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Rotork Fairchild Model 21 Adjustable Ratio Relay 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.

Operating Principles

The Model 21 consists of a signal chamber lever arm, a Model 20 output valve body, and a 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 the 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 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:

P_o = (Ps-K1) R+K2, where
P_o = Output pressure
P_s = Input signal
R = Ratio of setting
K1 = Input bias, (-) only
K2 = Output bias, (+) only

Model 21 Adjustable Ratio Relay Features

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
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
Adjustable from 30:1 dividing ratio to 1:30 multiplying ratio assures infinite pressure adjustments Venturi aspiration system compensates for downstream pressure loss
Available with both input and output adjustable bias allows versatility in applications
Floating seal ring isolates control chamber - increased stability by reducing effect of high flows
Panel or line mounting - easy installation and service in the field

Model 21 Adjustable Ratio Relay Specifications

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)
Sensitivity: 0.5” (1.27 cm) water column
Ambient Temperature Limits: -40 to +200 °F (-40 to +93.3 °C)
Materials of Construction:
- Body and Housing: Aluminum
- Trim: Stainless Steel, Brass, and Zinc Plated Steel Diaphragms: Buna N and Dacron
- Lever and Fulcrum: Hardened Steel

Model 21 Adjustable Ratio Relay Dimensions

Q&A Section with Expand/Collapse Icons

Model 21 Adjustable Ratio Relay Q & A

+ 1. What does the Model 21 Adjustable-Ratio Relay do?

The Model 21 relay is a pneumatic, adjustable-ratio relay: it converts an input (signal) pressure into an output pressure by using a lever-arm mechanism, letting you infinitely vary the ratio of output to signal from 1:30 down to 30:1 by turning a ratio-adjustment knob that moves the lever’s fulcrum. You can also introduce bias via set screws, so that the output can be slightly offset (positive or negative) from the pure ratio-based value.

+ 2. How precise and stable is the output?

The Model 21 uses a venturi aspiration system to compensate for downstream pressure loss, helping maintain a stable output even when the system pressure fluctuates. It also features a floating seal ring that isolates the control chamber, making it more stable by reducing the effect of high flow on the control diaphragm. Because of its lever-arm design, you get very fine control over output pressure based on your chosen ratio.

+ 3. What are the limits on input/output bias?

The M21 supports both input and output bias via set screws (in some variants, like the M21D). The maximum input bias is 3 psig, and the maximum output bias is 9 psig. When you include bias in the calculation, the output is governed by the formula: ???? = (???? ? ??1) × ?? + ??2, ?where ???? is the input signal, ?? is the lever-arm ratio, ??1 ?is the input bias, and ??2 is the output bias.

+ 4. What are the operating and flow specifications (pressure, flow, temperature)?

According to the spec sheet, the Model 21 can handle a maximum supply pressure of 250 psig. Its maximum output (signal) pressure is 150 psig. For the typical operating flow, it supports up to 40 SCFM (68 m3/hr) at standard conditions. The relay’s ambient temperature range is from –40°F to +200°F (–40°C to ~93°C), which makes it suitable for a wide variety of industrial environments.

+ 5. How is the Model 21 mounted, and what materials is it made from?

The Model 21 supports both panel mounting and inline (pipe) mounting, so you can install it where it best fits your system layout. In terms of construction, it has an aluminum body, with internal trim made of stainless steel, brass, and zinc-plated steel. The diaphragm is made from Buna-N (nitrile) on Dacron, and the lever/fulcrum assembly is made of hardened steel to ensure durability.

Download Rotork Fairchild Model 21 Adjustable Ratio Relay Datasheet

Rotork Fairchild Precision Control Product Overview Catalog

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