| Selection Guide to Thermocouples |
What is a Thermocouple?
A thermocouple is a device that is used for measuring temperature. It consists of two dissimilar metals that are joined to form a junction which when heated produce a thermoelectric voltage. this voltage changes as the temperature increases or decreases.
How Do I Begin to choose the Correct Thermocouple for My Application?
Before selecting the correct thermocouple option, you should know the following information.
1. Temperature ranges (high and low limits) that are to be measured.
2. The environment or atmosphere that the thermocouple will be exposed to. This is important because the metals used in thermocouple construction react differently in
different environments. Metals that work well in one environment, will quickly fail in another environment at the same temperature.
3. Physical space limitation, method of installation and type of instrument connection desired (termination option-see page 182).
4. Response time. Response time is a time required to reach the temperature being sensed. Response time is greatly influenced by the type of junction selected (grounded, ungrounded, and exposed). The diameter of the thermocouple also has a significant effect on response time. A smaller diameter results in a quicker response, but is not as durable as a larger diameter.
IMPORTANT:
When ordering thermocouples, be certain that the calibration type (K, J, T, etc. ) corresponds to that of the instrument being used. You can find this information on the face of the instrument. |
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Why are Thermocouples Protected with Mineral Insulation and/or Special Sheath Material?
If temperatures in excess of 800°F are to be measured, then mineral insulated thermocouples should be used. (See TCP’s Mineral Insulated Thermocouples). The mineral insulation helps protect the wire up to 2100°F. To measure temperatures in excess of 2100°F, special insulation and sheath material must be used. (See TCP’s High temperature Thermocouples on page 41). The High Temperature Section has thermocouples that are capable of reaching 3800°F.
When Do You Use a Protection Tube or Thermowell?
If the environment is corrosive or harsh, it is recommended that the thermocouple element be placed in a thermowell or protection tube. These wells and protection tubes can be made out of many different materials: Carbon Steel, 304SS, 316SS, 446SS, Monel, Inconel, etc.
What Type of Thermocouple or TCP Series Do I Select?
The Catalog’s Buyer’s Guide at the beginning of the catalog, can help you located the appropriate section. Also, the Table of Contents in the front and the Series No. Index on page 186 may assist you. If you need additional help, please contact our Customer Service Department. 800-727-5646.
Once you have determined the series that will work for your application, it becomes a simple matter of selecting the proper options. The following will assist you in determining the proper thermocouple options:
Selection Guide for Wire Calibration
Selection Guide for Sheath Material
Selection Guide for Junction Style
Selection Guide for Lead Wire
* Selection Guide for Thermowell Material
Selection Guide for Termination Style
* Selection Guide for Extension Configuration
* For Industrial Thermowell Assemblies and RTD assembly only. |
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Selection Guide to
Wire Calibration
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Thermocouples combine dissimilar metals or alloys to produce a voltage. They are classified by calibration types, and each has it’s own unique EMF (electromotive force) vs. temperature curves. An instrument or temperature controller will correlate the voltage to a particular temperature.
Type “J” - Iron vs. Constantan thermocouples can be used from 32°F to 1400°F.* Type J is particularly suited for use in reducing atmospheres. The iron leg tends to oxidize rapidly at temperatures above 1000°F. So large gauge wires inside of protection tubes are recommended at higher temperature.
Type “K” - Chromel vs. Alumel thermocouples can be used from 32°F to 2300°F.* Type K is particularly suited for use in oxidizing atmospheres. It is a reliable and accurate thermocouple and the most commonly used wire calibration type.
Type “T” - Copper vs. Constantan thermocouples can be used from -328°F to 700°F.* Type T can be used in either mildly oxidizing or
reducing atmospheres. It is particularly suited for use at low and cryogenic temperatures.
Type “E” - Chromel vs. Constantan thermocouples can be used from -328°F to 1600°F.* Type E can be used in either mildly oxidizing or reducing atmospheres. It has the highest EMF output of all the common thermocouple types.
Type “R”, “S” and “B” - R & S Platinum-Rodium vs. Platinum thermocouples can be used from 32°F to 2700°F.* Type B Platinum-Rhodium vs. Platinum-Rhodium thermocouples can be used from 1600°F to 3100°F. * Type R, S, & B are used in oxidizing atmospheres and are damaged by reducing atmospheres. These thermocouples are easily contaminated and therefore should be used inside of one or more protection tubes.
