5 Apparatus
Apparatus specifically constructed or assembled from stand-alone instruments can be used to realize this measurement method. The block diagram shown in Figure 2, and the following subclauses illustrate an appropriate equipment.
5.1 Test cell
Three terminal test cells designed according to the recommendations given in IEC 60247 are generally suitable for this measurement.
An additional type of cell in which there is no bridge made by any solid insulating material between the measurement electrodes, as shown in Figure 3, can be used. This type of cell often proves more accurate on highly insulating liquids.
The distance between the outer and the inner electrode is typically 4 mm; the minimum distance should not be lower than 1 mm. The material recommended for the electrodes is stainless steel. As an example, the diameter of the inner electrode is 43 mm, that of the outer electrode is 51 mm; the height of the electrodes is 60 mm; the diameter of the stainless steel vessel is 65 mm.
This type of test cell was designed to minimise the effects of contamination from the surfaces in contact with the liquid: although the surface in contact is large, the ratio X = "electrode surfaces"/"liquid volume" is rather small (X = 2,6 cm(-1)) due to the large volume of liquid (v = 200 cm3).
NOTE It is recommended to restrict the use of a given cell to a particular type of liquid.
5.2 Heating device
The heating device shall be adequate to maintain the temperature of the measurement cell within +/- 1 °C of the prescribed value. It may consist of a forced draught air oven or an oil-filled thermostatically controlled bath fitted with a shelf to support the cell.
The heating device shall provide screened electrical connections to the cell.
5.3 Square wave generator
The square wave generator shall deliver a highly stable quasi-rectangular voltage. The following characteristics are suitable:
- amplitude: 10 V to 100 V;
- frequency: 0,1 Hz to 1 Hz;
- ripple: < 1 %;
- rise time: 1 ms to 100 ms.
5.4 Measurement chain
The conduction current I R through the test cell is measured during the second part of each half-wave and averaged over a number of periods depending on the range of measurement. The measuring chain gives the conductance G of the test cell.
As an example, the range of measurable conductance values is 2 x 10(-6)S to 2 x 10(-14)S with a margin of error of less than 2 %.
The capacitance C of the test cell is deduced from the current measured during the rise of the voltage.
The measurable capacitance values are between 10 pF and 1000 pF with an uncertainty of less than 1 %.
As an example, for a liquid of relative permittivity εr = 2, a conductance value of 2 x 10(-14)S gives tanδ = 0,8 x 10(-6) at 50 Hz.
6 Sampling
The insulating liquid samples shall be taken in accordance with IEC 60475 by qualified personnel.
During transportation and storage the samples shall be protected from direct light.
7 Labelling
Insulating liquid samples shall be properly labelled before being dispatched to the laboratory.
The following information is necessary:
- customer or plant;
- identification of the liquid (type and grade);
- identification of equipment;
- date and time of sampling;
- temperature when sampling;
- point of sampling;
- other pertinent information.