Model 661, amplifying relay with bias
The Model 661 is a force-balance instrument that amplifies a pneumatic instrument signal. The output of the relay
can be either direct or reverse acting, depending on the unit supplied (see Model Designation). The output is
proportional to the input by the gain factor (e.g. 2:1, 1:2). Biasing is accomplished by setting the adjusting screw at
the top of the instrument.

Model 66, amplifying and reducing relay
The Model Series 66 Amplifying and Reducing Relays are used in control circuit to amplify and reduce pneumatic
signals. The output of a relay is proportional to its input by the fixed gain factor of that relay model.