Note: Duplex Calibration Thermocouple - A thermocouple constructed with two circuits. usually used when a backup circuit is desired.
* Wire size and sheath materials will also affect the upper temperature limits.
| Thermocouple Wire Names and Symbols |
Calibration Type |
Type of Thermocouple |
Temperature Range |
Comments |
J |
Iron (+)
Constantan (–) |
32°F to 1400°F
(0°C to 760°C) |
Reducing atmosphere recommended. Iron leg
subject to oxidation at elevated temperatures
- use larger gauge to compensate. |
K |
Chromel (+)
Alumel (–) |
32°F to 2300°F
(0°C to 1260°C) |
Well suited for oxidizing atmospheres.
Most commonly used calibration type. |
T |
Copper (+)
Constantan (–) |
-328°F to 700°F
(-200°C to 371°C) |
Most stable at cryogenic temperature ranges.
Excellent in oxidizing and reducing atmospheres within temperature range. |
E |
Chromel (+)
Constantan (–) |
-328°F to 1600°F
(0°C to 871°C) |
Highest output of basic metal thermocouples.
Not subject to corrosion at cryogenic temperatures. |
R |
Plat. 13% Rhod. (+)
Platinum (–) |
32°F to 2700°F
(0°C to 1482°C) |
Oxidizing atmosphere recommended.
Easily contaminated, requires protection. |
S |
Plat. 10% Rhod. (+)
Platinum (–) |
32°F to 2700°F
(0°C to 1482°C) |
Laboratory standard, highly reproducible.
Easily contaminated, requires protection. |
B |
Plat. 30% Rhod. (+)
Plat. 6% Rhod. (–) |
1600°F to 3100°F
(871°C to 1704°C) |
Easily contaminated, requires protection.
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Notes: Only Iron and Alumel wires are magnetic. Chromel & Alumel are registered trademarks of
Hoskins Manufacturing Co.
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Limits of Error for Thermocouples
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Calibration Type |
Type of Thermocouple |
Temperature Range |
Comments |
Standard |
Special |
J |
Iron
Constantan |
32°F to 530°F
530°F to 1400°F |
±4°F
± .75% |
±2°F
± .4% |
K |
Chromel
Alumel |
32°F to 530°F
530°F to 2300°F |
±4°F
± .75% |
±2°F
± .4% |
T |
Copper
Constantan |
-328°F to 32°F
32°F to 260°F
260°F to 700°F |
±2°F or ±1.5%
±2°F
± .75% |
—
±1°F
± .4% |
E |
Chromel
Constantan |
-328°F to 32°F
32°F to 500°F
500°F to 1600°F |
±3°F or ±1%
±3°F
± .5%
|
—
±2°F
± .4% |
R |
Plat. 13% Rhod.
Platinum
|
32°F to 1100°F
1100°F to 2700°F |
±2.5°F
± .25% |
±1°F
± .1% |
S |
Plat. 10% Rhod.
Platinum |
32°F to 1100°F
1100°F to 2700°F |
±2.5°F
± .25% |
±1°F
± .1% |
B |
Plat. 30% Rhod.
Plat. 6% Rhod. |
1600°F to 3100°F |
± .5°F |
— |
Notes: Chromel & Alumel are registered trademarks of Hoskins Manufacturing Co. Table courtesy of ASTM E230.
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| Selection Guide to Sheath Material |
The sheath serves to isolate and protect the wires and insulation from contamination and mechanical damage. There is no sheath material which is appropriate for all conditions. Temperature, corrosiveness, strength, service life cost must be considered when selecting a sheath material.
304 Stainless Steel: Maximum Temperature: 1650°F (900°C).
Very common low temperature sheath material. Lowest cost corrosion resistant material available. Used extensively in chemical, food, beverage, and other industries where corrosion resistance is required.
310 Stainless Steel: Maximum Temperature: 2100°F (1150°C).
Very good heat and oxidation resistance. The mechanical and corrosion resistance is better than 304SS.
316 Stainless Steel: Maximum Temperature: 1650°F (900°C).
Similar to 304SS with very good corrosion resistance. This alloy is also used extensively in the food and chemical industries.
446 Stainless Steel: Maximum Temperature: 2100°F (1150°C).
A stainless steel with good corrosion resistance, heat and oxidation resistance. Excellent for sulphur bearing environment.
