In a review paper on the impact of advances in technology on the measurement of thermal properties presented at the 1990 Vienna European Conference on Thermophysical Properties, I referred to the twenty plus year period of the early nineteen sixties to late eighties as being “The Golden Age” for those of us whose interests lie in the subject.
During this period, the stimulation of the increasing growth of technologies and associated and accompanying new and improved materials provided many opportunities for a significant increase in the advance, improvement and understanding of properties measurements.
However, this time period coincided with and was overshadowed by three world events that involved increasing developments of new technologies for potential solutions. These were “Cold War” and the “Space Race” between the western nations, primarily the USA, and the Soviet Union, and later and most importantly for the future of mankind the “Energy Crises”. Fortunately, looking on the bright side of life, these also provided the ultimate stimulant of adequate FUNDING, at least until the nineties, becoming available to address the many technical issues involved.
Personally, I could not have chosen a better time than the mid-sixties to switch employers by a “jump across the “pond” from the National Physical Laboratory a UK government organization to the Dynatech R/D company USA and commercial measurements laboratory. This research and develop company founded in the early sixties by Warren Rosen how and fellow MIT professors including the president J.P. Barger thus ensuring a heat transfer reputation second to none. With a staff consisting mainly of MIT graduates and undergraduates, many from overseas, it became known as the local league of nations. At one time, in a total of some eighty staff there were twenty of us from different countries in four continents.
With this background and from approaches to and by government defense department organizations and the large group of the supporting and supplier commercial companies, the company quickly became involved in developing and building special one-off systems to measure thermal conductivity and other related properties including specific heat and emittance for high temperature applications. These included for example, designs of guarded hot plate and comparative thermal conductivity methods for temperatures to 1300 kelvin and an adiabatic calorimeter for specific heat measurements to 800 kelvin.
It soon became obvious that the market for these large special research type systems was limited to fulfill only the needs of the large government departments and their major aerospace and similar companies. Correspondingly, it also became obvious that there was a significantly larger need for a reliable source that could provide reliable measurements service to the smaller service and supply companies.
Within a year or so of my arrival, a growing Measurement Laboratory service became available to all types of clients ranging from those requiring numerous measurements for a large project space or defense projects to a one-off test for a local thermal insulation company. In addition, the facilities and capabilities were used to support the instruments group for their development and improvement of its new and improved measurement systems.
The basis of the laboratory consisted of forms of required apparatus including internally built prototype models of, for example, a high temperature (>1200K) guarded hot and flat slab comparative apparatus together with a dilatometer and an adiabatic calorimeter. These were quickly supplemented with systems having vacuum and pressure capabilities to address increasing special requirements together with a quadrupled growth of staff.
The subjects for measurement studies were so many and varied. At any one time, the range could be from measuring high temperature thermal properties of carbon-carbon composites used for the leading edge and nose cone of the space shuttle to the cryogenic temperature properties of balsa wood used to insulate liquid natural gas tanks on LNG container ships to a single test on a composite filled polymer proposed for use as a heat sink in a computer system.
In the studies of some the “larger” projects where many measurements or materials were involved and where the subjects could be made public or were not commercially confidential, the client was approached with the suggestion, a joint paper be published, a paper on the results. Through this approach, it was possible to publish in excess of thirty paper in the open literature on a variety of subjects. Many of these contained not only good technical information but also interesting and entertaining results with the added function of being reliable marketing information for the instruments. I hope to address one or two of these in future blogs.
At the NPL conference I mentioned in the earlier blog, and in the resultant two volume Thermal Conductivity I edited, the subject of different transient methods was addressed both for solids and fluids. Both included what may be described only as a “very brief” reference to the existence of a promising new technique called the “flash method” to measure thermal diffusivity developed in the mid-sixties by Bill Parker and his colleagues. Some five or so years later this interest had exploded into becoming what I consider as the first of the two most significant events that basic contributed to “a paradigm shift” i.e. scientific progress accomplished by long periods of relative stability interrupted by a revolution after which nothing is ever the same, to start a new era for thermal properties measurements.
