Series 661 Amplifying Relays are fixed-gain force-balance instruments, which incorporate bias adjustment that amplify pneumatic instrument signals. For example, a 3-15 psi signal can be amplified to operate a 3-27 psi control valve.
The input pressure signal. acting upon the effective area of the upper diaphragm, produces a force opposed by the force produced by the output pressure over the effective area of the lower diaphragm and by the manually-set (constant) spring force. Any imbalance in the opposing forces will operate the pilot valve to throttle supply air to change the output until re-balance is achieved.