INTRO: Hard fired ceramic insulators are used to make rigid thermocouples for high temperature service. The insulator should have good mechanical properties and is commonly used without a sheath or external protection tube. It also needs good thermal shock resistance to deal with temperature gradients from insertion of one end into the hot zone. Electrical insulation requirements are relatively modest due to the low voltages (millivolt level) of a typical application.
Description:Thermocouple insulators are rigid tubes, rods, and multi-hole configurations. Standard configurations are readily available from many commercial sources. These are obtained in mullite and alumina compositions at reasonable prices and many options for size and lengths.
Routine furnacing requirements to 1500 C are readily serviced by these suppliers, and TATEHO OZARK uses outside supplliers for our own routine requirements. Note that it is common to obtain protection tubes as a companion to thermocouple insulators. The normal design is a long monolithic tube with a closed end.
Deferral: TATEHO OZARK cannot address inquiries for plain vanilla alumina or mullite hard fired thermocouple tubing. If you have special needs, please consider our limited menu of options described below.
Resource links for mullite and alumina standard insulators are below:
OZARK Alternatives:
High Temperature:
TATEHO OZARK is the supplier of choice for very high temperature applications which can use our Hafnium Oxide (HfO2) insulators. The upper temperature limit for alumina is about 1600 C, whereas HfO2 is in common use to 2000 C, and reported use to 2500 C by our customers.
Special Shapes and small orders
TATEHO OZARK is highly skilled at extrusion and is capable of complex designs that are not available as standard items on the market. We will be glad to offer our high density Alumina grade HLA for your special project.
link to HLA here
Very unusual environmental requirements
TATEHO OZARK represents the only option for the possibility of high density MgO extruded shapes. However, this remains a very narrow solution which might only be viable for exposure to materials which aggressively attack Alumina and where the other limitations of MgO are not mission critical. This should not be tried at home….
Limitations:
Each material option is discussed in the appropriate spec sheet. The common deficiency is that TATEHO OZARK firing limitations do not enable production of long, straight tubes. Such tubes are made by firing vertically iwith special purpose made kilns and equipment. The practical limit for TATEHO OZARK tubes is about 10” (250 mm), and a straightness of 1% is typical.
However, if you can use short, crooked tubes, consider TATEHO OZARK! (kidding! kind of :))
Material/Purity:
This category refers to our high density, hard fired extrusions. See individual specifications for the individual item of interest, there is no overlap in the applications. For simplicity, the analysis of the starting compounds and some reference properties are listed below.
The following are typical analyses or limits for powders used to produce extruded insulators. Most raw materials are high quality fused grain powders which are processed in-house to obtain optimum particle size distributions for end use as probes, cables, and heaters.
| Material | Al2O3 | HfO2 | MgO |
|---|---|---|---|
| GRADE | HLA | HUH | HRM |
| Purity incl. binary component (%) | 99.75 | 99.9* | 99.2* |
| binary species | - | ZrO2 | Y2O3 |
| binary content (%) | - | 0.5 max | 2% |
| Density Range | 3.6-3.8 | 7.5-9.1 | 3.0-3.2 |
| % Theor. Dens. | 90-95 | 78-94 | 84-90 |
| Temp limit (protected) | 1500 | 2200 | 1600 |