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. 

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: 

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.