Some of the hardest leaks I've chased over the years weren't caused by bad tubing or a sloppy make-up. They were caused by a metric part sitting in a fractional system, or the other way around. The sizes are close enough to fool you, and that's exactly the problem. Here's how to tell them apart and keep them out of the wrong line.
Why the Mix-Up Happens
A few common sizes are almost twins. 1/4" tubing is 6.35 mm across; 6 mm tubing is 6.00 mm. 3/8" is 9.53 mm against 10 mm. 1/2" is 12.70 mm against 12 mm. A 6 mm ferrule can slide onto 1/4" tube, and a nut can thread onto the wrong body far enough to feel normal. It may even hold pressure for a while. Then it weeps, or worse, the tube pulls out under load.
Mixed systems show up more than you'd think: skids built overseas, analyzer packages, OEM equipment, and plants that have been patched by several contractors over the years.
How to Tell Metric From Fractional
Measure the tube, don't eyeball it. Put calipers across the OD. 1/4" reads about .250"; 6 mm reads about .236". That small difference is the whole story.
Look at the fitting markings. On many two-ferrule fittings, metric nuts and bodies carry a stepped shoulder or an "M" in the part number. Check the catalog for the brand in your hand so you know what its metric mark looks like.
Read the part number and the paperwork. DK-Lok metric parts include "M" in the size code, such as DU-6M-S for a 6 mm stainless union. Your packing slip and the line's drawings should match what's in the bin.
Ask where the equipment came from. If it was built outside the U.S., assume metric until you prove otherwise.
Keep the Two Apart on the Job
- Store metric and fractional fittings in separate, clearly labeled bins. Never mix loose ferrules.
- Tag metric tubing runs and panels so the next tech knows what they're walking into.
- When a skid is metric, order replacement parts metric. Don't "adapt" with a fractional part that seems to fit.
- If you need to join the two, use an adapter or reducing fitting made for that exact size combination, not a forced connection.
Check the Make-Up, Then Check for Leaks
After make-up, run a gap inspection gauge around the nut to confirm it was pulled up far enough. Remember the gauge confirms make-up, not size, so verify size before you start. Gap inspection gauges and the Fitting Tool, which holds stainless tube fittings from 1/16" to 1/2" steady while you work, are on our tools page.
Once the line is pressurized per your site procedure, apply Gator Leak Detector all the way around each joint you touched and watch for bubbles or foam. On male connectors and other NPT-to-tube fittings, check the pipe-thread joint as well as the tube nut, since the NPT side often gets skipped. If something leaks, isolate and depressurize before putting a wrench on it. Never tighten a fitting under pressure.
A Word on Intermixing Brands
Intermixing brands is a different question from mixing sizes, and the answer is good news. DK-Lok tube fittings are 100% interchangeable and intermixable with Swagelok and Parker A-Lok, for like size and material, properly assembled per the DK-Lok catalog. In ASTM F1387 testing witnessed by TÜV Rheinland, 14 combinations of intermixed DK-Lok and Swagelok nuts, ferrules and bodies passed every test, with "No failure observed." DK-Lok also backs Parker A-Lok compatibility in writing (Parker A-Lok interchangeability letter). Same size is the key: metric to metric, fractional to fractional.
We're Here to Help
Not sure what you've got? Bring in a sample or a part number and we'll help you sort it out. A-Best Industrial Inc. Fittings, Valves, and Tubing stocks DK-Lok compression tube fittings in both fractional and metric sizes. Call 310-833-3353 or email sales@abesttube.com, Monday–Friday, 8:00 AM–5:00 PM Pacific, with after-hours emergency service available.