A system to determine and control the acid etching strength of a given process solution - Case Study
- High HF and High Acid (Low pH) resistant pH and Fluoride Element
- Custom Engineered reference system for acid etching media
- Menu driven Ion Selective Industrial transmitter and controller for ion sensor calibration and process control outputs and alarms (all values controlled in ppm)
A metal etching company needed to control the power of its fluoride etching bath. The quality of the etching solution depends on the activity of the fluoride and pH. The throughput depends on the total fluoride concentration of the bath and the speed of the line. Fluctuations in the fluoride activity will result in the incomplete etching of the parts, preventing further processing and the partial or complete dissolution of the aluminum parts. Since free fluoride ion concentration is a function of pH, they needed to monitor both the pH and fluoride ion concentration in order to properly control their bath.
Most commercially available pH sensors will be dissolved by fluorides. While the process is progressing, a crust is deposited on the surface of the pH and junction element reducing the sensitivity and elongating response time. The elimination of this phenomenon requires frequent cleaning and calibration that will eventually destroy the pH element. The large quantity of etching solution passing by the sensor will exhaust the reference element of its internal salts and cause constant drifting, thus requiring recalibration. The fluoride element used in these types of measurements also will be attacked by the acid/fluoride mixture. The acid is needed to activate the fluoride for the etching process. The acid also influences the available free fluorides and changes the readings in the control system. The fluoride ion selective sensors employed suffered many of the same difficulty as the pH sensors result in poor and sporadic process control.