7. Standardization of Apparatus
7.1 The thermal conductivity of the cell contents, KL, shall be expressed in terms of the following equation:
where A and B are essentially constants that depend on the dimensions of the cell and its materials of construction. If the cell is purchased the values of these constants should be certified by the manufacturer.
NOTE 2 - A, in fact, is a constant depending only upon cell geometry, type of glass and, to a lesser extent, the operation of the temperature bath and bath fluid. Within the present recommended accuracy of the method, B can be considered to be a constant.
7.2 To determine the absolute values of the cell constants, the various dimensions of the cell are obtained by appropriate measurements made during and after the construction of the cell. The conductivity of the glass cell body is obtained from the manufacturer's literature.
7.3 An additional relative calibration procedure may be used, with reliable thermal conductivity standards. A standard material, such as dimethyl phthalate (Table 2), is placed in the cell. For a given current, ΔT and the cell resistance are measured in the manner described in 8.1. By substituting B, as determined in 7.1, and the above-measured values into Eq 1, constant A can be determined with accuracy. Small errors in B have little effect on the calculated thermalconductivity. Hence, if exact dimensions for the calculation of the value of B are not available, a rough approximation will suffice in many instances.
7.4 The temperature-resistance relationship of the cell filament is determined by measurement of its resistance at various temperatures with the cell filled with a fluid of high thermal conductivity, such as water (Table 3). To eliminate the effects of self-heating during this calibration, the apparent resistance at each temperature selected is measured at several low bridge currents. The actual resistance is then obtained by extrapolation of the curve of apparent resistance versus current to zero current.