Equivalent diameter usually refers to the flow area of a tube. For a circular tube, the equivalent diameter will be equal to the ID.

Thanks for the answer, but I have a question.
If the equivalent diameter is related to flow area (cross section?), where does it come from that it is equal to the ID? As far as I understand, the ASTM E8M indicates that the gauge length must be equal to 5D where D is the external diameter.

I think your customer used the wrong terminology and this should be clarified with the customer. Equivalent and external seem like easy enough words to swap if English isn't a first language.

In the US people use 2" or 4D (outside) for tube GL.
In Europe it is more common to use 5D.
I have seen both OD and mean D used in the GL calculation.
At the end of the day it doesn't matter.
Annealed 316L will be up over 50% elong however they measure it.
Strain rate matters more.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

WHAT 316L material-tube specification, WHAT strain hardness and WHAT wall thickness?

Is this a structural tube or a fluid-flow ['hydraulic' quality] tube?

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

The tube is an ASTM A213. We have a lot of experience in manufacturing this type of tubes, however in this case the client requests that for the tensile test the elongation shall be taken using a gauge length equal to 5D where D is the equivalent diameter. What is not clear to me is the meaning of "equivalent diameter", that's all.
Thank you

The client doesn't know themselves.
I would use the OD.
And you need to also report the value with a 2" GL in order to comply with A213

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Theoretically, equivalent diameter = Squad (OD^2-ID^2)

Practically, you can use equivalent diameter = (OD+ID)/2. Or, simply use equivalent diameter = OD

Intrinsically, all probably donot matter much, as Ed pointed out, since 316 material would have a large elongation value (unless the deformation rate >95% during manufacturing).

The key is to clarify with the customer.

Ok thanks for the information. Obviously it is something that I asked the client, but sometimes the answers tend to come too late.