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Do you have bachelor’s degree in electrical engineering? Are you a frequent searches on the online portals? But could not able to find the right job that interests you the most? Then check out wisdom jobs online site which guides you for your right career? A Thermo couple is a sensor used to measure temperature. It produces a temperature-dependent voltage as a result of electrostatic effect, and this voltage can be interpreted to measure temperature. The role of thermocouple analyst is to have practical experience with industrial field control devices, thermocouple options, proximity sensors etc. So, avail the opportunity as thermo couple analyst, electrical engineer, thermal architect system engineer etc by looking into thermo couple job interview questions and answers given below.
Cold, or reference junction, is the end of a thermocouple that provides a reference point.
Thermocouples measure the difference in temperature between two junctions. They do NOT measure actual temperature. The sensing junction is where the thermocouple wires are welded (or otherwise connected) together, and located at a point where the temperature is desired. The other junction is typically located where it is connected to instrumentation (measuring device or transmitter). This is known as the cold or reference junction. Thermocouple millivolt tables and mathematical formulas are based on a cold junction temperature of 0°C. To determine actual temperature, the instrumentation must “adjust” for the difference between ambient temperature and 0°C. This adjustment is known as cold junction compensation.
Temperature Range: First, consider the difference in temperature ranges. Noble Metal Thermocouples can reach 3,100 F, while standard RTDs have a limit of 600 F and extended range RTDs have a limit of 1,100 F.
Cost: A plain stem thermocouple is 2 to 3 times less expensive than a plain stem RTD. A thermocouple head assembly is roughly 50% less expensive than an equivalent RTD head assembly.
Accuracy, Linearity, & Stability: As a general rule, RTDs are more accurate than thermocouples. This is especially true at lower temperature ranges. RTDs are also more stable and have better linearity than thermocouples. If accuracy, linearity, and stability are your primary concerns and your application is within an RTD’s temperature limits, go with the RTD.
Durability: In the sensors industry, RTDs are widely regarded as a less durable sensor when compared to thermocouples. However, REOTEMP has developed manufacturing techniques that have greatly improved the durability of our RTD sensors. These techniques make REOTEMP’s RTDs nearly equivalent to thermocouples in terms of durability.
Response Time: RTDs cannot be grounded. For this reason, they have a slower response time than grounded thermocouples. Also, thermocouples can be placed inside a smaller diameter sheath than RTDs. A smaller sheath diameter will increase response time. For example, a grounded thermocouple inside a 1/16” dia. sheath will have a faster response time than a RTD inside a ¼” dia. sheath.
Grounded Thermocouples: This is the most common junction style. A thermocouple is grounded when both thermocouple wires and the sheath are all welded together to form one junction at the probe tip. Grounded thermocouples have a very good response time because the thermocouple is making direct contact with the sheath, allowing heat to transfer easily. A drawback of the grounded thermocouple is that the thermocouple is more susceptible to electrical interference. This is because the sheath often comes into contact with the surrounding area, providing a path for interference.
Ungrounded Thermocouples (Or Ungrounded Common Thermocouples): A thermocouple is ungrounded when the thermocouple wires are welded together but they are insulated from the sheath. The wires are often separated by mineral insulation.
Exposed Thermocouples (or “bare wire thermocouples”): A thermocouple is exposed when the thermocouple wires are welded together and directly inserted into the process. The response time is very quick, but exposed thermocouple wires are more prone to corrosion and degradation. Unless your application requires exposed junctions, this style is not recommended.
Ungrounded Uncommon: An ungrounded uncommon thermocouple consists of a dual thermocouple that is insulated from the sheath and each of the elements are insulated from one other.
M.I. (Mineral Insulated) cable is used to insulate thermocouple wires from one another and from the metal sheath that surrounds them. MI Cable has two (or four when duplex) thermocouple wires running down the middle of the tube. The tube is then filled with magnesium oxide powder and compacted to ensure the wires are properly insulated and separated. MI cable helps to protect the thermocouple wire from corrosion and electrical interference.
