The present invention relates generally to the measurement of bulk thermal conductivity in a material, and more particularly pertains to a method for measuring such bulk thermal conductivity wherein a thermistor operates as both the temperature measurement sensor and the heat source.
Currently available methods and apparatus for making bulk thermal conductivity measurements typically employ the use of a special probe designed and built for that specific purpose, with such probe requiring special electronics also specifically designed to be used with the probe. These probes and their supporting electronics are relatively expensive and perform no function other than the reading of thermal conductivity. Additionally, if thermal conductivity measurements are required under conditions where the probe cannot be recovered, as for instance when the same is deeply buried in soil or in concrete while it is being "set up", use of these probes can become economically prohibitive.
For example, U.S. Pat. No. 3,938,383 to Sayer is directed to the detecting of thermal characteristics of a subsurface formation in sites by inserting a predetermined volume of a cryogenic material into the formation. From the information received by conducting energy tests and other calculations, the thermal conductivity of the formation can be determined. The apparatus of the invention includes the use of a specially designed vessel adapted to receive a selected quantity of a cryogenic material, such as liquid nitrogen, liquid helium or liquid argon, such substances being suited for the purpose of the invention in that they have extremely low temperatures and are caused to boil off or vaporize when subjected to higher temperatures. Further, the apparatus utilizes an array of detection equipment which includes a plurality of sensors or receivers adapted to receive the specific form of test energy discharged by a transmitter and in addition to the special probe vessel and receivers, the apparatus also includes the use of a multichannel amplifier, a multichannel recorder, a pulse generator and an operating electrical circuit. As such, it can be seen that this typical prior art thermal conductivity measurement apparatus and method is of a complex and uneconomical construction which is difficult to set up and operate.
Another example of a prior art apparatus for determining bulk thermal conductivity is to be found in U.S. Pat. No. 4,155,244, to Bhattacharyya, wherein a test specimen may be placed between hot and cold plates, and a heat flow sensing means may then be placed intermediate the test material so as to form two portions of the test specimen. A guarded hot plate measurement of the heat flow to the hot plate enables calibration of the sensitivity of the specimens and the heat flow sensing means. The apparatus is then calibrated for measurement of other specimens of like physical properties. As is apparent, this apparatus can be effectively utilized only in those situations where a material of similiar property to the material being tested is available so that a calibration of the apparatus can be achieved before the thermal conductivity measurement is taken.
Also of interest is U.S. Pat. No. 3,552,185, to Goode, Jr., et al, wherein a fixed thermal difference across a sample is accomplished by placing the sample between two surfaces whose temperatures are accurately controlled. The cold plate temperature is set and controlled by a thermoelectric heat pump driven by a high-gain proportional controller, while the hot plate is concurrently operated at a controlled temperature with the heat thereto being supplied from a close-coupled proportionally controlled low mass heater. The heat flow through the sample is measured with a heat flow transducer which produces an EMF that is proportional to the heat flow per unit time and area through the material, with the EMF thus derived being the voltage divided by a slide wire which is mechanically coupled to an in-place thickness indicator. Appropriate analog corrections are made to provide a signal for direct digital read-out of the thermal conductivity associated therewith. Inasmuch as the device utilizes a silicon controlled rectifier system, separate heaters, thermoelectric transducers, digital voltmeters and hot and cold thermometer probes, it can be seen that the same is of a complex construction which is uneconomical to manufacture and which is difficult to set up and utilize.
As can be appreciated then, there is a continuing need for new and improved apparatuses and methods for measuring bulk thermal conductivity which employ the use of simple and economical measuring equipment and associated techniques.