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Thermocouples


Thermocouples
The most widely used method of temperature measurement is the thermocouple.  Thermocouples are very rugged and in most cases inexpensive to replace.  We manufacture assemblies in a variety of Thermocouple Types that are available to meet nearly all industrial and scientific temperature ranges.
 
Thermocouples are available with flexible or “soft wire” insulations for applications that need the flexibility and where it is most practical such as surveys or applications that are conducive to this construction.  Tube and wire sheathed thermocouples are designed for mechanical strength or hold at the point of insertion into machine processes such as plastic injection molds and food applications among others.
  
The thermocouples in this section are considered to be low temperature 900°F (482°C) and below.   Insulations available are:
 Fiberglass 900°F (482°C)
 PFA Teflon® 500°F (260°C)
 FEP Teflon® 400°F (204°C)
Additional insulations are available that are not listed in this section of the catalog (see Wire section).
 
Teflon is a registered trademark of E. I. du Pont de Nemours and Company or its affiliates.
 
Performance 
Thermocouples can operate at temperatures from -454°F (-232°C) to 4,200°F (2315°C) and perform in extremely rugged applications.  They offer very fast response to temperature changes are small in physical size and are self powered.  Thermocouple accuracy has been established into Standard limits of error (+ 2.2°C) or .75% whichever is greater and Special limits of error
(+ 1.1°C) .4% whichever is greater.
 
Cold Junction Compensation
Thermocouples require a temperature reference to compensate for unwanted Emf at ambient temperature.  Traditionally reference to 0°C was accomplished by an ice bath where the positive and negative legs were immersed into the bath.  This method is impractical in most cases.  Today, temperature controllers, thermometers, indicators, recorders & transmitters compensate for the unwanted thermoelectric contributions.
 
ASTM thermocouple alloy combinations by letter type
Type J, Iron (+) Constantan (-), is the most commonly used calibration.  It is suitable for use in vacuum, inert, oxidizing or reducing atmospheres.  If unprotected the iron wire may be attacked by ammonia, nitrogen and hydrogen atmospheres.  Iron & Constantan wires can generate galvanic EMF between the two wires and should not be used in applications where they might get wet.  The positive leg is magnetic.
 
Type T- Copper (+) Constantan (-), is commonly used for subzero to 700°F temperatures.  Preferred to Type J for subzero applications because of copper’s higher moisture resistance, as compared to iron.  If unprotected, it will still function in a vacuum, inert, oxidizing or reducing atmospheres.  Copper-Constantan thermocouples are susceptible to conduction error, due to the high thermal conductivity of the copper.  Neither leg is magnetic.
 
Type K- Nickel-Chromium (+) vs.  Nickel-Aluminum (-), is generally used to measure high temperature to 2300°F (1261°C).  It should not be used for accurate temperature measurements below 900°F (482°C) or after prolonged exposure above 1,400°F (760°C).  If left unprotected it can be used only in inert or oxidizing atmospheres.  It has a short life in alternately oxidizing and reducing atmospheres particularly in the 1,500°F (816°C) to 1850°F (1010°C) range.  The negative leg is magnetic.
 
Type E- Nickel-Chromium (+) vs. Constantan (-), has the highest EMF output of any standardized metallic thermocouple.  If used unprotected, type E wire is not subject to corrosion at subzero temperatures.  Type E can be used in inert, oxidizing or reducing atmospheres.  Because they cover a wide range with a single calibration curve, type E thermocouples are preferred for computer applications.
 
Type N- Nicrosil (+) vs. Nisil (-) Superior oxidation resistance over type K due to the combination of a higher level of chromium and silicon in the N (+) (Nicrosil alloy) conductor.  Similarly, a higher level of both silicon and magnesium form a diffusion barrier to protect the N (-) (Nisil alloy).  Because of increased stability it’s recommended in nuclear applications.
 
Type S- Platinum -10% Rhodium (+) vs. 100% Platinum (-)
Type R- Platinum - 13% Rhodium (+) vs. 100 % Platinum (-), These thermo elements should always be protected from contamination by reducing oxides, metallic vapors or other oxides at high temperatures.  Insulation should be silica free to prevent contamination. Type S is frequently used for calibration standards. Type R has a slightly greater sensitivity and consequently is used more frequently in industrial applications.
Type B- Platinum - 30% Rhodium (+) vs.Platinum - 6% Rhodium (-). For use between 1000° and 3,175°F. It is intended to prevent the problems experienced with types S and R such as (1) weakening of the pure platinum leg due to excessive grain growth and (2) calibration shift due to the pure platinum wire picking up rhodium volatilized from the alloy wire at 2,732°F. The flatness of the temperature-millivolt curve at normal reference junction ambient temperature permits the use of copper extension wire.
Type C- Tungsten - 5% Rhenium (+) vs. Tungsten - 26% Rhenium - (-). These thermo elements possess excellent stability at temperatures in the 3,000 to 4,000°F range. For use at high temperatures a protective atmosphere must be provided, such as hydrogen, inert gas or vacuum. They are extremely sensitive to mechanical damage and should be handled carefully to prevent breakage.