A **ratio control system** is characterized by the fact that variations in the secondary variable do not reflect back on the primary variable. In the above diagram a ratio control system the secondary flow is hold in some proportion to a primary uncontrollable flow.

If we assume that the output of primary transmitter is A, and the output of the secondary transmitter is B, And that multiplication factor of the ratio relay is K, then for equilibrium conditions which means set valve is equal to measured valve, we find the following relation:

KA-B=O

or B/A = K, where ‘K’ is the ratio setting of the relay.