3/8 inch Instrumentation Tube Fittings: Stainless Steel & High-Nickel Alloy

Reviewed by the Crestflo Engineering Team

A 3/8 inch tube fitting is a compression fitting that grips the tube mechanically instead of relying on a flare or a weld. It seals onto 3/8-inch (0.375 in / 9.525 mm) outside-diameter instrumentation tubing, and the result is a connection that stays leak-tight and gas-tight. Crestflo makes 3/8 inch instrumentation tube fittings in 316/316L stainless and high-nickel alloys, offered in single- and double-ferrule designs with both imperial and metric end connections, and every one is tested to ASTM F1387. These aren't copper or plumbing compression fittings, push-to-connect couplers, or 3/8 NPT pipe fittings. They're instrumentation-grade stainless-steel and alloy tube fittings meant for measurement, sampling and process lines. Every part carries a heat/lot number and ships with an EN 10204 material test certificate, and the fittings are engineered for dimensional interchange with the major instrumentation brands. Standard items sit in ready stock; specials are made to order, and delivery runs 6-8 weeks.

3/8 inch Tube Fitting Names & Designations (Also Known As)

Buyers and specifiers call this part by many names, and each one points to the same 3/8-inch OD instrumentation fitting: 3/8 inch tube fitting, 3/8" tube fitting, 3/8 OD tube fitting, 3/8 tube connector, 3/8 instrumentation fitting, 3/8 compression fitting (in the instrumentation sense), 3/8 OD compression fitting, 3/8 stainless compression fitting, 3/8 double-ferrule or twin-ferrule fitting, 3/8 single-ferrule fitting, 3/8 ferrule, 3/8 nut-and-ferrule, and simply "fitting for 3/8 tubing." Every one of these describes a compression tube fitting that grips 3/8-inch outside-diameter tubing. The size names the tube the fitting seals onto. It doesn't name a thread.

What "3/8 inch" Means: 0.375 in / 9.525 mm Tube OD (Not 3/8 NPT)

"3/8 inch" is a tube outside-diameter designation: it's the OD of the tubing the fitting grips, not a thread size, and not the same as 3/8 NPT pipe thread. A 3/8 inch tube fitting seals a 0.375-inch (9.525 mm) tube, and it can carry an NPT, BSPT, JIC or metric thread on its other end, specified separately. 3/8 inch is an imperial fractional size, one step in the imperial fractional tube fitting range (1/8, 1/4, 3/8, 1/2 in and up), whereas the nearest metric size, 10 mm (0.394 in), is a distinct part with its own ferrules. Order the size that matches your tube OD.

A 3/8 inch tube fitting's size isn't defined by one figure alone. The tube OD it seals against, 0.375 in / 9.525 mm, is the first piece. The second is the set of common wall thicknesses, which set the pressure rating:

Values track the applicable standard, but they're customizable to your requirements. Want every OD? Check the full tube fitting size chart.

3/8 inch Tube Fitting Shapes: Union, Elbow, Tee, Connector, Reducer & More

Every instrumentation fitting shape is available in a 3/8 in. size. Click through to any shape's dedicated page for the full configuration detail:

Single vs Double Ferrule 3/8 inch Fittings

Crestflo offers 3/8 inch fittings in both single-ferrule and double- (twin-) ferrule designs. In a double-ferrule fitting, tightening the nut pushes a front ferrule forward to create the seal while a back ferrule grips and supports the tube. That separation of sealing from gripping is what makes repeated make-up and remake reliable. Single-ferrule designs, by contrast, ask one component to handle both jobs. Ferrules come hardened or non-hardened, depending on what the application calls for. Component-level detail lives in our pages on single-ferrule fittings, ferrules, and ferrule-less fittings. You can also compare the three approaches directly in our guide to single vs double vs ferrule-less.

End & Thread Connections for 3/8 inch Fittings: NPT, BSPT, BSPP, JIC, SAE-ORB & Metric

A 3/8 inch tube end works with national, international, or custom threaded ends alike. Two combinations show up most often: 3/8 in tube by 1/4 in NPT, and 3/8 in tube by 1/2 in NPT reducing connectors.

Values per applicable standard; customizable to requirement.

