certain the wires are nicely insulated and separated. MI cable facilitates to guard the thermocouple wire from corrosion and electric interference.

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Question 6. How Are The Thermocouple Sheaths Different?

Answer :

316SS (chrome steel): This is the maximum common sheath fabric. It is quite corrosion resistant and is cost effective.

304SS: This sheath isn't as corrosion resistant as 316SS. The value difference among 316SS and 304SS is nominal.

Inconel (registered trademark) 600: This cloth is recommended for exceptionally corrosive environments.

Question 7. What Do Thermocouple Wire Colors Indicate?

Answer :

A thermocouple can be identified by means of the coloration of its wire insulation. For example, in the United States a kind J thermocouple grade wire has one red twine and one white wire, commonly with a brown over jacket. A kind J extension grade twine additionally has one purple cord and one white wire, but it has a black over jacket. As a popular rule the red cord of a thermocouple or extension wire is negative and the high quality wire is color coded in step with the type of thermocouple. Different countries use special colour codes.

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Question 8. What Is The Difference Between Thermocouple Wire Cable And Extension Grade Wire?

Answer :

Thermocouple grade cord is used to manufacture thermocouple probes. Thermocouple grade twine is usually used for the junction and inside the stem sheath. This is due to the fact the thermocouple grade twine has a better accuracy specification than extension grade twine.

Extension grade wire is a less pricey, lower grade wire. It is used to increase the sign from the thermocouple probe to the manipulate system or virtual display. Extension grade wire is greater reasonably priced due to a lesser grade metallic getting used. Extension grade twine need to now not be used inside the manner itself and ought to now not see the temperature extremes & temperature biking as preferred grade twine.

Question 9. What Is Seebeck Effect?

Answer :

When two wires of various metals or steel alloys (thermo wires) are joined collectively in one stop (warm junction), a thermocouple is formed. If there's a temperature difference among the hot junction and the open ends, a thermal electromotive force (a thermal voltage) is created in the thermocouple. This is also known as the Seebeck effect.

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Question 10. What Is Cjc?

Answer :

A thermocouple size continually wishes information from joined twine give up (warm junction) and open wire cease (cold junction). The bloodless junction is also known as reference point. Variations of reference factor temperature are compensated with CJC measuring (Cold Junction Compensation). Temperature transmitters CJC measuring can be an internal characteristic or a measuring resistor incorporated in connectors. If the reference point is far faraway from the transmitter, a separate temperature measuring of that factor has to be implemented and wired to transmitter as reimbursement sign.

Question 11. What Is Compensating Cable And Extension Cable?

Answer :

Compensating cable is a thermocouple measuring circuit cable, that's identified by the letter C (e.G. For kind K cable KC). Wires of the compensating cable have the equal electrical features, but not the same materials, because the thermo wires of the TC sensor. Compensating cable is a more value effective answer than extension cable, however the maximum ambient temperature allowed is lower, about 100…two hundred °C depending at the insulation fabric.

Extension cable is a thermocouple cable that is recognized by way of the letter X (e.G. For type K cable KX). Wires of the extension cable are of precisely the same substances as the thermo wires of the TC sensor. These cables can acquire even the same ambient temperatures because the thermocouple can.

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Question 12. What Is Trace Heating / Sensor?

Answer :

Trace heating is a time period usually used for keeping pipelines and attached gadgets unfrozen. The important feature of trace heating is preserving stable temperature and glide rate of the materials flowing through the pipeline. The most commonplace implementation of trace heating is electrical, which gives good adjustability. However, for correct manner control and adjustment additionally specific temperature information is needed. For these packages we've got designed our excessive best hint heating sensors, which have already got been available for years, additionally for Ex applications.

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Question thirteen. What Size Thermowell Is Appropriate For My Application?

Answer :

Based at the design of your system you want to realize:

Operating temperature (°C)
Operating strain (bar), Specific quantity (m3/kg)
Velocity (m/s)
Once those are established then you definitely’re prepared to consult ASME preferred PTC 19.3 TW-2010 Thermowell section, which goes thru the calculation for the design of the well.

Question 14. Explain Pyrometer And Its Working?

Answer :

A Pyrometer is a non-contacting tool that intercepts and measures thermal radiations. Without making any contact with the radiating body and the process is called pyrometry. This device is beneficial for figuring out the temperature of an object’s floor.

Pyrometer strictly works at the precept of black body radiation. Here emissivity of the target performs an essential function, as it governs how shiny the goal seems to the pyrometer. Due to its high accuracy, speed, economy and specific advantages, it's miles extensively being used as a general system in lots of business programs.

Working:

An optical machine gathers the seen and infrared strength from an item and focuses it to the detector.
The detector receives the photon electricity from the optical gadget and converts it to an electric powered signal to power a temperature display or manipulate unit.
Question 15. Explain Thermocouples Measuring Junction?

