There are two types of SCR control systems:

• Open loop
• Closed loop

Each has its own particular advantages and benefits and their usage will depend on the job specifications. Simply stated, open loop control means the system has no feedback and results in minimal performance levels. With closed loop control, active measurement of NOx occurs before and after the SCR catalyst through the use of a NOx sensor. With this method, very precise delivery of the reagent can occur resulting in high levels of system performance.