Surface Temperature Measurements 

In order to obtain accurate measurements of rapidly changing surface temperatures, the presence of the sensor must have negligible effect on the temperature of the surface of interest. To meet such a condition when the surface is subject 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 may be met with the model TCS and TCF probes. Either of these probes may be installed in a wall so that its sensitive face is flush with the wall surface with the temperature to be measured. 

When installed as shown, the sensor will accurately record the surface temperature of the probe itself. In order for the probe surface temperature to be the same as the wall surface temperature, the probe materials should be selected so as to have values of thermal diffusivity, α, and thermal conductivity, k, that match the corresponding wall material properties as nearly as possible, and the probe diameter should be small compared to the wall thickness. Sometimes the probe may be constructed of the wall material itself, thus avoiding the necessity for seeking a compatible probe material. 

Internal Temperature Measurements 

The TCS series probes are ideally suited for use in obtaining temperature measurements at any location within metal walls. A typical installation is shown at right. 

Fast Response Heat Flux Measurements 

For test times that allow a sensor time constant of 10 ms or greater, a linear output direct indicating heat flux transducer or radiometer should be used to conveniently measure heating rates.  

When test times are very short or microsecond time constants are required, such as in wind tunnels, arc plasma tunnels, internal combustion engines, gun barrel bore surface temperatures, rocket engines, etc., the coaxial thermocouple and the thin film thermocouple have been used for over 40 years to obtain heat transfer rates computed from surface temperature-time history. 

For the coaxial thermocouple, surface heat flux is computed from the continuous or time sampled measurement of the true metal wall surface temperature versus time using computation methods. The fast response capability, along with the subminiature probe diameters, allows 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. 

The thin film thermocouple probes have a ceramic or Pyrex substrate and may be used to obtain valuable heat transfer data in short duration testing facilities where rapid response characteristics are necessary.  For testing times up to about 20 milliseconds, the recorded temperature history of the substrate surface, as measured by the film thermocouple, may be considered the surface temperature history of a semi-infinite solid and data reduction performed accordingly to infer the heat transfer rate at the probe surface. 

 We also produce platinum thin film heat flux gages (PTF Series), resistance thermometers on glass or ceramic substrates to obtain heat flux measurements in short duration tests.   

TCS SERIES - COAXIAL THERMOCOUPLE PROBE

The coaxial probes are used to measure the surface temperature of metal walls with microsecond response times. They are constructed by swaging a center wire (first thermo-element) in a thick-wall tube (second thermo-element). The thermo-elements are insulated from each other by a thin coating of special high-temperature insulation. The thermocouple junction between the two thermo-elements is formed by vacuum deposition of a metallic plating, one micron thick, at one end of the coaxial assembly. Advantages of the TCS coaxial probe over conventional probes are: 

• Response time of order of one microsecond when used for metal wall surface temperature measurements. 

• Location of plane of thermal junction within one micron. 

• Capable of convenient installation to measure surface temperatures without forming a protrusion on the surface. 

• Available in very small probe diameters (as low as .015”). 

• Inherent rugged construction. 

TCFW SERIES - FINE WIRE THERMOCOULE PROBES

The TCFW fine wire probes measure gas temperature with fast response time in difficult applications.  The unique feature of the fine wire probes is the patented method for mounting very fine thermocouple wires that reduces wire breakage by bending or vibration. The TCFW probes have a stainless steel sheathed MgO insulated stem with two fine wire thermo-elements, welded together at one end to form the thermal junction, and welded to the larger stem wires of the same materials. The delicate thermal junction is held in place by inserting the fine wires through small holes in the end of the larger wires and winding the fine wires around the larger wires before attachment by welding and encapsulation of the weld in ceramic cement. Thus, the rigid, easily broken conventional fine wire mount is eliminated and the rugged probe mounting-stem is retained. The advantages of the TCFW fine wire thermocouple probes over those of conventional fine wire probes are: 

• Greatly increased resistance to breakage of the fine wires at the point of connection with the probe body. 

• Rugged probe body capable of mounting by soldering or welding as well as by threaded connections or compression fittings. 

TCFW SERIES - FILM THERMOCOUPLE PROBES

The film thermocouple, for measurement of surface temperature of ceramic substrates, consists of two thermo-elements applied by vacuum deposition in thin films on an insulating substrate (usually ceramic or Pyrex). The vacuum deposited films form the thermoelectric junction where they overlap on the surface of the substrate. Two lead wires of the same material as the films are embedded in the substrate and are connected to each respective metal film, but remote from the thermocouple junction area. Advantages of the TCF film probe over conventional probes are: 

• Response time of less than one microsecond to measurement of the substrate surface temperature. 

• Calibration does not shift after exposure of the thermal junction to abrasive conditions (as opposed to platinum film resistance probes that experience calibration shifts with abrasion of the resistance element). 

• Precise location of plane of thermal junction (within one micron). 

• Lead wires are remote from the junction, minimizing conduction losses.