Thermocouple Selection

     ANSI, Application, Initial Calibration Tolerance Table, Junctions, Sheath Material

Thermocouples must be selected to meet the conditions of the application. Only general recommendations on size and type can be given. Some of the considerations involved are length of service, temperature, atmosphere, and desired response time. The temperature ranges of the commonly used thermocouple types are given in the Initial Calibration Tolerances Table. Smaller gauge sizes provide faster response at the expense of service life at the elevated temperatures. Larger gauge sizes provide longer service life at the expense of response time. See ANSI Circular MC96.1. As a general rule, it is advisable to protect thermocouple elements with a suitable protecting tube or drilled well.

When ordering thermocouple wire or elements, be certain that the type (K, J, E, etc.) corresponds to that of the instrument with which it will be used. This information can usually be found on the face of the instrument.

ANSI Letter Designations

Thermocouple and extension wires are generally ordered and specified by ANSI letter designations for wire type.  Positive and negative legs are identified by the appropriate letter suffixes P and N, respectively.

ANSI Letter	Description	Magnetics	Popular Generic / Trade Name
T	No	TP	Copper
	No	TN	Constantan, Cupron®, Advance®
J	YES	JP	Iron
	No	JN	Constantan, Cupron®, Advance®
E	No	EP	Chromel®, Tophel®, HAI KP®
	No	EN	Constantan, Cupron®, Advance®
K	No	KP	Chromel®, Tophel®, HAI KP®
	YES	KN	Alumel®, Nial®, HAI KN®
R	No	RP	Platinum 13% Rhodium
	No	RN	Pure Platinum
S	No	SP	Platinum 10% Rhodium
	No	SN	Pure Platinum
B	No	BP	Platinum 30% Rhodium
	No	BN	Platinum 6% Rhodium
N	No	NP	Nicrosil
	No	NN	Nisil

TYPE E: The type  E  thermocouple is suitable for use at temperatures up to +1652°F (+900°C) in a vacuum, inert, mildly oxidizing or reducing atmosphere. At cryogenic temperatures, the thermocouple is Not subject to corrosion. This thermocouple has the highest EMF output per degree of all the commonly used thermocouple.

TYPE J: May be used, protected or unprotected, where there is a deficiency of free oxygen, but, again for cleanliness and generally longer life, a protecting tube is recommended. Since JP (iron) wire will oxidize rapidly at temperatures over +1000°F (+538°C), it is recommended that larger gauge wires be used to compensate. Maximum recommended operating temperature is +1400°F (+760°C).

TYPE K: Due to its reliability and accuracy, type  K  is used extensively at temperatures up to +2300°F (+1260°C). It is good practice always to protect this type of thermocouple with a suitable metal or ceramic protection tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric furnaces, tube protection is Not always necessary when other conditions are suitable; however, it is recommended for cleanliness and general mechanical protection. Type  K  will generally out last type  J  because the JP (Iron) wire rapidly oxidizes, especially at the higher temperatures. Standard wire finishes are 14 B & S gauge and larger, oxidized; 16 B & S gauge and smaller, bright annealed.

TYPE T: This thermocouple can be used in either oxidizing or reducing atmospheres but for cleanliness and generally longer life, a protecting tube is recommended. Because of its stability at lower temperatures, this is a superior thermocouple for a wide variety of applications in low and cryogenic temperatures. Operating range -328°F to +662°F (-200°C to +350°C) but can be used to -452°F (-269°C) boiling helium.

TYPE S, R, and B: Maximum recommended operating temperature for type  S  or  R  is + 2642°F (+1450°C); type  B  is recommended for use at as high as +3092°F (+1700°C). These thermocouple are easily contaminated. Reducing atmospheres are particularly damaging to the calibration. Noble metal thermocouples should always be protected with gas-tight Serv-Rite ceramic tubes, a secondary tube of porcelain, and silicon carbide or metal outer tubes as conditions require. Types  S  and  R  thermocouples and thermo elements are provided in accordance with IPTS-68, unless IPTS-48 is specifically request. Type  B  thermocouples and thermo elements meet both IPTS-48 and IPTS-68.

TYPE C: Tungsten 5% Rhenium/Tungsten 26% Rhenium-This refractory metal thermocouple may be used at temperatures up to + 4200 °F (+2315 °C). As it has No oxidation resistance its use is restricted to vacuum, hydrogen or inert atmospheres.

TYPE N: Nicrosil/Nisil nickel-based thermocouple alloy used primarily at high temperature up to +2300°F (+1260°C). While Not a direct replacement for type  K , type  N  provides better resistance to oxidation at high temperature and longer life in applications where sulfur is present. It also out performs type  K  in  K's  aging range.