3/8 inch Tube Fitting Materials: 316/316L Stainless, Duplex, Inconel, Monel & Hastelloy

316/316L stainless steel is the standard choice for 3/8 inch instrumentation tube fittings. Crestflo doesn't stop there, though: the shop machines the entire corrosion-resistant-alloy lineup, covering aggressive, sour and high-temperature service. Each grade gets its own explanation on the material pages, and for instrumentation, that means the 3/8 inch fitting.

Compositions follow the applicable ASTM/UNS designations. In the US, the instrumentation scope covers stainless and alloy only; brass isn't included. You can browse all instrumentation fitting materials.

3/8 inch Tube Fitting Pressure Ratings: High-Pressure Walls, Temperature & Standards

What sets the working pressure of a 3/8 inch line is the tube: its wall thickness and the material it's made from, not the fitting body. Allowable working pressure gets calculated using the ASME B31.3 allowable-stress method. A heavier wall simply rates higher. When the application calls for high-pressure 3/8 inch service, specify a heavier wall (0.065 in or 0.083 in) or step up to a high-strength alloy; either move lifts the rating accordingly. A high-pressure 3/8 inch tube fitting, then, is built by matching wall and grade to the duty, not by swapping out the fitting body. The values shown below are representative, computed for annealed 316/316L tubing.

Values follow the applicable standard and can be adjusted to what the job needs, so confirm the wall thickness, temperature de-rating, and grade for your service. It's the tube and ferrule material that set the temperature limits, not the fitting itself. Sour service is different: 3/8 in. fittings come in NACE MR0175 / ISO 15156-compliant alloys. Crestflo pulls from several standards here: ASME B31.3, ASTM A269 for tubing, ASME B1.20.1 for NPT, ASTM F1387 for fitting performance, and NACE MR0175.

Certifications, Testing & Traceability

Crestflo holds ISO 9001, 14001, and 45001 certification along with PED, and it carries the standard approvals that apply to compression tube fittings, including ASTM F1387 performance qualification. Testing is thorough: hydraulic proof, burst and impulse, pneumatic, flexure-fatigue, vibration, thermal-cycling, pull-out, and vacuum, all backed by positive material identification (PMI) and in-house NDT. Destructive testing takes place at ISO/IEC 17025-accredited labs. Third-party inspection can be arranged on request. Each 3/8 in. part carries its own heat/lot number and ships with an EN 10204 3.1 material test certificate; a 3.2 is available if it's needed. For more detail, see traceability and testing & inspection. Datasheets and CAD models are available to customers who ask for them.

Why Source 3/8 inch Tube Fittings from Crestflo

Crestflo is the instrumentation division of a four-decade-old export house recognized by the Government of India, with a long track record of delivery across the Middle East. Our 3/8 in fittings are built for dimensional interchange with Swagelok, Parker, Hoke, Hy-Lok and Fitok tube fittings, and we don't just claim that interchange works: it's backed by ASTM F1387 performance qualification. Standard items ship from ready stock; specials are made to order. Either way, delivery runs 6-8 weeks, a fraction of what the majors typically take. We keep minimum order quantities small, run private-label and made-to-print programs, and offer both imperial and metric sizes on every fitting. Material is of Indian origin, though US and European melt is available on request, and every batch carries full material traceability. We Engineer Confidence. Explore our lead time, private-label and custom manufacturing capabilities, and US origin compliance.

How to Order 3/8 inch Tube Fittings & Request a Quote

Give us the shape, material grade, ferrule design, and end connection for each 3/8 in fitting; we'll return a quote from our engineering team, along with the applicable ordering codes. Switching a competitor line over to Crestflo? Ask our team for a Swagelok, Parker, Hoke, Hy-Lok or Fitok cross-reference, the cross-reference page and the Swagelok cross-reference page both cover it. US sales can be reached at sales@crestflousa.com or +1 346 594 9005 (Houston, TX), or you can simply request a quote. Distributors interested in partnering should visit become a distributor, while OEMs and EPCs can send along a print through custom manufacturing.

Related Tube Fitting Sizes

Curious how the neighboring sizes compare? Look at 1/8 inch, 1/4 inch, 1/2 inch, 5/8 inch, 3/4 inch, and 1 inch. On the metric side, there's 6 mm, 8 mm, 10 mm, and 12 mm. Or just head back to the tube fittings pillar page.

3/8 inch Tube Fittings FAQ