Ratio Control

Ratio control the ratio of one flow rate to another is controlled for some desired outcome.

Ratio control is used to ensure that two or more flows are kept at the same ratio even if the flows are changing.


Many industrial processed also require the precise mixing of two or more ingredients to produce a desired product. Not only do these ingredients need to be mixed in proper proportion, but it is usually desirable to have the total flow rate subject to arbitrary increases and decreases so production rate as a whole may be altered at will. 


Applications of Ratio Control:

• Blending of two or more flows to produce a mixture with specified composition.
• Blending two or more flows to produce a mixture with specified physical properties.
• Maintaining correct air and fuel mixture to combustion.

A ratio-controlled system may be implemented in several different ways with pneumatics or electronic hardware. The four basic hardware procedures are the following:


1. Ratio Relay - a manually set dial on the ratio relay establishes the ratio (multiplier) to use.
  2. Ratio Controller - the ratio really has been incorporated in the controller, and a  ratio dial replaces the normal setpoint dial. The controller, normally called the single-station ratio controller, usually costs less than a ratio relay plus a remote-set controller, and occupies a less mounting space.

3. Multiplying Relay - this system consists of a manual-loading station, supplying the ratio (multiplier) for a multiplying unit. This system offers the advantage of being adaptable to having the ratio established by some other source.

4. Dividing Relay - the actual ratio between the wild and controlled flow is computed by dividing relay. The  output of the divider becomes the measurement a standard, manually set controller. The controller's setpoint dial is calibrated in terms of the computed ratio range. The controller manipulates the controlled variable until its measurement is equal to the setpoint. This system has the advantage of allowing the actual ratio to be recorded or indicated, and permits alarms to shown abnormal conditions.

Cascade Control

Cascade Control uses the output of the primary controller to manipulate the setpoint of the secondary controller as if it were the final control element.


Reasons for cascade control:

• Allow faster secondary controller to handle disturbances in the secondary loop.
• Allow secondary controller to handle non-linear valve and other final control element problems.
• Allow operator to directly control secondary loop during certain modes of operation (such as startup).

Requirements for cascade control:

• Secondary loop process dynamics must be at least four times as fast as primary loop process dynamics.
• Secondary loop must have influence over the primary loop.
• Secondary loop must be measured and controllable.

Reasons not to use cascade:

• Cost of measurement of secondary variable (assuming it is not measured for other reasons).
• Additional complexity.

Examples of cascade control:

Control of heat exchanger outlet temperature using steam flow as secondary loop.

Examples of Cascade Control :