316SS (stainless steel): This is the most common sheath material. It is relatively corrosion resistant and is cost effective.
304SS: This sheath is not as corrosion resistant as 316SS. The cost difference between 316SS and 304SS is nominal.
Inconel (registered trademark) 600: This material is recommended for highly corrosive environments.
A thermocouple can be identified by the color of its wire insulation. For example, in the United States a type J thermocouple grade wire has one red wire and one white wire, typically with a brown over jacket. A type J extension grade wire also has one red wire and one white wire, but it has a black over jacket. As a general rule the red wire of a thermocouple or extension wire is negative and the positive wire is color coded according to the type of thermocouple. Different countries use different color codes.
Thermocouple grade wire is used to manufacture thermocouple probes. Thermocouple grade wire is normally used for the junction and inside the stem sheath. This is because the thermocouple grade wire has a better accuracy specification than extension grade wire.
Extension grade wire is a less expensive, lower grade wire. It is used to extend the signal from the thermocouple probe to the control system or digital display. Extension grade wire is more economical due to a lesser grade metal being used. Extension grade wire should not be used in the process itself and should not see the temperature extremes & temperature cycling as standard grade wire.
When two wires of different metals or metal alloys (thermo wires) are joined together in one end (hot junction), a thermocouple is formed. If there is a temperature difference between the hot junction and the open ends, a thermal electromotive force (a thermal voltage) is created in the thermocouple. This is also called the Seebeck effect.
A thermocouple measurement always needs information from joined wire end (hot junction) and open wire end (cold junction). The cold junction is also called reference point. Variations of reference point temperature are compensated with CJC measuring (Cold Junction Compensation). Temperature transmitters CJC measuring can be an internal function or a measuring resistor integrated in connectors. If the reference point is far away from the transmitter, a separate temperature measuring of that point has to be implemented and wired to transmitter as compensation signal.
Compensating cable is a thermocouple measuring circuit cable, which is identified by the letter C (e.g. for type K cable KC). Wires of the compensating cable have the same electrical features, but not the same materials, as the thermo wires of the TC sensor. Compensating cable is a more cost effective solution than extension cable, but the maximum ambient temperature allowed is lower, approximately 100…200 °C depending on the insulation material.
Extension cable is a thermocouple cable which is identified by the letter X (e.g. for type K cable KX). Wires of the extension cable are of exactly the same materials as the thermo wires of the TC sensor. These cables can achieve even the same ambient temperatures as the thermocouple can.
Trace heating is a term usually used for keeping pipelines and attached devices unfrozen. The important function of trace heating is maintaining stable temperature and flow rate of the materials flowing through the pipeline. The most common implementation of trace heating is electrical, which offers good adjustability. However, for accurate process control and adjustment also precise temperature data is needed. For these applications we have designed our high quality trace heating sensors, which already have been available for years, also for Ex applications.
Based on the design of your system you need to know:
Once those are established then you’re ready to consult ASME standard PTC 19.3 TW-2010 Thermowell section, which goes through the calculation for the design of the well.
A Pyrometer is a non-contacting device that intercepts and measures thermal radiations. Without making any contact with the radiating body and the process is known as pyrometry. This device is useful for determining the temperature of an object’s surface.
Pyrometer strictly works on the principle of black body radiation. Here emissivity of the target plays an important role, as it governs how bright the target appears to the pyrometer. Due to its high accuracy, speed, economy and specific advantages, it is widely being used as a standard procedure in many industrial applications.
An exposed (measuring) junction is recommended for the measurement of flowing or static non-corrosive gas temperature when the greatest sensitivity and quickest response is required.
An insulated junction is more suitable for corrosive media although the thermal response is slower. In some applications where more than one thermocouple connects to the associated instrumentation,insulation may be essential to avoid spurious signals occurring in the measuring circuits. If not specified, this is the standard.
An earthed (grounded) junction is also suitable for corrosive media and for high pressure applications. It provides faster response than the insulated junction and protection not offered by the exposed junction.