Answer :

An exposed (measuring) junction is suggested for the measurement of flowing or static non-corrosive fuel temperature while the finest sensitivity and quickest reaction is required.

An insulated junction is more appropriate for corrosive media despite the fact that the thermal reaction is slower. In some applications in which more than one thermocouple connects to the related instrumentation,insulation may be important to keep away from spurious signals going on within the measuring circuits. If now not specific, that is the usual.

An earthed (grounded) junction is likewise appropriate for corrosive media and for high strain packages. It offers faster reaction than the insulated junction and protection no longer provided by means of the exposed junction.

Thermocouple Standards

ASTM E 235: Standard Specification for Thermocouples, Sheathed, Type K and Type N for Nuclear or for different High-Reliability Applications.
ASTM E 839: Standard Test Methods for Sheathed Thermocouples and Sheathed Thermocouple Materials.
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 steel thermocouple cables.
ASTM E 608: Standard specification for compacted MI, MS, base metallic thermocouples.
ASTM E 696: Standard specs for tungsten – rhenium alloy thermocouple wire.
ASTM E 1652: Standard specification for Magnesium oxide & Alumina oxide powder & crushable insulators used in metal sheathed PRT’s, base steel thermocouples & noble steel thermocouple.
IS 12579: Specification for Base Metal Mineral Insulated Thermocouple Cables and Thermocouples.
GB/T 1598- 2010: Chinese fashionable for platinum thermocouples.
IEC 584: International fashionable for thermocouples.
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Question 16. What Are The Applications Of Thermowells?

Answer :

Thermowells offer protection for temperature probes towards adverse running situations such as corrosive media, bodily impact (e.G. Clinker in furnaces) and high strain fuel or liquid. Their use also allows quick and clean probe interchanging without the need to “open-up” the manner.

The primary utility regions are:

Protection tube are used in thermocouples
Insures integrity in excessive pressure applications
Smaller wells are used in low strain programs
Straight wells are utilized in corrosive and erosive environment
For applications where a short reaction to adjustments in temperature is needed, fabricated pockets may be fitted with a discounted tip.
One-piece Thermowells are proper to the very best manner hundreds, depending on their layout. Thus inside the petrochemical industry, one-piece thermocouples are now used almost exclusively.
Question 17. What Are The Types Of Thermocouple Constructions?

Answer :

There are  types of thermocouple creation used most commonly.These are Mineral Insulated (M.I.) Thermocouples & Non M.I. Thermocouples.

Mineral Insulated Thermocouples:

Magnesium Oxide insulated thermocouples, are used in many technique and laboratory packages. They are rugged in nature and bendable, and their fairly high temperature rankings make MgO thermocouples a popular preference for a mess of temperature measuring applications.
MgO sensors are constructed by using putting an element or elements into a sheath of a suitable fabric and size, insulating the factors from themselves and the sheath with unfastened crammed or crushable Magnesium Oxide powder or insulators, after which swaging or drawing the crammed sheath down to its very last reduced size. The swaging method produces an detail with tremendously compacted MgO insulation and offers high dielectric electricity insulation among the elements themselves and their sheath.
Mineral insulated Thermocouples include thermocouple cord embedded in a densely packed refractory oxide powder insulate all enclosed in a unbroken, drawn metallic sheath (commonly chrome steel).
At one quit cores and sheath are welded from a “warm” junction. At the opposite quit, the thermocouple is hooked up to a “transition” of extension wires, connecting head or connector.
Non M.I. Thermocouples:

In Non-M.I. Thermocouples, thermocouple wires are both insulated with ceramic beads or after insulation of ceramic, protected by means of a metallic sheath (usually stainless steel) and a few shape of termination (extension lead, connecting head or connector for instance) is supplied. In this type of production thermocouple wires are covered from the measuring environment while a sheath protection is supplied. The sheath fabric is depending on the measuring surroundings typically stainless steel is used. According to the corrosive surroundings sheath selection is changed.
This creation does now not provide flexibility & now not observed in small sizes. Not too right mechanical strength.
In Non M.I. Construction sheath may be of ceramic or metal as per suitability.
Exposed, Grounded and Ungrounded all forms of junctions are fashioned in both the M.I, & Non M.I. Construction.
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Question 18. What Are The Principal Factors That Affect The Life Of A Thermocouple?

Answer :

Temperature: Thermocouple lifestyles decreases via about 50% whilst an boom of fifty °C takes place.

Diameter: By doubling the diameter of the wire, the lifestyles increases by way of 2-three times.

Thermic biking: When thermocouples are uncovered to thermic biking from room temperature to above 500°C, their existence decreases by about 50% as compared to a thermocouple used constantly at the equal temperature.

Protection: When thermocouples are covered by means of a protecting sheath and placed into ceramic insulators, their life is significantly prolonged.

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Question 19. How To Choose A Thermocouple ?

Answer :

Because a thermocouple measures in wide temperature levels and can be tremendously rugged, thermocouples are very often used in enterprise.