Inconel 600: Maximum Temperature: 2200°F (1175°C).
Most commonly used high temperature sheath material. Good corrosion resistance and very good heat and oxidation resistance. Not intended to be used in a sulfur bearing environment.
Note: Other sheath material are available. |
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| Selection Guide to Junction Style |
Thermocouple wires are joined at the sensing end of the thermocouple to form a measuring or ”Hot” junction. The most common method for joining the wires is welding.
A general ”Rule of Thumb” is as follows:
Grounded Junctions are 4 times Faster than Ungrounded–Junctions exposed to moving hot water
Exposed Junctions are 50 times Faster than Ungrounded–Junctions exposed to moving hot water
Note: ”Tube Temp” junction styles are designated by a ”G” for grounded and a ”U” for ungrounded.
Grounded Thermocouple Junctions have wires and sheath tip joined together to form an integral junction. Grounded junctions have a faster response time than ungrounded junctions because the wires are in close proximity to the environment but service life may be affected accordingly. Care should be taken to avoid electrical interference and ground loops when using grounded junctions.
Ungrounded Thermocouple Junctions have joined wires that are electrically isolated from the sheath tip by compacted insulation. Ungrounded junctions produce the slowest response time, yet are the most durable because the wires are completely insulated from the environment. This junction style is particularly suited for harsh environmental conditions, temperature cycling and electrical interference.
Exposed Thermocouple Junctions have joined wires that are totally exposed from the sheath. Exposed junctions provide the fastest response time but are the least durable because the wires are directly subjected to mechanical and environmental conditions.
The rounded junctions styles (W2 & W4) are the most commonly used. When a flat surface needs to be sensed, flat junctions styles (W1 & W3) should be used to provide the most efficient transfer of heat from a hot surface perpendicular to the junction. |
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Selection Guide to
Lead Wire |
Fiberglass: Fiberglass insulation with silicone resin over each wire and overall. 900°F maximum temperature. ANSI color coded. Fair abrasion resistance. Good moisture and chemical resistance.
Teflon: Extruded FEP teflon insulation over each wire and overall. 400°F maximum temperature. ANSI color coded. Excellent abrasion, moisture and chemical resistance.
SS Overbraid: Stainless steel braiding greatly improves abrasion resistance and extends life of leadwire.
SS Flexhose: Stainless flexible hose or armor improves abrasion resistance and protects leadwire from mechanical damage such as pinching and crushing. |
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| How to Select Thermowell Protection Tube Material |
| General Material Application Data |
Material |
Recommended Maximum Temperature |
Comments |
Carbon Steel |
1000°F (540°C) |
Satisfactory in any except corrosive atmospheres. |
Monel
(67% Nickel,
30% Copper) |
900°F (480°C) |
Used where high strength and resistance to corrosion are required, such as sea water, dilute sulfuric acid and strong caustic solutions. |
304SS
(18% Chromium,
8% Nickel) |
1600°F (870°C) |
Resistant to oxidation and corrosion. Generally used in wet-process applications such as steam lines, oil refineries and chemical solutions. Resists nitric acids well, halogen acids poorly, and sulfur acids moderately. |
316SS
(18% Chromium,
8% Nickel,
2% Molybdenum) |
1600°F (870°C) |
Superior to 304 SS in corrosion resistance.
Resists pitting in phosphoric and acetic acids. |
446SS
(28% Chromium, Iron) |
2000°F (1090°C) |
Excellent resistance to oxidation and corrosion at high temperatures. Used extensively in general-purpose alloy tubes. Highly resistant to sulfur attack. |
Nickel |
2000°F (1090°C) |
Resistant to attack by many chemicals at high temperatures.
Principally used for hot caustic and molten-salt baths.
Should NOT be used where sulfur is present. |
Inconel 600
(80% Nickel,
15% Chromium) |
2100°F (1150°C) |
For general high-temperature use. Has greater mechanical strength than 446SS. Should not be used where sulfur is present.
