On a small-bore line the fitting is rarely the weak point. The tube is. Instrumentation connections fail because the tube was too hard for the ferrule to bite, or cut at an angle, or scratched along its length, and none of that is visible once the nut is tightened.
This guide covers how a twin-ferrule fitting works, what the tube has to be, how to gauge the pull-up, why mixing brands is a real risk rather than a sales argument, and where isolation and needle valves belong on a measuring line.
Why small bore is its own discipline
A leak on a 6 mm instrument line does not look dramatic. It still means:
- A measurement that is wrong rather than absent, which is worse, because the reading is believed.
- Gas at pressure in an enclosed space, where a small continuous leak accumulates.
- A sample that no longer represents the process, in analyser work.
The connections also live on vibrating skids, get remade during calibration, and sit in racks where a fitter cannot get a spanner square. The system has to tolerate all of that.
How a twin-ferrule fitting holds and seals
Two ferrules, doing two jobs:
- The front ferrule seals. It is driven into the body's taper and against the tube, and the seal is metal to metal at both contacts.
- The back ferrule grips. It hinges inward, bites the tube surface and holds against pressure and vibration. It also acts as a spring, which is what gives the joint its resistance to shock and thermal cycling.
The nut supplies the force. Tightening is by turns from finger-tight, not by torque: a nominal 1¼ turns on most sizes, with the manufacturer's figure taking precedence for small sizes and for the first assembly of a soft material.
A gap inspection gauge, placed against the nut after pull-up, shows whether the joint has been made up far enough. It is a five-second check and the only practical way to verify an assembly before pressure.
The tube decides everything
Most failures traced back to the fitting start here:
Hardness. The tube must be softer than the ferrule so the ferrule can bite. For 316 tube with 316 ferrules the usual requirement is fully annealed tube, and the datasheet states the maximum hardness. Hard-drawn tube bought on price will not take the bite, and the joint will leak under vibration.
Cut and finish. Square cut with a tube cutter, deburred inside and out. A saw cut leaves the end out of round.
Surface. Longitudinal scratches are a leak path under the ferrule. Circumferential marks are tolerable; a scratch down the tube axis is not.
Dimensions. Outside diameter to tolerance, with a wall thickness heavy enough that the tube does not collapse under the ferrule. Thin wall needs an insert.
Metric or imperial, never both. A 6 mm tube is not a ¼ inch tube (6.35 mm). The fitting will assemble and it will not seal reliably. Keep the two systems physically apart in the store, and mark the racks.
Mixing brands
Twin-ferrule fittings look interchangeable. The taper angle, ferrule geometry and pull-up allowance differ between manufacturers, and the combination that matters is a body from one maker with ferrules from another.
The pragmatic rule: standardise on one system per installation, keep the ferrules with the bodies, and if a remake is needed on an existing joint, use the ferrules already swaged onto that tube. They are conditioned to it.
Remakes are the system's strength. A joint can be broken and remade many times as long as it goes back onto the same tube with the same ferrules, tightened to a light nip past the original position.
Valves on a measuring line
A gauge without an isolation valve cannot be calibrated or replaced without shutting the process down, and that is how out-of-date gauges stay in service.
- Ball valve for isolation. Full bore or reduced, quarter turn, either open or shut. Not for throttling: a partly open ball valve wears the seat.
- Needle valve for throttling, metering and for damping a gauge that is being hammered by pulsation. The stem is tapered, so the flow curve is fine over a long stem travel.
- Manifolds (two-valve, three-valve, five-valve) where a differential pressure transmitter has to be isolated and equalised in a defined order. The equalise valve is what stops the cell being over-ranged during commissioning.
For the difference between these and the 24° cone system used on hydraulic tube, see DIN 2353 fittings explained. For the port threads on the process side, BSP against NPT covers the thread forms you will meet.
Materials
316/316L stainless is the baseline, and covers most process, laboratory and analyser work.
Higher alloys where the medium is chloride-bearing or hot: 6Mo, Alloy 825 and Alloy 400 each have their case, and the case is made on the specific chloride concentration and temperature rather than on the general reputation of stainless steel.
Brass for compressed air, water and instrument air panels, where the economy is real and the medium is benign.
Oxygen service changes the specification rather than only the material: cleaning to a stated standard, degreased components, and packaging that keeps them clean until assembly. Ask for the cleaning certificate.
A specification checklist
- Tube material, outside diameter and wall thickness, with the hardness condition stated as annealed.
- Metric or imperial, chosen once for the installation and kept apart in the store.
- Fitting system, one manufacturer per installation, ferrules kept with bodies.
- Pressure and temperature at the connection, noting that tube rating falls with wall thickness and temperature.
- Medium, for body alloy and for any cleaning requirement.
- Vibration exposure, which decides whether tube clamps are needed near the fitting.
- Isolation and equalisation, so gauges and transmitters can be serviced live.
- Gauging, with an inspection gauge available to whoever makes the joints.
- Tube support, because an unsupported run transfers its weight into the ferrule.
Where to look
The precision fittings section holds 1,510 items, of which the LET-LOK twin-ferrule range covers both metric and inch tube. Isolation and throttling are in precision ball valves and precision needle valves. Send us the tube size, the medium, the pressure and the temperature, and we will confirm the tube specification alongside the fittings: request a quotation.