Inline static mixing of acid or caustic into a process stream ensures uniform pH at the measurement point, preventing control loop instability. The low additive volume fraction (typically 0.01–0.5% v/v) demands a well-positioned injection quill and sufficient mixing length to achieve Coefficient of Variation (CoV) ≤0.05 upstream of the pH probe.
Coefficient of Variation (CoV) ≤0.05 at mixer discharge or within 3–5D downstream. Injection ratio typically 0.01–0.5% v/v (acid to main flow). Injection velocity ratio 1.5–3:1 jet-to-pipe velocity. Minimum 6 elements for 10:1 turndown. Velocity Gradient (G-value) not critical — concentration uniformity governs.
Preferred MOC: 316 Stainless Steel standard. FRP or PVDF-lined for fluorosilicic acid (H₂SiF₆) and aggressive mineral acids. Rubber-lined CS for lime (Calcium Hydroxide / Lime (Ca(OH)₂)) slurry duty.
Injection quill mandatory — centreline penetration, angled 45° downstream. KE-R removable elements for lime and scale-forming reagents. Wide turndown (10:1) — verify CoV at minimum flow.