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Model 21 Adjustable Ratio Relay

Brand: Fairchild Industrial Products
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The Rotork Fairchild Model 21 Adjustable Ratio Pneumatic Relay has a signal chamber lever arm, whereby 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 which moves the fulcrum.

Output pressure is a function of signal pressure times the ratio of lever arm lengths on either side of the fulcrum. Bias may be introduced by means of the set screws. 

The Model 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, and K2 = Output bias, (+) only.
Features
  • Adjustable from 30:1 dividing ratio to 1:30 multiplying ratio
  • Venturi aspiration system
  • Optional input and output biasing
  • Floating seal ring isolates control chamber
  • Panel or Line Mounting
The Rotork Fairchild Model 21 Adjustable Ratio Pneumatic Relay has a signal chamber lever arm, whereby 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 which moves the fulcrum.

Output pressure is a function of signal pressure times the ratio of lever arm lengths on either side of the fulcrum. Bias may be introduced by means of the set screws. 

The Model 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, and K2 = Output bias, (+) only.
Features
  • Adjustable from 30:1 dividing ratio to 1:30 multiplying ratio
  • Venturi aspiration system
  • Optional input and output biasing
  • Floating seal ring isolates control chamber
  • Panel or Line Mounting
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Specifications

Brand

Actuator Type

  • Tamper-Proof

Ambient Temperature Range

  • -40° to 93° C (-40° to 200° F)

BSP Connection

  • 1/4” BSPT Female
  • 3/8” BSPT Female

Diaphragm Material

  • Buna-N
  • Dacron

Enclosure, Body Material

  • Aluminum

Exhaust Capacity - Relieving Units Only

  • 5.5 scfm (9.4 m³/hour)

Flow Capacity

  • 40 scfm (68 m³/hour)

NPT Connection

  • 1/4” NPT Female
  • 3/8” NPT Female

Sensitivity

  • 1/2” WC (1.27 cm)

Signal or Output Pressure - Maximum

  • bar: 10
  • kPa: 1,000
  • psig: 150

Supply Pressure - Maximum

  • bar: 17
  • kPa: 1,700
  • psig: 250

Supply Pressure Effect

  • psig: 0.1 (bar: 0.007, kPa: 0.7) for a psig: 100 (bar: 7, kPa: 700) Change in Supply Pressure

Trim Material

  • Brass
  • Stainless Steel
  • Zinc-Plated Steel
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