The post What Control is Used When a Specific Blend of Two Different Materials is Required? first appeared on the ISA Interchange blog site.
Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control 
System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for information about the CCST program.
What kind of control is often used when a specific blend of two different materials is required?
A) ratio
B) cascade
C) feedforward
D) feedback
E) none of the above
A ratio controller maintains a predetermined ratio between two or more variables or maintains the magnitude of a controlled variable at a fixed ratio to another variable. In this figure, the hydrocarbon line receives the air’s flow rate. Then the FF − flow ratio (flow fraction) − tells the FC (flow controller) how much hydrocarbon to pass into the mix. So if the predetermined ratio is 10 parts air to one part hydrocarbon, the FF takes the input from above and tells the FC below to allow one-tenth that amount of hydrocarbon to pass.
The correct answer is A.
Source: ISA News
