Four connection types. Each one correct for a specific set of conditions. Each one wrong — sometimes seriously wrong — for conditions it wasn't designed for.
The Decision Framework
Tube fitting selection for UHP gas systems is a decision about three variables:
- Disassembly frequency — how many times will this connection be broken over its service life?
- Purity requirement — what is the acceptable contamination contribution from the connection itself?
- Installation context — is pre-fabrication possible, or must the connection be made in the field?
The same gas, same pressure, and same tube size can require different fitting types at different points in the same system.
The Four Connection Types
VCR metal gasket face seal
Correct for: process gas isolation points, cylinder and source connections, any connection that will be broken more than once during service life, corrosive gas service, high-purity applications requiring all-metal sealing.
The gasket is a single-use consumable. Every disassembly requires a new gasket. This is the feature, not the limitation — the gasket absorbs the wear from disassembly, protecting the precision sealing faces on the fitting bodies.
VCO O-ring face seal
Correct for: instrument connections, analyzer sample points, gauge connections, and locations requiring frequent access where inert or compatible gas service allows elastomer sealing.
Not correct for: corrosive gas service, ultra-high-purity applications where elastomer outgassing is unacceptable, or any service where the specific gas is incompatible with the O-ring material.
Compression fittings
Correct for: general instrumentation and utility gas connections where tube-to-fitting alignment is variable and field assembly convenience outweighs the higher contamination contribution relative to face-seal fittings.
Generally avoided on critical UHP process gas wetted paths because face-seal or welded connections offer better control of leakage and contamination risk.
Orbital weld
Correct for: permanent connections in UHP process gas systems — tube runs, valve-to-tube connections, any point that will never require disassembly for maintenance. No gaskets, no O-rings, no ferrules, no interfaces to leak.
Not correct for: any connection point that will require disassembly. Cutting out a welded connection for maintenance access is a major operation.
The Practical Allocation in a Gas Stick
- Cylinder connection: VCR — broken at every cylinder changeout
- Inlet isolation valve to tube: orbital weld — permanent
- Tube runs between components: orbital weld — permanent
- Valve to regulator connections: VCR — broken during PM and component replacement
- Instrument taps and gauge connections: VCO — frequent access, inert service
The pattern: weld what won't move, use VCR where maintenance access is needed, use VCO only on inert instrument connections.
Two Specification Errors From the Field
Compression fittings on process gas lines
A gas delivery system built using compression fittings throughout — common in general industrial gas systems — showed persistent particle control problems and higher leak rates in UHP process gas service. The system required complete rebuild with appropriate fittings.
VCR fittings at high-frequency maintenance points
A filter housing connected with VCR fittings required replacement every 90 days. Over two years, the gasket and labor cost at this single point exceeded the cost of redesigning the connection. The fitting type was not wrong — VCR is correct for a connection that gets broken — but the maintenance frequency had not been factored into the total cost of the selection.
Bottom Line
The correct tube fitting for a given connection point is determined by what will happen to that connection over its service life. Weld permanent connections. Use VCR where the connection will be broken for maintenance. Use VCO only where gas chemistry and purity requirements permit elastomer sealing.
View KTIC's complete fitting range: VCR fittings, VCO fittings, weld fittings, and tube fittings. Contact us for selection support.
Related: VCR vs. VCO — Choosing the Right Face-Seal Fitting Related: Common Installation Mistakes in UHP Gas Systems