Salts, minerals, and even dissolved gases contribute uniformly to the conductivity of a solution. This means that the conductivity can be used as an indicator of the amount of dissolved materials in a solution. TDS can be used fairly accurately when comparing the status of a single source, such as NaCl, but error can arise when trying to compare two different types of solutions. It is necessary to calibrate the meter using the same dissolved materials that are in the test solution Conductivity meters and cells should be calibrated to a standard solution before using. When selecting a standard, choose one that has the approximate conductivity of the solution to be measured. Most conductivity meters have a two-electrode cell available in either dip or flow-through styles. The electrode surface is usually platinum, titanium, gold-plated nickel, or graphite. The four-electrode cell uses a reference voltage to compensate for any polarization or fouling of the electrode plates. The reference voltage ensures that measurements indicate actual conductivity independent of electrode condition, resulting in higher accuracy for measuring pure water.