PTB News 3/2000 (554 kB) PTB News 3/2000, English edition, Issue December 2000External walls essentially determine the heating energy consumption of a building. Poor insulation results in higher heating costs and increased CO2 emission from the heating system. Decisive for the heat transfer of a wall is its thermal conductivity. PTB has developed a novel instrument to determine thermal conductivities, in particular, for masonry units. The new procedure is less time consuming and more economic than conventional ones.
Under the German Construction Products Act, the thermal conductivity must be determined for each thermal insulating masonry unit type according to fixed directives. Measurements are to be carried out on wall samples or on brick material which are continuously heated on one side and cooled on the other side for this purpose. The ensuing stationary heat flow is the measure of the quantity under test. This procedure is time consuming, tedious and, thus, expensive.
The situation can now be considerably improved with a measuring instrument developed by PTB. A thin strip of nickel placed between two halves of a brick cut for the measurement serves as thermal conductivity sensor. By means of a 4-point resistance measurement at high current, the temperature rise in the strip is determined over a period of about five minutes. The thermal conductivity results from the slope of this signal. This so-called THS (transient hot strip) instrument requires, in addition to the strip, only a current source, a voltmeter and a PC, so that the technical outlay is insignificant. The instrument allows to measure the thermal conductivity with an uncertainty of 2,6% and has meanwhile been officially approved for construction supervision of perforated bricks.
This development will contribute to improve the competitiveness of both test institutes and brick manufacturers. In addition, the large measurement range of the THS instrument opens up further fields of application as, for example, the investigation of insulating materials and plastics for window frames.