A pneumatic relay uses a pilot pressure signal and a force-balance mechanism to control separate supply, output and exhaust paths. Pressure acting on an effective diaphragm area gives one force term; it does not by itself determine spool travel, port state, output pressure or flow amplification.
Pilot-Diaphragm Ideal Force Explorer
Enter pilot gauge pressure and effective diaphragm diameter. The worksheet reports circular effective area and the ideal pressure resultant in newtons, decanewtons and pound-force, before springs, friction, seats and output-pressure feedback are considered.
Equation Used
- Pilot pressure is gauge pressure applied uniformly over the declared circular effective area.
- The entered diameter represents effective pressure area, not necessarily physical outside diameter.
- The force is an ideal resultant before spring, friction, seat, linkage and feedback forces.
- The illustrated diaphragm and relay member remain mechanically connected, but their small cyclic flex is not a predicted displacement.
- No cracking pressure, spool position, port state, output pressure, flow coefficient, gain, response, leakage, noise, rating or service life is predicted.
Retains exact pressure-area arithmetic while removing the invented opening threshold and all unsupported output, gain and performance claims.
A relay is a force-balance device
Emerson's pressure-balanced relay uses diaphragm areas, supply and exhaust valves, restrictions and feedback. At equilibrium its inlet and exhaust ends are closed. A signal change disturbs the force balance; supply or exhaust can then open until output-pressure feedback returns the assembly to a new equilibrium. Pilot pressure on one area is therefore only one term in the mechanism.
Device architecture determines the function
Pneumatic relays, pilot-operated regulators, valve positioner boosters and twin-piston pressure boosters are not interchangeable names for one universal device. Their area ratios, springs, feedback paths, seats and porting determine whether they relay, regulate, boost pressure or increase flow capacity. Use the specific manufacturer schematic and performance data for the actual device.
The worksheet computes only ideal pressure force
For bar and square millimetres, the direct conversion is F in newtons equals P in bar times A in square millimetres times 0.1. This result is not an amplification ratio and does not include any opposing or feedback force.
A transparent pressure-area example
At 6.0 bar gauge and 40 mm effective diameter, area is 12.57 square centimetres. The ideal pressure resultant is 754.0 N, equivalent to 75.40 daN or 169.5 lbf, before opposing forces.
What the force ledger omits
Actual motion requires spring preload and rate, friction, seat areas, diaphragm stiffness, linkage geometry, supply pressure, exhaust and output feedback. Flow performance additionally requires port geometry, pressure ratios, gas state and tested coefficients. The former universal opening threshold, flow-amplification ratios, pressure-booster claims, clearances, response times, lifetimes, field examples, diagnostics and sizing advice are removed.
Pneumatic relay worksheet questions
Does ideal force predict when the spool opens?
No. Opposing and feedback forces are missing.
Does diameter determine pressure gain?
No. A documented multi-area force balance may establish a ratio for a particular relay.
Are flow boosters and pressure boosters the same?
No.
Can this select or size a relay?
No. Use device-specific schematics and tested performance data.
Pneumatic relay references
- Emerson Fisher 2502 Controller Manual: pressure-balanced double-diaphragm relay, area ratio and closed equilibrium.
- Emerson Fisher Positioner Manual: supply/exhaust relay sequence and output-pressure feedback.
- Festo Pressure-Regulator Principles: diaphragm, spring and output-pressure force balance and operating distinctions.
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