The following criteria are used in selecting a thermocouple:

Temperature range
Chemical resistance of the thermocouple or sheath cloth
Abrasion and vibration resistance
Installation requirements (may need to be like minded with existing device; present holes may additionally determine probe diameter).
Question 20. What Is The Response Time Of A Thermocouple?

Answer :

A time steady has been defined as the time required by way of a sensor to reach sixty three.2% of a step trade in temperature under a unique set of situations. Five time constants are required for the sensor to approach a hundred% of the step exchange cost. An uncovered junction thermocouple gives the quickest response. Also, the smaller the probe sheath diameter, the quicker the reaction, however the maximum temperature can be lower. Be aware, but, that occasionally the probe sheath cannot resist the entire temperature variety of the thermocouple type.

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Question 21. How Do We Know Which Junction Type To Choose?

Answer :

Sheathed thermocouple probes are to be had with one among three junction sorts: grounded, ungrounded or uncovered. At the top of a grounded junction probe, the thermocouple wires are physically attached to the inner of the probe wall. This results in exact warmness switch from the out of doors, via the probe wall to the thermocouple junction. In an ungrounded probe, the thermocouple junction is indifferent from the probe wall. Response time is slower than the grounded fashion, however the ungrounded offers electric isolation.

Question 22. How To Choose Between Thermocouples, Resistance Temperature Detectors (rtd’s), Thermistors And Infrared Devices?

Answer :

You need to recall the characteristics and expenses of the numerous sensors in addition to the to be had instrumentation. In addition, Thermocouples commonly can degree temperatures over wide temperature ranges, inexpensively, and are very rugged, however they may be no longer as correct or stable as RTD’s and thermistors. RTD’s are strong and feature a fairly wide temperature variety, but are not as rugged and less expensive as thermocouples. Since they require the usage of electric contemporary to make measurements, RTD’s are difficulty to inaccuracies from self-heating. Thermistors tend to be greater correct than RTD’s or thermocouples, however they have a much more constrained temperature range. They are also concern to selfheating. Infrared Sensors may be used to measure temperatures better than any of the alternative devices and achieve this without direct touch with the surfaces being measured. However, they may be typically not as accurate and are touchy to floor radiation efficiency (or greater precisely, surface emissivity). Using fiber optic cables, they could measure surfaces that aren't inside a direct line of sight.

Question 23. What Are The Thermocouple Types ?

Answer :

Thermocouples are to be had in distinct combos of metals or calibrations. The maximum commonplace are the “Base Metal” thermocouples called Types J, K, T, E and N. There are also high temperature calibrations – als referred to as Noble Metal thermocouples – Types R, S, C and GB.

K Type Thermocouples are known as wellknown motive thermocouple due to its low cost and temperature variety.

This excessive temperature twine/thermocouple is produced the use of a huge range of substances, and is to be had with PVC, FEP, TFE, PFA, Fiberglass, and Ceramaflex insulation. However, in case you see it referenced by means of “type” on a spec sheet, the description is regarding the type of metal alloy used for the twine’s conductor.

Type E = Chromel/Constantan
Type J = Iron/Constantan
Type K = Chromel/Alumel
Type N = Nicrosil/Nisil
Type T = Copper/Constantan
When an “X” follows the letter indicating which alloy changed into used inside the cable, the cable is extension grade. In those instances, the forms of cable are listed as Type EX, Type JX, Type KX, Type NX, or Type TX.

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Question 24. What Is The Difference Between Thermocouple Grade Wire And Extension Grade Wire?

Answer :

Thermocouple grade twine is wire that is used to make the sensing point of the instrument, in which extension grade twine is best used to increase a thermocouple sign from a probe again to the instrument reading the sign.

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Question 25. Explain Thermocouple Material Selection ?

Answer :

The various sheath materials are based upon the utility. The following list will help you are making the fine choice:

304 SS

Maximum temperature of 1650°F (900°C) and is the maximum extensively used low temperature sheath material. It gives accurate corrosion resistance however is challenge to carbide precipitation inside the 900°F to 1600°F (480 to 870°C) range.

310 SS

Maximum temperature of 2100°F (1150°C) and gives suitable mechanical and corrosion resistance similar to 304 SS. Very top heat resistance. Not as ductile as 304 SS.

316 S

Maximum temperature of 1650°F (900°C) and has the fine corrosion resistance of the austenitic stainless steels. Subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C)

Inconel®

Maximum temperature 2150°F (1175°C) and is the most widely used thermocouple sheath material. Good excessive temperature electricity, corrosion resistance and is proof against chloride-ion pressure corrosion, cracking and oxidation. Do now not use in sulfur bearing environments.

Hastelloy X

Maximum temperature 2200°F (1205°C) extensively used in aerospace applications. Resistant to oxidizing, reducing and neutral atmospheric situations. Excellent excessive temperature energy.