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TCS Series – Coaxial Thermocouple Probes

Product Details

Measure surface temperature with a response time as little as 1 microsecond 

Our co-Axial thermocouple probes are coaxial thermocouples consisting of a small wire of one thermocouple material coated with a .0005″ thickness of a special ceramic insulation of high dielectric strength swaged securely in a tube of a second thermocouple material.  The insulation resistance between the wire and the tube is effective to more than 1500°C at steady state (over 1700°C for transient temperatures).  The thermocouple junction is formed by vacuum depositing a metallic coating of one to two microns thickness over the sensing end of the probe, forming a metallurgical bond.  A variety of mounting configurations and transition pieces for lead wire connections may be installed on the coaxial probe assembly. 

Fast Response Metal Wall Surface Temperature Measurements 

Our co-Ax surface thermocouple probes offer microsecond response times in the measurement of rapidly varying metal wall surface temperatures such as encountered inside combustion chambers, gun barrels, bearings, die casting dies, and apparatus for heat transfer studies. The through-the-wall mounting technique for the Co-Ax thermocouples lends itself to measurement of wall surface temperatures where it is important not to disturb the surface geometry, such as rocket nose cones, cannon projectiles, or wind tunnel models.  The wall temperatures measured in this way are often used to compute pulses of heat flux absorbed by the surface, such as aerospace models installed in shock tunnels.  The probes may also be used for measurement of internal wall temperatures where the location of the sensing element must be precisely known, such as in conduction heat transfer studies. 

The Co-Ax surface thermocouple is ideal for the applications described above because the thermal junction is formed within a one or two micron thickness at the end of the probe, thus allowing positioning of the junction within one or two microns.  The probe can be installed through an accurately drilled hole in the test surface such that the junction forms a continuous part of the test surface, or it can be installed in a “blind” hole in the test wall to measure internal temperature at a precisely known location. 

Fast Response Surface Heat Flux Measurements 

Surface heat flux is computed from the continuous or time sampled measurement of the true metal wall surface temperature versus time using computation methods reported by a number of authors since the 1950’s, several of which are described by Beck, Blackwell, and St. Clair in Inverse Heat Conduction, Wiley Interscience, New York, 1985.  The fast response capability, along with the subminiature probe diameters, allow the measurement of heat transfer rates at many points on a very small metal model in just a few milliseconds of test time.  This approach can make substantial cost savings in hypersonic tunnel heat transfer research programs. 

Since each coaxial thermocouple should be suited in configuration and in material properties to its application, a wide variety of configurations is offered.  You may specify mounting configuration by “designing your own” thermocouple probe as described on the facing page.  You may specify one of the standard configurations on the back page of this brochure, select from the hundreds of standard configurations available or provide us details of your desired configuration. 

Features 

  • Microsecond Response to Metal Wall 
  • Surface Temperature Changes 
  • Allows Very Short Duration Testing 
  • Available Probe Diameters from .015 Inch 
  • Precise Location of Thermocouple 
  • Rugged Renewable Junctions 
  • Withstand High Pressure and Heat Flux 

Applications 

  • Wind Tunnel Model Surface Temperature 
  • And Heat Flux Measurement 
  • Inside Surface Temperatures of engine cylinders, air compressors, gun barrels, bearings 
  • Die Casting Dies 
  • Blast Effects on Structures 
  • Laser Target Surface Temperatures 

Standard Configurations 

Over the years, four standard configurations have been found to cover a wide variety of applications. The dimensions are shown below: 

Model No. Mounting Thread
A 
Probe Diameter B Probe Length
C 
Thread Length
D 
Transition Length 

E 

Transition Hex 

F 

Total Length 
TCS-101 #2-56UNC 0.061 0.062 0.12 0.25 1/4 HEX 0.44 
TCS-102 #3-56UNF 0.031 0.125 0.50 0.50 3/16 HEX 1.12 
TCS-103 1/4-28UNF 0.061 0.250 1.50 0.75 3/8 HEX 2.5 
TCS-104 #4-48UNF 0.031 1.00 0.50 0.50 1/4 HEX 2.0 

In order to obtain the most accurate measurements of rapidly changing surface temperatures, it is necessary that the presence of the sensor have negligible effect on the temperature of the surface of interest. To meet such a condition when the surface is exposed to convective heating or cooling, it is also necessary that the probe not disturb the surface continuity so that the convective heat transfer rate will not be altered. These conditions are met with the Co-Ax probe when installed in a wall so that the thermocouple junction is flush with the wall surface. When so installed, the sensor will accurately indicate the surface temperature of the probe itself. In order for the probe surface temperature to be at nearly the undisturbed wall surface temperature, the probe material should be selected to have values of the thermal diffusivity, α, and the thermal conductivity, k, that match the corresponding wall material value of √kρc (or k/√α) as nearly as possible, and the probe diameter should be small compared to wall thickness. Approximate values at 212°F are: 

Materialk

Btu/ft hr °F

α

ft2/hr

√kρc

Btu/ft2 √sec °F

Copper2194.31.76
Aluminum1193.31.09
Platinum420.980.69
Iron330.630.69
Carbon Steel (C1020)270.530.62
Alumel170.260.56
Type 410 Stainless Steel140.270.45
Constantan130.230.45
Chromel P110.190.42
Type 302 Stainless Steel9.40.160.39

Probes may also be used to measure surface temperature in sliding contact areas such as bearing faces. In many such abrading applications, the thermal junction at the end of the probe continuously re-establishes itself due to the action of the facing surface in forming new “slivered” junctions of the thermocouple material across the small insulation gap. This sliver junction technique is often used to establish junctions on contoured model installations and to renew those junctions when necessary, while the probes are in place in the test article. Complete instructions for completing these junctions are available from the factory. The advantages of the probe when installed are: 

  • Time Constant: The small thickness and mass of the thermocouple junction allows a nominal time constant of 1 microsecond for the vapor deposited junction. The time constant is the theoretical time to reach 63.2% of a step change in surface temperature for a 2 micron thick junction. The observed time constant for sliver junctions is usually less than 10 microseconds. 
  • Accuracy: ISA Standard Limits of Error materials are used. Conduction losses are similar to the  undisturbed wall because the thermocouple materials become part of the wall. 
  • No Surface Protuberance: Convective flow over the surface is undisturbed. 

Design Your Own Probes 

Order “design your own” probes from the page below and the above standard models by the number as shown above. To designate the thermoelectric materials, add the ISA material designation code as – “M” at the end of the model number. For example, a model TCS-103 probe with a constantan inner thermoelement and iron outer thermoelement is designated model TCS-103-J, a model often used in die casting dies. The platinum 10% rhodium vs. platinum is not available in 0.061 diameter. 

Special Co-Ax Surface Thermocouple Probes 

The Co-Ax surface probes can be routinely customized in many ways. In addition to the shapes presented here, many units are available to install from the outside surface of a model. The sensitive end of the probe may be shaped at installation to fit the contour of virtually any metal surface. Special mounting provisions, probe sizes, and high temperature MgO insulated sheathed lead wire is available. Thermocouples can be provided to fit existing mounting holes for quartz crystal pressure transducers and spark plugs. Models with back side thermocouples are available for steady state heat flux measurements. Special materials may be used with appropriate calibration of their thermoelectric properties. Many customers have us install the probes in their test articles at our facility. Triaxial thermocouples are available which offer good thermal connection but provide electrical isolation of the thermocouple junction from its surroundings (ungrounded junction).  

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