Ceramic Thermocouple Protection Tubes

General Information

Ceramic thermocouple protection tubes are available in a variety of sizes and materials with Alumina, Mullite, and Zirconia being the most commonly used. Ceramic thermocouple protection tubes are typically used to protect Platinum thermocouple elements at high process temperatures above 2200°F.

When choosing a ceramic protection tube material, careful consideration should be taken into account in regards to its unique thermal shock characteristics and physical strength.

Alumina

Alumina has a maximum service temperature of 3450°F or 1900°C in both oxidizing and reducing atmospheres. Thermocouple protection tubes made from Alumina are considered to have better mechanical strength (twice the tensile strength of Mullite) and better chemical resistance than Mullite protection tubes. They also have excellent abrasion resistance. Secondary metal tubes can be added for additional protection from mechanical damage. Since Alumina protection tubes are considered to be gas tight, they make excellent primary protection tubes. All tubes, which are used in horizontal installations and at elevated temperatures, should be supported to prevent mechanical breakage. Alumina tubes can be used for primary and secondary protection tubes on Noble Metal (Platinum based) thermocouples. This material has a very low silica content so they will not contaminate Platinum thermocouple elements above (2200°F or 1200°C) as a Mullite protection tube.

Composition - 99.8% Aluminum Oxide.

Specifications

• Maximum Service Temperature of 3450°F or 1900°C
• Excellent Mechanical Strength
• Excellent for Noble Metal Thermocouples
• Tube sizes from 3/16" to 1 7/8" O.D.
• Available with Couplings, Hex Fittings, Steel Fittings, Sleeves, or Threaded Nipples.
• Available with Cemented Support Collars.
• Tolerance for O.D. +/- 5% or .025"
• Tolerance for Tube Length +/- .062"

Mullite

Mullite Porcelain has a maximum service temperature of 2900°F or 1590°C. It is considered to be superior to Alumina for thermal shock resistance. Mullite protection tubes should be used with care, since they have poor mechanical shock resistance. Secondary metal outer tubes can be added for protection from any mechanical damage. Mullite Thermocouple protection tubes are considered gas tight a high temperatures. All Mullite protection tubes, which are used in horizontal installations and at elevated temperatures, should be supported to prevent breakage. Mullite tubes should not be used for primary protection tubes on noble metal thermocouples (Platinum based) if the temperature will exceed 2200°F (1200°C). Above these temperatures the silica in the Mullite can contaminate the platinum thermocouple elements if there is not enough air circulation in the tube.

Composition - 63.5% Aluminum Oxide, balance is Silicon Dioxide.

Specifications

• Maximum Service Temperature of 2900°F or 1590°C
• Excellent Thermal Shock Resistance
• Tube sizes from 3/16" to 1 7/8" O.D.
• Available with Couplings, Hex Fittings, Steel Fittings, Sleeves, or Threaded Nipples.
• Available with Cemented Support Collars.
• Tolerance for O.D. +/- 5% or .025"
• Tolerance for Tube Length +/- .062"

Zirconia

Zirconia has a maximum service temperature of 4172°F or 2300°C in oxidizing, reducing atmospheres, and vacuum environments. This material is yttria (10.5%) stabilized. Zirconia is chemically inert and has excellent corrosion resistance. Zirconia thermocouple protection tubes should not be used in contact with Alumina above 1112°F (600°C). This material is gas tight, but above 1112°F (600°C) Oxygen will permeate through its wall structure. To prevent thermal shock. heat or cool Zirconia at a rate less than 302°F (150°C) per hour.

Composition - 89.5% Zirconia, 10.5% Yttria.

Specifications

• Maximum Service Temperature of 4172°F or 2300°C
• Excellent Corrosion Resistance
• Excellent Chemically Inert
• Tube sizes from 3/16" to 1" O.D.
• Available with Couplings, Hex Fittings, Steel Fittings, Sleeves, or Threaded Nipples.
• Available with Cemented Support Collars.
• Tolerance for O.D. +/- 5% or .025"
• Tolerance for Tube Length +/- .062"