Thermocouple, Manufacturer of Thermocouple, Thermocouple Type, Thermocouple Assemblies, Resistance Temperature Detectors, India

Nutech Engineers
143-D Government Industrial Estate, Kandivli West, Mumbai - 400 067
Phone : 91-22-28683549 / 28684580,  Fax : 91-22-28674580
email : nutechen@vsnl.com , nutechen@mtnl.net.in,
info@nutechengineers.com,  sales@nutechengineers.com

Temperature Sensing Devices

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Magnesium Oxide (MgO) Insulated Thermocouples

Thermocouples with magnesium oxide insulation are recommended where the thermocouple is immersed in liquids, high moisture, corrosive gases, or high pressures.  The thermocouples can be formed to reach otherwise in-accessible areas.  the magnesium oxide has a high dielectric strength, responds quickly to temperature changes, and is very durable.

Magnesium oxide insulated thermocouple wire is manufactured from premium quality wire incased in pure magnesium oxide and processed into a 

chemically clean outer metal sheath.  The wires are individually selected and matched and are of uniform cross section with smooth surfaces.  Finished stock is warranted to meet ANSI standard Limits of Error set forth in MC 96.1.  The unique preparation of magnesium oxide insulated thermocouple wire produces a uniform thickness of insulation with high density.  The result is a product that is mechanically strong and resistant to penetration of corrosive gases and moisture.  The diameters of 0.040 and 1/16 inch are useful for applications requiring fast response.

MgO Specifications

Insulation MgO (magnesium oxide) densely packed, 99.4% purity or better
Minimum Bend Diameter Two times the outside diameter of the sheath

CONTINUOUS MAXIMUM TEMPERATURE RATING OF SHEATH IN OXIDIZING ATMOSPHERES

304 S.S: +1650Of -- good corrosion characteristics and resistance to oxidation, generally regarded as a standard sheath material
Inconel 600 : +2100oF -- good high temperature resistance to corrosion; not suitable for use in presence of sulfur above 1000oF.
316 S.S.: +1700oF -- has excellent acid corrosion resistance, highly resistant to pitting type corrosion.
310-S.S: +2100oF -- good resistance to oxidation and corrosion at high temperatures.

*Not necessarily a recommended temperature.  Sheath wall thickness, contaminants, abrasion, and erosion must be considered.

JUNCTION CONSTRUCTION

Grounded  Thermocouple welded to the sheath.  Fast response with thermocouple protected.
Ungrounded (Insulated)   Thermocouple insulated from sheath with magnesium oxide.  Stray EMF's are prevented from affecting the reading.  Response from rapid or frequent temperature cycling is slower than for grounded style.
Exposed  Thermocouple junction is not protected by welded cap.  Used for quick response, but is susceptible to early corrosive failure.

Dual Element Common  Two Thermocouple with junctions welded.
Dual Element Isolated  Two thermocouples electrically separate in the same sheath, provides isolation where instrumentation necessitates.

TIME CONSTANTS

The time constant is the time required for a thermocouple to indicate 63.2% of a step change in temperature in surrounding media.  Some of the factors influencing the measured time constant are sheath wall thickness, degree of insulation compaction, and distance of junction from the welded cap on the ungrounded style.  In addition, the velocity of liquid or mass velocity of a gas past the thermocouple probe greatly influences the time constant measurement.  In general, time constants for measurement in a gas can be estimated to be 10 times as long as those for measurement in a liquid.  The time constant also varies inversely proportional to the square root of the velocity of the media.

Approximate Time Constants in Seconds for Different Sheath Diameters in water for a step change from 0oC to 100oC

Sheath Diameter Grounded Junction Ungrounded Junction Exposed Junction
0.040 0.2 0.7 0.1
0.063 0.3 0.8 0.2
0.125 0.5 1.3 0.3
0.188 1.0 2.5 0.5
0.250 2.3 4.3 0.6
 
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By JPN