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Control of the caustic concentration in the bath is important for the uniform and consistent quality of the mercerized textile. Proper caustic control ensures the fabric will have an even and predetermined caustic exposure, based on weight and feed rate. It also helps in controlling the neutralization steps by avoiding overexposure to caustic that can prolong the washing and neutralization part of the process.
INSTRUMENTATIONElectrical conductivity is an economical and convenient method for controlling the caustic bath concentration. Toroidal conductivity sensors are recommended to resist chemical attack by the strong caustic and fouling by accumulated solids. The versatile Model 228-02 sensor can be inserted through a 1½" ball valve or lowered into the bath from above. The large-bore Model 226 sensor is the ultimate in high-solids conductivity measurement and is usually lowered into the bath from above. The Flow- Through Model 242 sensor is also available for locations that cannot be easily accessed from above. Analyzers such as the easy to use Model 54eC and multi-parameter Model 1055 are ideal for this application because they can show readings directly in percent NaOH (caustic).
The relationship between caustic concentration, temperature, and conductivity is somewhat complicated. However, over the range of 20-28% caustic, the conductivity decreases in a relatively linear manner. Note that conductivity is a non-specific measurement and that a buildup of salts in the process may produce some background conductivity that may interfere with the concentration measurement.