This rapid, few seconds, dynamic transient technique, that uses small 10 to 20mm diameter disc specimens, generally less than 5 mm thick with blackened surfaces now named the “laser flash”, due to the universal use of a laser as the power source appeared to be the answer to “the maidens prayer” for an “ideal” technique. Initial verification was undertaken using then existing metallic reference materials by a comparison of known values to those derived conductivity values obtained from the measured diffusivity, using density and accepted literature values for specific heat.
For materials where the solid conduction mode is the major heat transmission, the method is relatively simple and has been shown to have an uncertainty of +/- 4% over a broad temperature range in excess of 1300kelvin elevated temperatures. However, this can only be attained when later developed corrections and data analyses are employed and comparison of individual techniques and analyses are evaluated. While several individual comparisons with steady-state methods indicated that there is good agreement between measured and derived values, there have been many occasions where such results for a particular material were in disagreement due not only to the test accuracy but more often the uncertainty of the specific heat used.
Ultimately, the method has become so popular that it was “overused” or misapplied, for example, on materials having significant gaseous and/or radiative heat transmission modes. In addition, the small specimen size does restrict its use for lower thermal conductivity, inhomogeneous materials and especially thermal insulation and similar type materials that are known to require larger specimens to be considered representative of the material.
The second event contributing to this significant paradigm shift was the development the heat flow meter measurement method for thermal insulations. This event was stimulated by the pressing needs of the thermal insulation industry to address issues arising as result of the two Energy Crises in the early seventies. The requirement was the need for a rapid technique to supply reliable values (+/-3% to 4%) of the thermal resistance of insulation products used in buildings. This specification was necessary to satisfy both the Quality Control and Performance Assurance requirements of the industry and more importantly the regulatory bodies becoming involved in energy conservation.
This simple method in concept and operation, can be described as a modification of the guarded hot-plate but where the absolute measurement of heat flow is replaced by use of a calibration procedure involving a heat-flux transducer(s) plus one or more reference materials. It was first proposed in the late fifties and recommended for use as an industrial tool by Pelanne and Bradley of Johns Manville Company. Soon, much improved as a commercially available system by Dynatech R/D with additional modern technology, it became the invaluable tool to satisfy the immense increase in requirements for thermal insulations.
In addition to speed of measurements (tens of minutes instead of hours) low initial and use costs the real advantage is that the apparatus is “self-guarding” providing the thickness (thermal resistance) of the calibration and test specimens are held similar. However, claims of 1% have often been made for some other commercial systems despite the uncertainty of the reference material provided by national measurements laboratories being at best +/- 2%
It is not an exaggeration to say that the introduction of this method was a “revolution in measurements on insulations”. National and international standardization was quickly established. It was the first thermal properties method for which commercial apparatus was manufactured in quantity. Many of original models have been in operation in the laboratories and plants of a manufacturers and services companies worldwide for over 30 years proving the adage “keeping things simple” is best.
An important factor that arose early in the period was the technical and economic necessity that the reliability of a measured value be verified in some way. The driving forces led to a need for relative simplicity of concept and operation, plus speed of measurement. National and international trade has expanded such that the need for “leveling of the playing field”. These requirements led to corresponding increases in the need both for definitive material specifications and development of national and international test standards, plus provision of standard reference materials for final verification purposes. This resulted in a corresponding increase of involvement in standards development both in ASTM and ISO providing to mix with the major experienced workers from many countries so that appropriate regulations and standards were available well before the end of the era.
In conclusion, I have to admit that from my coincident arrival in the USA at the start of this period of dramatic change to when I left twenty eight years later, I was lucky to experience this phenomenon from start to close to end. I had so many opportunities to meet, visit, participate and work with so many of the “experts” that I “earned my spurs” in the who, what where, why and how of good thermal measurements of thermal properties.