PMI: Limit of Detection (LOD) 1

[AWeld1]

Greetings!

I would like to know what LOD means in PMI? How do we judge the readings that shows LOD? For example:

Material is P91 the reading is:
Titanium: <LOD> 0.0245 ; wherein ASME II , Part A – SA-213/sa-213M (P91–Ti = 0.01 max)

How can I explain LOD in the easiest way?

THanks.

 

[weldstan]

You will need to discuss this issue with the PMI equipment manufacturer. Standard XRF equipment will not provide the accuracy for Ti that you may require.

 

[mrfailure]

LOD = Limit of detection. You would report Ti <0.0245%.

Portable PMI XRF units are not intended to detect compliance as they are only semi-quantitative in nature. They are used to instead determine the bulk material. Low elemental amounts like the one you presented for Ti may be a requirement but will not show up in such testing, as Weldstan noted.

 

[EdStainless]

PMI is for verification of the grade in service.
If the resolution of the equipment is good enough to assure that the material is not another grade, then it works.
There is no intention of verifying specification compliance.
The term PMI appears in an API standard. Outside of that context it has no meaning unless you have written a specification to cover your facility and application.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[metengr]

Section I of the Boiler and Pressure Vessel Code also has a Nonmandatory B Appendix on the Practice of PMI.
An excerpt from the Introduction is below

B-1 INTRODUCTION
As part of his material control system, a Manufacturer
may determine that a situation warrants positive material
identification for a specific material or item. This may include
material used by the Manufacturer for pressure retaining
boiler parts and their associated support materials
and may also include components covered by PG-11 or by
ASME standards listed in Table A-360. This Nonmandatory
Appendix is provided as a guide for use by the Manufacturer
in developing a Positive Material Identification
Practice (PMIP) that may be applied to address the material
or item of concern.

 

[Stdsp]

LOD= Limits of Detection (LODs)

You may take a look at

http://www.olympus-ims.com/en/applications/handheld-pmi/

 

[TOVmania]

PMI accuracy, especially for a XRF type, depends on the surface preparation and the way the ray is exposed to the surface without interference of its detection mode.

 

[Guest102023]

PMI is very useful for differentiating alloys that are distinct from each other, such as the Cr-Mo family or stainless steels, where alloy additions tend to come in big chunks. Its also useful as a quality control check (in my experience as a substitute for actual quality control).

You cannot trust PMI for detecting minor elements in P91, which has a large number of them at low concentrations. But as mentioned above, contact the manufacturer for details. Keep in mind that radioactive sources degrade over time, increasing the LODs.

It does not help matters that PMI has been deceitfully promoted as "chemical analysis" for so long that many folks believe it. I don't blame the makers, I blame the chain inspection companies for whom PMI is a gold mine.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"