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THERMOTECH USA - Thermocouples

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THERMOTECH USA - Thermocouples

Ceramic Beaded TCRemote Head Thermocouple with Flanged ThermowellRemote Head Thermocouple

Style A -- Ceramic Beaded.................................................. Style B --Metal Sheathed MgO..............................
Thermotech USA- Thermocouple Styles

 

INTRODUCTION TO MgO THERMOCOUPLE

 

GENERAL INFORMATION

These unique thermocouples are manufactured by a swaging process which compresses the magnesium oxide around the wires and literally flows the metallic sheath into place around the ceramic. This method produces an assembly free of cracks or ruptures and assures the exact placement of the wires in the sheath. Because of this superior manufacturing process, MgO thermocouples are extremely rugged and are used in conditions of severe vibration, temperature, pressure and corrosive atmospheres. The sheath can be bent to conform to virtually any configuration to permit easy installation.

ADVANTAGES:

Precision formed junctions for high accuracy
Insulated for high temperatures
Field bendable to any configuration
Made from calibrated thermocouple wire
Moisture proof-insulation sealed in metal
Rapid response
Small in size
Hygienically clean
Weldable

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MAGNESIUM OXIDE (MgO) THERMOCOUPLES

MgO or magnesium oxide thermocouples are the most generally used temperature sensing devices throughout all industries. They are recommended where moisture proof, non-porous insulation is required, for special temperature or pressure conditions, for flexibility in forming, to reach inaccessible areas and fast response.

MgO thermocouples are obtainable in a wide selection of calibrations, sheath material and diameters.

The following guidelines have been established to help in the proper selection of a sensor best suited to your application.

Standard MgO Calibrations:

Thermocouple calibrations are expressed in standard ANSI type designations. The table below will indicate the calibration, ANSI designations and total temperature range of a given calibration.

Specifications:

Temperature Range: The temperature range should not exceed the limits outlined in thermocouple specifications for the specific sheath material or thermocouple type

Limits of Error: Meets or exceeds ISA specifications.

Sheath Material: 304, 316, 310, 321 Stainless Steels, Inconel and others for special applications

Insulation: 99.5% pure Magnesium Oxide (MgO)

Insulation Resistance: Minimum resistance from wire to wire or wire to sheath is 1.5 megohms at 500 vdc for all diameters.

Pressure Range: Up to 50,000 PSI at 1200F for all closed metal sheaths

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How a Thermocouple Works:

The TC is a very simple device that is simply the junction of two dissimilar metals. The junction of the two metals produces an electric voltage. The voltage produced is a very small direct current voltage and is actually in the millivolts ( mvDC ) range. Any two dissimilar metal wires will produce this voltage, however, certain types of thermocouples are selected as standard thermocouples because of the characteristics of the voltage output. The mvDC must be sufficient in magnitude to amplify so other instrumentation can utilize it for display and control of temperature. In addition to being a strong enough signal, it also should be somewhat linear over a range of temperature.

Standard Thermocouple Types:

Industry accepted standard thermocouples are: J, K, E, T, R and S.
 There are more types of thermocouples, however these are the most common with J and K 
being by far the most commonly used type. (see Thermocouple Specifications for more information)
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Thermocouple Hot Junctions:

The hot junction is the tip of the thermocouple at which the heat measurement is sensed. Magnesium Oxide (MgO) Thermocouples are available in four basic hot junction styles as follows:

"G" - GROUNDED JUNCTION

Wires welded directly to the sheath material.

Rapid response time.

Protects T/C wires from corrosive environment.

Recommended for high pressure gases and liquids.

"U" - UNGROUNDED JUNCTION

Sensing junction completely insulated from sheath material with MgO packing. (Slightly slower response time than grounded)

Recommended where readings may be affected by electrical or magnetic fields.

"E" -EXPOSED JUNCTION

Instantaneous response time as T/C wires are welded outside sheath.

The Thermocouple is sealed with a special compound against moisture.

Not recommended for corrosive environments or high temperatures.

"I" -UNGROUNDED ISOLATED

Ungrounded and also separated or isolated from each other.

(Applies to dual thermocouples only)

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Thermocouple Specifications

Type J: Iron / Constantan:
Useful range of temperature is -300F to 1200F. Maximum temperature 1600F. 
Possible problems: Oxidizes rapidly due to the iron wire. The use of the stainless steel
metal sheathed MgO style of construction has overcome some of this problem and is 
much preferred over the beaded bare wire style of thermocouple. 
Type K: Chromel / Alumel 
Useful range of temperature is -300F to 1800F. Maximum temperature 2300F.
Type E: Chromel / Constantan
Useful range of temperature is -300F to 1800F. Maximum temperature 1000F
Type T: Copper / Constantan
Useful range of temperature is -300F to 700F. Maximum temperature 700F.
Type R and S: Platinum / Platinum-Rhodium
Useful range of temperature is 40F to 3000F. Maximum temperature 2300F.
Both the R and S thermocouples are used for very high temperatures. These couples 
are relatively expensive compared to other thermocouples since they are made of platinum.
These thermocouples must not touch the sheath if a metal sheath is used for construction. 
Normally a ceramic protective tube and ceramic beads are used for construction for both 
high temperature reasons and to prevent contamination of the noble metal. 
(See Sheath Metal Temperature Limits)

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SHEATH MATERIAL LIMITS

Continuous Maximum Temperature Ratings F, in oxidizing atmospheres

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