A dimension used to identify a connection need not describe the passage available to the air.
Read the two diameter fields
SMC's identified tubing table lists outside and inside diameters separately. Preserve both rather than shortening the tubing description to one number. Primary reference: SMC tubing dimension fields.
The actual fitting specification may use outside diameter to identify its tube interface. Internal diameter defines a different geometric passage. A dimensional match is useful evidence for its own question but does not automatically establish material compatibility or the complete circuit's flow capacity.
A hypothetical equal-outside-diameter pair
Assume two invented tubes have 8 mm outside diameter, with internal diameters 5 mm and 6 mm. Their ideal circular bore areas are approximately 19.63 mm² and 28.27 mm². The 6 mm bore has 1.44 times the area because the ratio is(6/5)². The equal outside dimensions do not make those passages equal.
This geometric comparison establishes no actual pressure loss or flow rating. Keep inside diameter, length, fittings and applicable conditions in separate rows for the complete flow review. The values are not SMC catalog dimensions or HM tubing specifications.
Preserve a complete tube designation
Ask which dimension the connector requires and which bore evidence the circuit calculation uses. Retain manufacturer part identity, units, material and applicable tolerance information with those fields. Avoid treating an unexplained 8 mm note as a complete tube description.
If a spare changes wall thickness while retaining outside size, update the bore comparison and relevant duty evidence. This guide helps a buyer identify which dimension answers which question; it supplies no universal tube-flow equation, fitting approval or instruction to substitute installed tubing solely from the area calculation.
Customer Questions
Does equal outside diameter prove equal bore?
No. Retain inside diameter separately.
Does 1.44 times bore area prove 1.44 times actual flow?
No. The complete pressure-loss and operating basis remains necessary.
Is a diameter alone the whole fitting approval?
No. Other interface conditions also apply.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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