     Initial Calibration Tolerances for Thermocouples

Reference Junction 0°C (32°F)
Type	Temperature Range		Tolerances~
	°C	°F	Standard	Special
			(Whichever is Greater)
B	+870°C to +1700°C	+1598°F to +3092°F	±0.5%
E	0°C to +900°C	+32°F to +1652°F	±1.7°C or ±0.50%	±1.1°C or ±0.4%
J	0°C to +750°C	+32°F to +1382°F	±2.2°C or ±0.75%	±1.1°C or ±0.4%
K	0°C to +1250°C	+32°F to +2282°F	±2.2°C or ±0.75%	±1.1°C or ±0.4%
N	0°C to +1250°C	+32°F to +2282°F	±2.2°C or ±0.75%	±1.1°C or ±0.4%
R/S	0°C to +1450°C	+32°F to +2642°F	±1.5°C or ±0.25%	±0.6°C or ±0.1%
T	0°C to +350°C	+32°F to +662°F	±1.0°C or ±0.75%	±0.5°C or ±0.4%
Cryogenic Ranges
E*	-200°C to 0°C	-328°F to +32°F	±1.7°C or ±1.0%	**
K*	-200°C to 0°C	-328°F to +32°F	±2.2°C or ±2.0%	**
T*	-200°C to 0°C	-328°F to +32°F	±1.0°C or ±1.5%	**

*Thermocouples and thermocouple material are Normally supplied to meet the tolerances specified in the table for the Normal specified range. The same materials, however, may Not fall within the cryogenic tolerances in the second section of tables. If materials are required to meet the cryogenic tolerances, the purchase order must so state. Selection of materials usually will be required. Tolerances indicated in this table are Not necessarily an indication of the accuracy of temperature measurements in use after initial heating of the materials.

** Little information is available to justify establishing special tolerances for cryogenic temperatures. Limited experience suggests the following tolerances for types  E  and  T  thermocouples. Type  E  -200°C to 0°C 1.0 °C or ±0.5% (whichever is greater); Type  T  -200°C to 0°C 0.5°C or ±0.8% (whichever is greater). These tolerances are given only as guide for discussion between purchaser and supplier. Due to the characteristics of the materials, cryogenic tolerances for type  J  thermocouples and special cryogenic tolerances for type  K  thermocouples are Not listed.

~ Where tolerances are given in percent, the percentages applies to the temperature being measured in degrees Celsius. For example, the standard tolerance of type  J  over the temperature range is +277°C to +750°C is ±0.75 percent. If the temperature being measured is +538°C, the tolerance is ±0.75 percent or 538, or ±4.0°C. To determine the tolerance in degrees Fahrenheit, multiply the tolerance in degrees Celsius times 1.8.

Junctions

Exposed Junction (E)-Thermocouple wire are butt welded. Insulation is sealed against liquid or gas penetration. This junction style provides the fastest possible response time but leaves the thermocouple wires unprotected against corrosive or mechanical damage.

Grounded Junction (G)-The sheath and conductors are welded together forming a completely sealed integral junction. Recommended in presence of liquids, moisture, as or high pressure. The wire is protected from corrosive or erosive conditions. In the Grounded Junction, response time approaches that of the Exposed Junction.

Ungrounded Junction (U)-Thermocouple junction is fully insulated from welded sheath end. Excellent for applications where stray EMF's would affect the reading and for frequent or rapid temperature cycling. In the Ungrounded Junction, response time is slightly longer than for the Grounded Junction.

Sheath Material

Virtually any malleable metal can be used as XacTpak sheath material Some of the more commonly used materials and their maximum continuous operating temperatures in an oxidizing atmosphere are: Alloy 600 +2100°F (+1149°C), 304 Stainless Steel +1650°F (+899°C), 316 Stainless Steel +1700 °F (+927 °C).

Conductor Material-The thermocouple material used in XacTpak meets or suppresses all applicable ANSI Standard.

Insulation-The insulation used in XacTpak is high (99.4%) or standard (96%) purity metallic oxide. During the manufacturing process, the insulation is highly compacted, which excludes air from the sheath, retards moisture absorption, and prevents  powdering out . The high degree of compaction achieved also ensures high thermal conductivity and maximum dielectric strength.

Thermocouple Selection Guide    |     Wire Selection Guide Wire Selection Guide

Specifications subject to change without notification.  Chromel® and Alumel® are registered trademarks of Hoskins Manufacturing Company. XpacTpak® is a registered trademark of Watlow. Cupron ® , Nial ® and Tophel ® are registered trademarks of AMAX.   Advance ® , HAI KP ® , and HAI KN ® are registered trademarks of Harrison Alloy Company.