BS EN ISO 15091:2020 pdf download – Paints and varnishes – Determination of electrical conductivity and resistance
To compare the resistance of the sample with a reference resistance, a bridge circuit is used in which the resistances are balanced so that the bridge current becomes zero. The resistance of the sample is calculated from the ratio of the resistances of the bridge circuit. Because the bridge current is zero, errors which can result from the existence of a load on the voltage source when the voltage/current measurement method is used are avoided. The only contributions to the overall measurement error are any uncertainty in the reference resistance and any uncertainty in the adjustable resistance. See Figure 4.
4.2 Avoidance of electrolysis and polarization effects
In order to avoid electrolysis or polarization effects which would falsify the measurement, measurements of the resistance are usually carried out using alternating current. The frequency of the voltage applied to the measuring cell should, however, be as low as possible in order to minimize the contribution made by the reactance of the measuring cell, which acts as a capacitor.
5.1 Measuring instrument
Use a resistance- or conductivity-measuring instrument calibrated as described in Annex A.
5.2 Measuring cell
The measuring cell consists of electrodes insulated from each other, with a known cell constant. The electrodes should preferably consist of a material that is easy to clean and inert (e.g. stainless steel, platinum, graphite, titanium) in order to make sure that the measurement will not be invalidated by changes in the electrodes. For examples of suitable electrodes, see Figure 5. It is important to ensure that the measuring cell is completely immersed in the liquid. The exact depth will depend on the type of electrode. The cell geometry shall be chosen so that the possibility of contamination by dirt is minimized.
6 Sampling
Take a representative sample of the product to be tested, as described in ISO 15528.
Examine and prepare each sample for testing, as described in ISO 1513.
7 Procedure
7.1 Test conditions
Carry out the test at a temperature of (25 ± 1) °C, unless otherwise agreed. The deviation from any agreed temperature shall also be not more than ±1 °C.
The dependence of the conductivity on the measurement temperature is shown in Annex B.
7.4 Measurement of the electrical resistance or the electrical conductivity
Take the required quantity of the paint sample and immerse the electrodes in it, avoiding the formation of bubbles. If required, homogenize the material before doing this. Prior to the measurement, check the measuring cell for cleanliness and remove any particles of dirt. Connect the measuring cell to the measuring instrument. For measurements using a direct-current measuring instrument, e.g. a Wheatstone bridge, read the value after a waiting time of 10 s in order to allow the electrical potential to become constant. For measurements using an alternating-current measuring instrument, read the value after an agreed waiting time. When using continuously measuring instruments, the value shall only be read if it is constant for more than 20 s. Immediately after the test, thoroughly clean the measuring cell.