VCR and VCO look identical from the outside. Same hex body, same tube connection, same face-seal geometry. The difference is what happens at the sealing interface — and that difference determines where each fitting belongs.
What VCO Actually Is
A VCO fitting is an O-ring face seal connection. The seal is formed by a captive elastomeric O-ring seated in a groove on one fitting body, compressed against the flat face of the mating component when the fitting is made up. No metal gasket. No plastic deformation of a sealing element. The O-ring provides the seal through elastic compression.
This is functionally different from VCR, where a soft metal gasket is plastically deformed between two precision-machined faces. In VCR, the gasket is consumed by the sealing act — it cannot be reused. In VCO, the O-ring can be replaced without affecting the fitting body, and the fitting faces themselves are not the sealing surfaces.
Where VCO Is Used in Practice
VCO fittings appear most commonly at instrument and analyzer connections — locations where the fitting will be opened repeatedly for maintenance, calibration, or component swap, and where the gas service is compatible with elastomer sealing.
Typical VCO locations in a semiconductor gas system:
- Gas chromatograph and mass spectrometer sample connections — analytical instruments require periodic disconnection for maintenance and recalibration
- Pressure transmitter and gauge connections — instrument replacement during PM cycles
- Gas analyzer cabinet connections — frequent access for sensor replacement and calibration
- Sample points — connections opened periodically for grab samples
The common thread: these are locations where the fitting will be broken more than once, where the speed and convenience of O-ring assembly matters, and where the gas service — typically inert or low-reactivity — is compatible with the O-ring material.
Why VCO Is Faster and More Forgiving Than VCR
VCR assembly requires a new gasket at every disassembly, correct gasket seating, and sufficient torque to plastically deform the gasket to the correct thickness. Done incorrectly, the fitting leaks. Done correctly, it provides a reliable all-metal seal.
VCO assembly is more forgiving. The O-ring self-locates in its groove. Makeup torque requirements are lower. The fitting can be assembled and disassembled quickly without consumables management — the O-ring is reusable until it shows visible degradation, though periodic replacement is recommended.
For technicians performing routine instrument maintenance, VCO reduces the skill requirement and the consumables cost per maintenance event.
The Two Installation Mistakes That Cause Most VCO Failures
O-ring contamination at assembly
The O-ring sealing surface must be clean at assembly. Touching the O-ring with bare hands transfers skin oils and particulate to the sealing surface. In UHP applications, this contamination enters the gas stream. Handle O-rings with cleanroom gloves. Inspect the O-ring groove for debris before assembly.
Overtorquing
VCO fittings have a lower makeup torque specification than VCR fittings of equivalent size. Installers who work with both fitting types regularly sometimes apply VCR torque to VCO connections. The result: the O-ring is extruded out of its groove, and the connection leaks. Use torque specifications specific to VCO fittings.
Where VCO Does Not Belong
Corrosive and reactive gas service
O-ring material compatibility must be verified for the specific gas, concentration, temperature, and pressure. Standard elastomers are not compatible with halogen and halide gases. Even high-performance elastomers — Kalrez, Chemraz — have limits that must be evaluated against specific service conditions. For corrosive gas service where long-term elastomer compatibility cannot be confirmed, VCR metal gasket sealing is the correct specification.
Ultra-high-purity applications with sub-ppb impurity requirements
Elastomers outgas. For process applications where acceptable impurity levels are below what elastomer outgassing contributes, VCO introduces an impurity source that VCR does not.
High-pressure service
VCO pressure ratings are lower than VCR ratings of equivalent tube size. Verify the VCO pressure rating against actual system operating and test pressure before specifying.
VCO vs. VCR: The One-Question Selection Guide
Will this connection be broken during service, and is the gas compatible with elastomer sealing?
If yes to both: VCO is a legitimate choice.
If the gas is reactive, corrosive, or requires all-metal sealing for purity reasons: VCR regardless of maintenance frequency.
If the connection will never be broken after initial installation: orbital weld is the correct answer.
Looking for VCO or VCR fittings? View KTIC's VCO fittings or contact us for specifications.
Related: VCR vs. VCO — Choosing the Right Face-Seal Fitting for UHP Gas Systems Related: Metal Seal vs. Elastomer Seal Connections in UHP Gas Systems Related: How to Select the Right Tube Fitting for UHP Applications