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What is a Thermocouple

A thermocouple is used to calculate temperature. They are made of two conduction wires (semiconductors), that is housed in a protective box. Each wire is made from different metals, so that when joined, they will create a conjunction. These semiconductors take an accurate temperature by turning heat into an electrical current. The temperature is accurately read by using a thermocouple reference table.

What are thermocouples used for?

The manufacturing and Bio-industries use thermocouples often because the temperature sensor withstands extreme cold and heat much better than thermometers. Thermocouples temperatures can be read from a computer, which is much easier than reading the degrees on a thermometer. These industries have also found that using thermocouples is more economical because there are reasonably inexpensive.

How a thermocouple works

A thermocouple is made of two wires welded to a sensor and a protective metal case. The thermocouple sensor is lowered into the area of temperature where the reading is needed. The sensor then records the temperature range of both thermocouple wires and sends the results to a computer controlled device.

Thermocouple wiring

There are several types and quality levels of thermocouple wiring; each made of different metals. The combination of metals is what gives each thermocouple its own action and response time.

Thermocouple grade wire. Thermocouple wire made with thermocouple grade wiring so that they can withstand the heat inside the sheath. Thermocouple grade has a higher specification for accuracy than extension grade wiring.

Extension wire.  Extension wire is lower quality, so it is less expensive. It is not used to help detect the temperature within the thermocouple. Extension wire is solely used to delivery currency from the sensor to the display module on the control system.

Color-coded thermocouple wire. Like most electric devices, thermocouple wiring is often color-coded. These color help to pinpoint what part of the mechanism the wiring starts from, and where it leads to. This way if there is trouble with certain areas of the thermocouple, it is easier to detect, and fix.

M.I. coating. Mineral Insulation (M.I.) coating is a hollow cable used to insulate thermocouple wire. The M.I. Coating helps to protect the wiring from corrosion and an extreme temperature range.

Learning thermocouple types

There are about eight different types of thermocouples; each with its own unique mechanism.

Type K thermocouple. This thermocouple makes use of one Nickel-Chromium, and one Nickel-Alumel wire. Type K thermocouple is the most commonly used, despite being the most expensive. HVAC Experts often recommend it, because it tends to be more accurate, lasts longer, and has a wider temperature range.

Type J Thermocouple. It is made of an Iron, and a Constantan wire. The Type K has about the same price range and reliability as the Type K thermocouple. However, it has a slightly short lifespan, when trying to keep up with high temperatures.

Type T Thermocouple. It uses Copper and Constantan wires. Type T is often used in the cold because it can maintain stability even in below freezing temperatures.

Type E Thermocouple. This Nickel-Chromium and Constantan wired thermocouple has better accuracy and reading signal than Types K, and J at temperatures a thousand degrees or below.

Type N Thermocouple. This type makes use of metal wires made of Nicrosil and Nisil. Type N Thermocouple is somewhat more expensive than Type K. However it works about the same at every temperature level.

Type S Thermocouple. It has one Platinum Rhodium wire, and the other is made of 10 percent Platinum. Type S Thermocouple is commonly used in Bio-labs because of its versatility, and accuracy in both very high and low temperatures.

Type R Thermocouple. This device has one wire that is made of thirteen percent Platinum Rhodium, and one that is pure Platinum. Because of the higher percentage of Platinum, it is slightly more expensive than the Type S Thermocouple. While its function is very much like Type S, it tends to be more stable and accurate in colder temperatures.

Type B Thermocouple. This thermocouple has two Platinum Rhodium conduction wires. One is thirty percent Platinum Rhodium, and the other is only six percent. Type B Thermocouple can withstand higher temperatures than any other thermocouple. Because of its tolerance for heat, it is used solely for high-temperature range.

Thermocouple junction categories

There are four different design categories, which makes each thermocouple have a unique tolerance to extreme temperature range.

Grounded Thermocouples. This thermocouple junction has two wires welded together with a thermocouple sheath. Since the thermocouple wire is directly connected to the sheath, the grounded thermocouple has a very good response time, allowing for faster high temperature transfer. The grounded thermocouple does seem to have one problem. Because of the grounded sheath, the thermocouple can create an electrical interference.

Thermocouples that are ungrounded. Being ungrounded means that the thermocouple wires are still welded together, but they have a protective mineral insulation to withstand a wider temperature range. The insulation around the wires keeps them separated from the sheath.

Bare-wire Thermocouples. Wires that are not protected with a mineral insulation are directly welded into the thermocouple junction. Bare-wire thermocouples tend to have a very quick response time, but the wires can become corroded more easily than insulated thermocouple wire.

Uncommon Thermocouples. This means that the thermocouple wire and elements are all fully insulated separately from each other, and the sheath.

Measuring a thermocouple electromagnetic field (EMF)

Thermocouples produce a minor electromagnetic field, also referred to as an (EMF) reading. The EMF production from a thermocouple is so minor, which it takes highly sensitive instruments to detect its existence. The two most commonly used thermocouples are a galvanometer and a potentiometer. The potentiometer tends to be the most preferred because it automatically balances the voltage sounds. The thermocouple wiring produces a humming sound; that is so low the naked ear cannot hear it. However, if the EMF device being used does not have an automatic balance, the voltage sound can be quite deafening.

Thermocouple shapes

Thermocouples come in many shapes and sizes to accommodate different levels of need. The most used thermocouple shapes are beaded and probed. The most expensive form of the thermocouple is the bead type, but it is also thought to have the most effective temperature sensor. The probe shaped thermocouples are the most commonly used in the medical, science, and industrial fields because they have the ability to endure extreme temperatures.

Selecting a thermocouple can be quite difficult, but there are many aspects to consider. Always think of the level of temperature needing to be measured, the response time, and thermocouple accuracy.  

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