Early Guarded-Hot-Plate Apparatus

 

The earliest work at NIST (formerly the National Bureau of Standards) on heat transfer through thermal insulation and building materials began about 1910 following a request by the American Society of Refrigerating Engineers to provide the coordinated and usable data pertaining to heat transmission in insulation needed for design purposes. At the time, however, a precise method for measuring heat transmission through insulation was unavailable. In 1912, Dickinson conceived and built the first guarded hot plate apparatus at NIST for this purpose. Later, while traveling in Europe, he learned that Poensgen in Germany had been using a guarded hot plate for thermal conductivity measurements since 1910. Prior to the development of the guarded hot plate, heat transfer through insulation had been determined by methods in which heat was transferred through panels of insulation from warm air on one side to cool air on the other. The results of tests of this type were in many cases expressed as thermal conductivities, but are now defined as thermal transmittance values.

The first important publication in this field by Dickinson and Van Dusen in 1916 was described as containing accurate determinations of heat flow through air spaces and through 30 insulating materials. This publication also promoted the usage of standard terminology for thermal transmission measurements obtained by means of the hot plate method. Subsequent measurements of insulating materials were reported by Van Dusen in 1920 and Van Dusen and Finck in 1928 using similar apparatus. During these years, NIST continued to improve and standardize the hot plate method. About 1929, Van Dusen built what was to be the final version of this type of guarded-hot-plate apparatus. This particular apparatus operated consistently for NIST for more than fifty years until 1983. In 1987, the apparatus was officially relocated under the guardianship of the NIST Museum for preservation and display.

In 1945, the American Society for Testing and Materials formally adopted the guarded-hot-plate method as a standard test method based, in part, on NIST's design. In 1947, Robinson and Watson extended the temperature range of the guarded hot plate apparatus and in the next few years completed the first interlaboratory comparison of thermal conductivity tests of insulations among laboratories, jointly sponsored by the American Society of Heating and Ventilating Engineers and NIST. This series of tests clearly demonstrated the need for suitable means to calibrate the apparatus of industrial and other laboratories. Shortly thereafter a program was devised for supplying to industry measured samples of suitable insulating materials for calibration purposes. By 1977, more than 300 laboratories had been served, resulting in considerable improvement in the quality of thermal conductivity data on insulating and building materials reported in technical journals and handbooks.

Line-Heat-Source Guarded-Hot-Plate Apparatus

In 1964, Robinson first presented the basic design of the line-heat-source guarded hot plate to a thermal conductivity conference sponsored by the National Physical Laboratory in England. The design was reported in Nature (1964) as follows:

H.E. Robinson (U.S. National Bureau of Standards) discussed forms of line heat sources that could be used as heaters in apparatus for measurements at lower temperatures on insulating materials in disk and slab form. These new configurations lend themselves more readily to mathematical analysis, they are more simple to use and would appear to be able to yield more accurate results.

The design was novel. In contrast to a (conventional) guarded hot plate that used uniformly distributed heaters, line-heat-source guarded hot plates utilized circular line-heat sources at precisely specified locations. By proper location of the line-heat-source(s), the temperature at the edge of the meter plate can be made equal to the mean temperature of the meter plate, thereby facilitating temperature measurements and thermal guarding. The benefits offered by a line-heat-source guarded hot plate included: simpler methods of construction; improved accuracy; simplified mathematical analyses for calculating the mean surface temperature of the plate as well as determining the errors resulting from heat gains or losses at the edges of the specimens; and, use under vacuum conditions.

In 1971, Hahn conducted an in-depth analysis of the line-heat-source concept and investigated several design options. The design, mathematical analysis, and uncertainty analysis for a prototype line-heat-source guarded-hot-plate were published in 1973 by Hahn, Robinson (posthumously), and Flynn. Construction of the prototype apparatus was completed in 1978 and described by Powell and Siu. The performance and uncertainty analysis were published in 1981 by Siu and Bulik. Because of the promising results from the prototype, NIST began plans for a second, larger line-heat-source guarded hot plate apparatus. Construction of this apparatus was dramatically hastened due to a ruling by the U.S. Federal Trade Commission in 1980 concerning the labeling and advertising of home insulation.

Near the end of 1980, the second line-heat-source guarded-hot-plate apparatus was completed under the efforts of Hahn and Peavy of NIST and Ober, a guest worker from private industry. Almost immediately, measurement services for the public began early in 1981 with the laboratory providing the first full thickness reference materials comprised from a low-density glass-fiber thermal insulation. From 1981 to 1996, more than 75 measurements have been provided. This apparatus eventually replaced the earlier guarded hot plate apparatus that was constructed in 1929 and is still in service today. In 1996, the American Society for Testing and Materials (ASTM) formally adopted the line-heat-source concept as a standard practice based, in part, on NIST's design. The ASTM standard also includes an adjunct with blueprints of both the NIST prototype and second-generation line-heat-source guarded hot plate apparatus.

 In addition to measurement services for the public, the guarded-hot-plate apparatus has been used to develop Standard Reference Materials (SRMs) for thermal resistance. The Standard Reference Materials Program at NIST provides a valuable service for achieving measurement quality and traceability to national and international standards by distributing over 1300 Standard Reference Materials (SRMs) including several thermal insulation SRMs. The motivation for thermal insulation SRMs began in the 1970's when the American Society for Testing and Materials Committee C-16 on Thermal Insulation published a recommended plan advocating the establishment of an SRM thermal insulation program. In response, from 1979 to 1987 NIST, in a coordinated effort with the U.S. Department of Energy, completed measurements for characterizing three thermal insulation SRMs including fibrous-glass board, fibrous-glass blanket, and fumed-silica board. More recently, in 1996, NIST has established an expanded polystyrene board SRM for standard window test methods. These thermal insulation SRMs were established based on data obtained from the guarded hot plate apparatus described above.

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