The Fairchild 21 Adjustable Ratio Relay, by means of a movable fulcrum design, allows infinite signal multiplication/division over the range of 1:30 to 30:1 by turning a simple adjustment knob. Preset input and output biasing are also possible with this high quality unit.
The Fairchild 21 consists of a signal chamber lever arm, a Fairchild 20 output valve body, and pivot assembly for lever arm adjustment. The ratio of output pressure to signal pressure is infinitely adjustable. The adjustment range permits signal amplification of 1:30 or signal reduction of 30:1 by rotation of the ratio adjustment knob.
The signal pressure acting on the signal chamber diaphragm transmits a force through a lever to the control diaphragm, thus setting output pressure. The lever fulcrum is adjustable.
Output pressure is a function of signal pressure times the ratio of lever arm lengths on either side of the fulcrum. A bias may be introduced by means of the set screws.
The Fairchild 21D is available with both input and output adjustable bias. Maximum input bias is 3 psig, with a maximum output bias of 9 psig. The basic mathematical expression for the bias in this relay is:
Po = (Ps-K1) R+K2, where
Po = Output pressure
Ps = Input signal
R = Ratio of setting
K1= Input bias, (-) only
K2 = Output bias, (+) only
40 SCFM (68 m³/HR) 100 psig, [7.0 BAR], (700 kPa) supply, 20 psig, [1.5 BAR], (150 kPa)
5.5 SCFM (9.4 m³/HR) (downstream pressure 5 psig, [.35 BAR], (35 kPa) above set pressure)
250 psig, [1.7 BAR], (1700 kPa) Maximum
Supply Pressure Effect
Less than .1 psig, [.007 BAR], (.7 kPa) for 100 psig, [7.0 BAR], (700 kPa) change
Signal or Output Pressure
150 psig, [1.0 BAR], (1000 kPa) Maximum
30:1 through 1:30 (signal pressure: output pressure)
Operating Pressure (minimum)
0.5 psig, [0.03 BAR], (3.5 kPa)
0.5” (1.27 cm) Water Column
Ambient Temperature Limits
-40°F to +200°F, (-40°C to +93.3°C)
Materials of Construction
Body and Housing: Aluminum
Trim: Stainless Steel, Brass, and Zine Plated Steel
Diaphragms: Buna N and Dacron
Lever and Fulcrum: Hardened Steel