Model 14 Positive and Negative Bias Relay

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The Fairchild 14 Positive and Negative Bias Relay is designed for applications that require an output pressure that is the sum of a controlled input signal plus or minus a fixed bias.


  • Control sensitivity of 1/2" water column allows use in precision applications.
  • A balanced Supply Valve minimizes the effects of supply pressure variation.
  • An Aspirator Tube minimizes downstream pressure droop under flow conditions.
  • A separate Control Chamber isolates the diaphragm from the main flow to eliminate hunting and buzzing.
  • Mounting Bracket is available
  • Canadian Registration Number (CRN) certification for all territories and provinces.

Operating Principles

The output of the relay is the sum of the spring bias, set with the Range Screw, plus a pneumatic input signal. (Po = Ps ± K); where Po is output pressure, Ps is signal pressure, and K is the combined spring constant. The signal pressure exerts a force against the top of the Signal Diaphragm that creates a downward force on the Diaphragm Assembly and opens the Supply Valve. Output pressure flows through the Outlet Port and the Aspirator Tube to the Control Chamber where it creates an upward force on the bottom of the Control Diaphragm.

When the setpoint is reached, the force that acts on the bottom of the Control Diaphragm balances with the force that acts on the top and bottom of the Signal Diaphragm.

When the output pressure increases above the setpoint, the increase is transmitted through the Aspirator Tube to the Control Diaphragm. The increased pressure that acts on the Control Diaphragm moves the Diaphragm Assembly upward to seat the Supply Valve, move the Relief Seat away from the Relief Valve, and let downstream air exhaust through the port in the Ring Spacer.


Supply Pressure
250 psig, [17.0 BAR], (1700 kPa) Maximum
Flow Capacity (SCFM)
40 SCFM (68 m³/HR) @ 100 psig, [7.0 BAR], (700 kPa) supply and 20 psig, [1.5 BAR], (150 kPa) setpoint
Exhaust Capacity (SCFM)
5.5 SCFM (9.35 m³/HR) where downstream pressure is 5 psig, [.35 BAR], (35 kPa) above 20 psig, [1.5 BAR], (150 kPa) setpoint
Signal or Output Pressure
150 psig, [10.0 BAR], (1000 kPa) Maximum
Supply Pressure Effect
Less than 0.1 psig, [.007 BAR], (.7 kPa) for 100 psig, [7.0 BAR], (700 kPa) change in supply pressure
Less than 1/2” (1.27 cm) Water Column
Ambient Temperature
-40°F to +200°F, (-40°C to 93.3°C)
Hazardous Locations
Acceptable for use in Zones 1 and 2 for gas atmosphere; Groups IIA and IIB and Zones 21 and 22 for dust atmospheres.
Materials of Construction
Body and Housing: Aluminum
Trim: Stainless Steel, Brass, Zinc Plated Steel
Diaphragms: Nitrite on Dacron

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

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