One of the most common gap gauge questions we get is "why doesn't this gauge work?" Usually the answer is that the gauge doesn't match the fitting size. A gap inspection gauge is a no-go check cut for specific fitting sizes, so the first job is matching it to your tube OD. Here's how the DK-Lok DIG gauges are sized and how to pick the right one.
What a Gap Inspection Gauge Does
On a new two-ferrule tube fitting, the gap between the nut and the body hex closes as you pull the nut up. The gauge tells you if you went far enough. Per DK-Lok's instructions: if the gauge does not fit into the gap between the nut and the body hex, the fitting is tightened enough on initial assembly. If the gauge fits into the gap, the fitting needs more tightening.
The gauge is for the initial make-up only. When you remake a fitting, tighten back to your mark and go slightly past it instead of gauging it.
Which DK-Lok DIG Gauge Covers Which Size
For most instrument work, one gauge does the job: the DK-Lok DIG-468. It works on eight sizes: 1/4", 3/8", 1/2" and 3/4", plus 6, 10, 12 and 18 mm. One gauge in the bag covers the common instrument sizes, so there's no digging for the right one.
Two sizes fall outside it, and DK-Lok makes a separate gauge for each:
A-Best Industrial Inc. Fittings, Valves, and Tubing offers all three gauges in stainless steel. Running DK-Lok Z series single-ferrule fittings? Those use their own DIG-Z gauges, so tell us which series you have before you order.
Pair It With a Tube Depth Marking Tool
The gap gauge checks the nut. It can't tell you if the tube was bottomed in the fitting. That's what the DK-Lok DTM tube depth marking tool is for. Push the tube into the DTM until it bottoms, mark the tube at the top of the tool, then push the tube into the fitting until it bottoms on the shoulder. If you can see the mark above the nut, the tube isn't fully bottomed. We carry DTM tools from 1/4" to 1" (for example, the DTM-4 for 1/4" tube).
Using It on the Job
Start with a square cut and a clean end: a RIDGID tube cutter made for stainless, the GRIP-IT TOOL to keep the tube from spinning while you cut, and a deburring tool inside and out. Hold the body steady while you pull up the nut. On fittings from 1/16" to 1/2", the Fitting Tool holds the fitting so it doesn't turn. Then slide the gauge up to the gap. All of these are on our tools page.
Mixing With Swagelok or Parker?
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, front ferrules, back ferrules and bodies passed 100% of the testing, with "No failure observed." DK-Lok confirms Parker in its Parker A-Lok interchangeability letter. Browse our compression tube fittings.
Leak Test Every Joint You Touched
A passing gauge check doesn't replace a leak check. Once the line is pressurized per 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 is often skipped. If something leaks, isolate and depressurize before putting a wrench on it. Never tighten a fitting under pressure.
Need the Right Gauge?
Tell us your tube sizes and fitting series and we'll match the gauge. A-Best Industrial Inc. Fittings, Valves, and Tubing also offers a free Safety training course for new and existing customers. Call 310-833-3353 or email sales@abesttube.com. We're open Monday–Friday, 8:00 AM–5:00 PM Pacific, with after-hours emergency service.
Swagelok and Parker A-Lok are trademarks of their respective owners.