Model 21 Relay

Enquire about this product:

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.


  • Venturi aspiration compensates for downstream pressure losses.
  • Optional input and output biasing allows versatility in applications.
  • Adjustable from 30:1 dividing ratio to 1:30 multiplying ratio assures infinite pressure adjustments.
  • Floating seal ring isolates control chamber which increases stability by reducing effect of high flows.
  • Panel or Line Mounting.
Operating Principles

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


Flow Capacity
40 SCFM (68 m³/HR) 100 psig, [7.0 BAR], (700 kPa) supply, 20 psig, [1.5 BAR], (150 kPa)
Exhaust Capacity
5.5 SCFM (9.4 m³/HR) (downstream pressure 5 psig, [.35 BAR], (35 kPa) above set pressure)
Supply 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
Ratio Range
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

Catalogue PDF: application/pdf
Operational PDF: application/pdf
Located in: Relays

Latest News

FIPA presents: Oval vacuum cups

Introducing the newly developed oval vacuum cups series.

The vacuum cups are characterised by a very long cycle life, which has also been successfully demonstrated against vacuum cups of competitors in practical operation. The long service life is enabled by means of a new high wear- and oil-resistant NBR material and a special vacuum suction cup construction which minimises the load on the vacuum cup during handling.


FIPA presents: New ID grippers for parts with different diameters

Flexible and service friendly

The pneumatically operated, single-acting ID grippers from FIPA GmbH have already established themselves as the alternative for workpieces with holes or cut-outs or for when vacuum cups or grippers aren’t just right for handling goods. FIPA has completely overhauled its portfolio and now offers users maximum flexibility with minimal set-up time.

The small ID grippers are now equipped with food-safe gripper heads made of polyamide (PA) and are suitable for light and small parts with openings measuring 3 to 10 millimeters in size. If necessary, the gripper heads can be easily exchanged in a non-destructive manner.