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| Notes: Only specific options are offered in each catalog section. |
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| How to Select Termination Style |
Termination Styles |
Termination Style |
Description |
Code |
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Termination Style |
Description |
Code |
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111 Standard Plug |
10 |
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Waterproof Cover (1020W) |
51 |
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131 Miniature Plug |
20 |
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Fiberglass Head (1016) |
60 |
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109 Standard High Temp Plug (Ceramic) |
30 |
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Ceramic Head (1015) |
61 |
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Standard 3 Inch Pig Tail (Bare Wire End) |
40 |
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151 Standard RTD Plug (3 Wire) |
70 |
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3 inch Pig Tail with #6 Ring Rerminal and BX Connector |
41 |
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Cast Aluminum Head (1018 Series) |
AL |
Cast Iron Head (1018 Series) |
CI |
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Dust Cover (1020D) |
50 |
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Polypropylene Head (1028 Series) |
P |
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| How to Select Extension Configuration |
| Extension Configurations |
Code |
 |
Head* |
1 |
 |
Head and Nipple** |
2 |
 |
Head and Nipple/Union* |
3 |
 |
Head and Nipple/Union Nipple** |
4 |
*For use with Protection Tube Assemblies only
**For use with Thermowell Assemblies only. |
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| Thermocouple Alloys |
Resistance of Thermocouple Alloy Wires Per Foot
(OHMS per foot of Both Wires)
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Temperature-Millivolt Relationship Curves |
B&S Gauge |
Inch Equiva-lents |
K OHMS |
J OHMS |
E OHMS |
T OHMS |
Copper (2 wires) |
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2 |
.2576 |
.00912 |
.00533 |
.01088 |
.00454 |
.000312 |
6 |
.1620 |
.0230 |
.01345 |
.0274 |
.01145 |
.000790 |
8 |
.1285 |
.0366 |
.02135 |
.0437 |
.0182 |
.001256 |
11 |
.09074 |
.0736 |
.0429 |
.0876 |
.0366 |
.002520 |
14 |
.06408 |
.1470 |
.0859 |
.1753 |
.0733 |
.005050 |
16 |
.05082 |
.2339 |
.1363 |
.2785 |
.1163 |
.008032 |
18 |
.0403 |
.372 |
.2175 |
.444 |
.1851 |
.012770 |
20 |
.03169 |
.591 |
.3450 |
.704 |
.294 |
.02030 |
22 |
.02535 |
.942 |
.550 |
1.122 |
.465 |
.03228 |
24 |
.02010 |
1.499 |
.874 |
1.783 |
.746 |
.05134 |
26 |
.01594 |
2.380 |
1.388 |
2.835 |
1.182 |
.08162 |
28 |
.01264 |
3.79 |
2.21 |
4.52 |
1.885 |
.12980 |
30 |
.0100 |
5.91 |
3.45 |
7.04 |
2.94 |
.20640 |
See Codes on Chart Below
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| ANSI* Symbols |
Code |
Comments |
T |
Copper-Constantan For temperatures below 600°F (316°C). Frequently used to determine temperatures of generators, alternator windings, and lead or solder melt-point apparatus. |
J |
Iron-Constantan Although these wires may be used at temperatures as high as 1600°F (871°C), they are most suitable for service between 400°F (204°C) and 1200°F (649°C). For applications requiring a low-priced protection tube, a plain stainless steel tube will be satisfactory. The iron and constantan wires are almost universally used to measure diesel engine exhaust gas temperatures. If they are used in an oxidizing atmosphere at elevated temperatures, they should be encased in a protection tube. |
K |
Chromel-Alumel® May be used for continuous duty for temperatures between 1200°F (649°C) and 2000°F (1093°C) and for intermittent service as high as 2300°F (1260°C). Plain iron protection tubes should not be used with these alloys because the iron will cause carbonization that may affect the accuracy of the thermocouple. It should have an alloy protection tube when used in a reducing atmosphere. |
E |
Chromel-Constantan This alloy follows the temperature millivolt curve shown above in the "Temperature-Millivolt Relationship Curves" graph. |
S |
Platinum 10% Rhodium-Platinum Used for temperatures between 2000°F (1093°C) and 2800°F (1538°C). It should always be used with a gas-tight ceramic protection tube. Supplied for Pyrometers only when specified. |
R |
Platinum 13% Rhodium-Platinum Used for temperatures between 2000°F (1093°C) and 2800°F (1538°C). Its higher millivolt output makes it more desirable than an S thermocouple. It should always be used with a gas-tight ceramic protection tube. |
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Positive Elements
Iron, Chromel, Copper, Platinum-Rhodium
Negative Elements
Alumel, Constantan, Platinum |
* American National Standards Institute.
® Registered Trademark of Hoskins Manufacturing Co. |
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