3A vs DIN Tri-Clamp Fittings – Same Clamp, Different Tube Sizes

December 3, 2025 at 11:31 pm, No comments

In many catalogues everything is just called “1.5" Tri-Clamp” and that’s it.
On the shop floor it is not that simple.

You clamp two ferrules together – they seal, the gasket fits, everything looks fine.
Then you check the weld from the inside and see a small step in the bore, or you notice that one line is built from 38 mm tube and another from 41 mm, although both were sold as “1.5" Tri-Clamp”.

Historically the split was clear:

3A / ASME BPE sanitary tubing in the US and pharma,
DIN / EN hygienic tubing in Europe.

Today the reality is much more mixed. A large share of stainless steel fittings in Europe now comes from suppliers who follow 3A-style / ISO clamp dimensions and combine them with different tube series. That includes the fittings we supply on our internet shop: they are based on 3A dimensions up to 4" and are widely used in European food and beverage plants alongside DIN tube.

This guide explains what “3A Tri-Clamp” and “DIN Tri-Clamp” actually mean, using real tube dimensions (OD and ID) so you can see why “1.5 inch” is not always the same thing – especially when you start mixing 25, 32, 38 and DIN sizes in one system.

1. What do we really mean by “3A Tri-Clamp” and “DIN Tri-Clamp”?

First, a quick clarification.

Tri-Clamp (Tri-Clover) describes the connection:

two ferrules with flat flanges,
a gasket between them,
and a clamp that presses the flanges together.

Different standards then define:

flange dimensions,
surface finish and hygiene requirements,
and which tube series these ferrules are welded to.

3A side – the fittings I supply

On the 3A side you typically see imperial sanitary tube sizes:

1", 1.5", 2", 3", 4"…

with fixed outer diameters (OD):

1.5" → OD = 38.1 mm
2" → OD = 50.8 mm
3" → OD = 76.2 mm
4" → OD = 101.6 mm

In practice, for process lines up to 4" most suppliers in Europe and China (including me) use about 1.5 mm wall thickness as a de-facto standard. That gives you a 1.5" 3A tube with a bore around 35 mm.

Ferrules and tubes in this family are designed to follow 3A Sanitary and ASME BPE / ASTM A270 requirements: weldability, cleanability, defined OD and wall thickness and a hygienic surface finish.

When I write “3A 1.5" Tri-Clamp fitting” in my shop, I mean:

A Tri-Clamp ferrule with a standard clamp flange, welded to 1.5" sanitary tube (38.1 mm OD, ≈1.5 mm wall), compatible with 3A-style hygienic tube.

DIN side – the original European metric series

On the DIN side the tube is metric and defined as DN:

DN25, DN32, DN40, DN50, DN65, DN80, DN100…

Each DN has its own OD and typical wall thickness. For example:

DN40 → OD ≈ 41.0 mm, wall ≈ 1.5 mm → ID ≈ 38 mm
DN50 → OD ≈ 53.0 mm, wall ≈ 1.5 mm → ID ≈ 50 mm

The matching clamp-type ferrules are defined in DIN 32676, which ties clamp flange sizes to these DIN tube series.

So when someone says “DIN DN40 Tri-Clamp”, they usually mean:

A Tri-Clamp-style ferrule according to DIN 32676, welded to DIN hygienic tube (for example DN40: ~41×1.5 mm).

Key point so far

On the outside, 3A and DIN ferrules for “1.5" / DN40” can share the same clamp flange diameter (so the clamp and gasket are the same).
On the inside, they are welded to different tube series – 38.1 mm OD for 3A, ~41 mm OD for DIN.

Unsurprisingly, that difference shows up in the weld and in the flow.

2. The classic comparison: 1.5" 3A vs DN40 DIN

(and where 25 mm and 32 mm come into the picture)

Let’s start with the combination you see almost every day: 1.5" Tri-Clamp.

1.5" in the 3A world

A typical 3A 1.5" process tube:

OD = 38.1 mm
Wall ≈ 1.5 mm
ID ≈ 38.1 – 2×1.5 ≈ 35.1 mm

So a 1.5" 3A line has a bore of roughly 35 mm.

DN40 in the DIN world

A typical DIN DN40 tube:

OD ≈ 41.0 mm
Wall ≈ 1.5 mm
ID ≈ 41.0 – 2×1.5 ≈ 38.0 mm

A DN40 line therefore has a bore of about 38 mm.

How they actually fit together

Now put a piece of 1.5" 3A tube next to a piece of DN40 tube.

3A 1.5" tube: 38.1 mm OD
DN40 tube: ≈38 mm ID

If you try it in the workshop, the 3A tube will slide neatly inside the DN40 tube. The fit is surprisingly close – close enough that it is tempting to weld them together.

From a standards point of view, however:

the smaller 3A tube becomes a kind of inner sleeve,
the larger DIN tube becomes a socket.

There is no “official” joint like this in 3A or DIN tables, it is simply what happens when the two systems meet.

If you butt-weld them directly, you almost always end up with a step in the bore. That step is small, but it is real: it affects flow and cleaning and is exactly the kind of detail that never shows up on the sales drawing labelled “1.5" Tri-Clamp”.

And then there are 25 mm and 32 mm “1.5 inch” lines

In real installations you will also see 25 mm and 32 mm tube welded to 1.5" Tri-Clamp ferrules:

1.5" clamp ferrule on 25×1.5 mm tube → ID ≈ 22 mm
1.5" clamp ferrule on 32×1.5 mm tube → ID ≈ 29 mm

Machine builders like these combinations on:

small filling lines,
CIP return lines,
sampling and utility lines,

because the 1.5" clamp hardware (50.5 mm flange) is cheap and universal, while the tube OD is chosen just for the required flow.

These 25 mm and 32 mm “1.5 inch” solutions do not belong to 3A or DIN hygienic tube series in the strict sense. They are practical hybrids: “whatever clamp size the factory uses” + “whatever metric tube was convenient”.

The result is that under the same label “1.5" Tri-Clamp” you can meet at least four different tube bores:

~22 mm (25 mm tube),
~29 mm (32 mm tube),
~35 mm (1.5" 3A tube),
~38 mm (DN40 tube).

Which brings us to the clamp flange itself.

3. Where 3A and DIN really match: the Tri-Clamp ferrule flange

Up to now we looked at tube OD and ID.
The ferrule flange is the part that actually sees the clamp and gasket.

For the most common process sizes, 3A / ISO / ASME BPE and DIN 32676 use very similar clamp flange diameters:

1.5" / DN40 → flange OD around 50.5 mm
2" / DN50 → flange OD around 64 mm
3" / DN80 → flange OD around 91 mm
4" / DN100 → flange OD around 119 mm

This is why, in practice, you can:

take a 3A 1.5" ferrule on 38.1 mm tube,
take a DIN DN40 ferrule on ~41 mm tube,
put a standard 1.5" clamp and gasket between them,

and the joint will seal and work.

From the outside everything looks compatible: “same clamp, no leaks, all good”.

From the inside, the situation is less tidy:

the clamp flange is standardised and compatible between the systems,
the tube series behind the flange is different and can create:
- a step in the bore,
- a small pocket where product can sit,
- different flow velocities at the same volumetric flow.

So a practical summary for the reader:

If two ferrules close with the same 1.5" clamp, they are “Tri-Clamp compatible” at the flange.
If the tubes behind them come from different families (3A 38.1 mm, DIN 41 mm, or 25 / 32 mm hybrids), they will not be the same line internally.