Thermowells provide protection for temperature probes against unfavorable operating conditions such as corrosive media, physical impact (e.g. clinker in furnaces) and high pressure gas or liquid. Their use also permits quick and easy probe interchanging without the need to “open-up” the process.
The main application areas are:
There are two types of thermocouple construction used most commonly.These are Mineral Insulated (M.I.) Thermocouples & Non M.I. Thermocouples.
Mineral Insulated Thermocouples:
Non M.I. Thermocouples:
Temperature: Thermocouple life decreases by about 50% when an increase of 50 °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 500°C, 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.
Because a thermocouple measures in wide temperature ranges and can be relatively rugged, thermocouples are very often used in industry.
The following criteria are used in selecting a thermocouple:
A time constant has been defined as the time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to approach 100% of the step change value. An exposed junction thermocouple offers the fastest response. Also, the smaller the probe sheath diameter, the faster the response, but the maximum temperature may be lower. Be aware, however, that sometimes the probe sheath cannot withstand the full temperature range of the thermocouple type.
Sheathed thermocouple probes are available with one of three junction types: grounded, ungrounded or exposed. At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style, but the ungrounded offers electrical isolation.
You have to consider the characteristics and costs of the various sensors as well as the available instrumentation. In addition, Thermocouples generally can measure temperatures over wide temperature ranges, inexpensively, and are very rugged, but they are not as accurate or stable as RTD’s and thermistors. RTD’s are stable and have a fairly wide temperature range, but are not as rugged and inexpensive as thermocouples. Since they require the use of electric current to make measurements, RTD’s are subject to inaccuracies from self-heating. Thermistors tend to be more accurate than RTD’s or thermocouples, but they have a much more limited temperature range. They are also subject to selfheating. Infrared Sensors can be used to measure temperatures higher than any of the other devices and do so without direct contact with the surfaces being measured. However, they are generally not as accurate and are sensitive to surface radiation efficiency (or more precisely, surface emissivity). Using fiber optic cables, they can measure surfaces that are not within a direct line of sight.
Thermocouples are available in different combinations of metals or calibrations. The most common are the “Base Metal” thermocouples known as Types J, K, T, E and N. There are also high temperature calibrations – als known as Noble Metal thermocouples – Types R, S, C and GB.
K Type Thermocouples are known as general purpose thermocouple due to its low cost and temperature range.
This high temperature wire/thermocouple is produced using a wide range of materials, and is available with PVC, FEP, TFE, PFA, Fiberglass, and Ceramaflex insulation. However, if you see it referenced by “type” on a spec sheet, the description is referring to the kind of metal alloy used for the wire’s conductor.
When an “X” follows the letter indicating which alloy was used in the cable, the cable is extension grade. In these cases, the types of cable are listed as Type EX, Type JX, Type KX, Type NX, or Type TX.
Thermocouple grade wire is wire that is used to make the sensing point of the instrument, where extension grade wire is only used to extend a thermocouple signal from a probe back to the instrument reading the signal.
The various sheath materials are dependent upon the application. The following list will help you make the best selection:
Maximum temperature of 1650°F (900°C) and is the most widely used low temperature sheath material. It offers good corrosion resistance but is subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C) range.
Maximum temperature of 2100°F (1150°C) and offers good mechanical and corrosion resistance similar to 304 SS. Very good heat resistance. Not as ductile as 304 SS.
Maximum temperature of 1650°F (900°C) and has the best corrosion resistance of the austenitic stainless steels. Subject to carbide precipitation in the 900°F to 1600°F (480 to 870°C)
Maximum temperature 2150°F (1175°C) and is the most widely used thermocouple sheath material. Good high temperature strength, corrosion resistance and is resistant to chloride-ion stress corrosion, cracking and oxidation. Do not use in sulfur bearing environments.
Maximum temperature 2200°F (1205°C) widely used in aerospace applications. Resistant to oxidizing, reducing and neutral atmospheric conditions. Excellent high temperature strength.
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