How to determine whether the inner wall roughness of the stainless steel pipe meets the requirements

To determine whether the inner wall roughness of the stainless steel pipe (with the core indicator being the Ra value, measured in μm) meets the requirements of the laboratory gas pipeline, a three-layer logic of "document verification (at the purchasing end) + visual inspection (at the site end) + professional testing (at the accuracy end)" needs to be adopted. This approach avoids relying solely on subjective judgment and ensures that the data precisely matches the experimental requirements (for example, high-purity gas requires Ra ≤ 0.8 μm, and ordinary inert gas requires ≤ 1.6 μm). The specific methods are as follows: 

1. Step 1: Prioritize verification of the "document evidence" provided by the supplier (basic and crucial)

When purchasing stainless steel pipes from the laboratory, the primary basis is the official test report provided by the supplier. This is the "paper evidence" for determining whether the roughness meets the standards. Three core pieces of information need to be focused on:

1. Report type and qualifications

It is necessary to require the supplier to provide a "surface roughness test report" instead of a general "material report".

The report must be authoritative: for high-purity gas pipelines (such as chromatography carrier gas, semiconductor gas), the report should include the CMA (China Metrology Certification) or CNAS (China National Accreditation Service for Conformity Assessment) qualification mark (the validity of the qualification can be checked on the official website of the State Administration for Market Regulation), to avoid "self-made reports" being forged.

2. Core data: Ra value and testing standards

Clearly specify the specific value of "inner wall roughness" in the report (such as Ra = 0.6 μm), and it must be completely consistent with your requirements (for high-purity gas, it should be ≤ 0.8 μm).

Confirm the standard for testing: in laboratory scenarios, it should comply with GB/T 3505-2009 "Surface Roughness Terminology, Definitions and Parameters" or international standard ISO 4287:1997, avoiding using non-pipe-specific roughness standards (such as sheet and profile standards, with different indicator definitions).

3. Testing location and sampling method

The report should indicate the testing location (such as "randomly select 3 sections of the pipe wall, and measure 2 points on each section"), avoiding only testing "the pipe mouth" (the pipe mouth is prone to polishing, and the middle section of the inner wall may not meet the standards).

Sampling ratio: For batch purchases (such as ≥ 50 meters), it is required to "sample and test 1 out of every 10 pipes", rather than only providing "sample pipe report" (the sample may differ from the actual supply). 

II. Step 2: On-site visual inspection (initial screening, rapid elimination of obvious non-conforming products)

After receiving the pipe materials, you can preliminarily determine whether the inner wall roughness is qualified by "looking, touching, and inspecting" without the need for professional instruments. This method is particularly suitable for eliminating "severely excessive" inferior pipes:

1. Visual inspection (check the gloss and flatness)

Qualified pipes (Ra ≤ 0.8 μm): The inner wall has a uniform mirror-like gloss, with no obvious scratches, pits, or oxide scale (the oxide scale appears as dark brown spots, which can cause the roughness to soar);

Unqualified pipes: The inner wall is dark, with obvious longitudinal or transverse scratches (can be felt by hand), or there are "foggy" areas (mostly due to incomplete polishing).

Observe under the light (such as with a flashlight shining on the inner wall):

Note: Avoid "only looking at the pipe end" - some manufacturers only polish the pipe end, while the middle section of the inner wall may be rough, and the pipe material needs to be cut off (or inserted with an endoscope) to check the middle section of the inner wall.

2. Touch inspection (feel the smoothness, pay attention to cleanliness)

Qualified pipes: The hand feels uniform and smooth, without a "grainy" or "bumpy" sensation;

Unqualified pipes: Can clearly feel scratches, protrusions (such as metal debris from polishing), or a "fuzzy" area (usually due to unremoved oxide layer);

Wear clean powder-free latex gloves (to avoid hand oil contamination of the inner wall), and gently insert your hand into the inner wall (only for short pipes or near the pipe end):

Warning: This method is only for "initial screening" and cannot replace professional testing (such as the difference in feeling between Ra = 1.2 μm and Ra = 0.8 μm is very small, making it impossible to distinguish).

3. Endoscope inspection (for long pipes / curved pipes, verification at key positions)

Focus on inspecting "welding positions" (such as TIG welding interfaces): Qualified pipe weld seams are smooth, without weld bulges or oxide layers; Unqualified pipe weld seams are rough, even with residual welding slag (which can cause the local Ra value to far exceed the standard);

Inspect "middle section of the pipe segment": Avoid situations where the supplier only processes the pipe end, and the middle section is not polished.

If the pipe material is a long pipe (such as 6 meters / per piece) or has a curved pipe (such as laboratory pipes at the corner), use an industrial endoscope (diameter ≤ 5 mm, with lighting function) to insert into the inner wall for observation: 

III. Step 3: Professional Instrument Testing (Precision Verification, Required in High-Requirement Scenarios)

If the gas pipeline is of high purity (such as over 99.999% purity), clean room pipeline, or critical experimental gas pipeline, professional instrument testing for the Ra value must be conducted to ensure accurate data. There are two common methods:

1. Contact-type surface roughness instrument (high precision, straight pipe preferred)

Principle: Place a sensor with a diamond probe (probe tip radius ≤ 2μm) gently against the inner wall. The probe moves up and down along the undulations of the inner wall, and the instrument calculates the Ra value based on the displacement signal (in accordance with GB/T 3505 standard).

Operation points:

Before testing, clean the inner wall: Use high-purity alcohol (99.9%) to wipe the inner wall to remove dust and oil (impurities can cause the probe to get stuck and data distortion);

Testing positions: Select 3 points on the inner wall of the pipe in the "axial (length direction)" and "circumferential (circular direction)" directions, and take the average value (to avoid local compliance but overall non-compliance);

Applicable scenarios: Straight pipe (bent pipe cannot be inserted into the probe), with a precision of ±0.02μm, fully meeting laboratory requirements.

2. Non-contact laser roughness instrument (flexible, suitable for bent pipes / complex structures)

Principle: Use a laser beam to illuminate the inner wall, receive the "scattered signal" of the reflected light, and use optical principles to calculate the degree of inner wall undulation, converting it into the Ra value (no need to contact the inner wall, avoiding damage to the pipe);

Operation points:

Ensure the inner wall is clean (dust will affect laser reflection), and there is no strong reflection on the inner wall of the pipe (this can be solved by adjusting the laser angle);

Applicable scenarios: Bent pipes, irregular pipes (such as the inner wall of the three-way joint in the laboratory pipeline), or thin-walled pipes (such as 316L pipes with a wall thickness of 1.0mm) that are prone to being scratched by the probe;

Note: The precision is slightly lower than that of the contact type (±0.05μm), but it fully meets the laboratory's detection requirement of Ra ≤ 0.8μm. 

IV. Key Considerations (Avoiding Misjudgments)

1. Do not blindly pursue "the lower the better"

The Ra value is not always the smaller the better: excessively low Ra values (such as ≤ 0.2 μm) require ultra-mirror polishing, which will significantly increase the cost (approximately twice that of Ra = 0.8 μm), but in most laboratory scenarios (such as non-semiconductor experiments), such high precision is not necessary. It is necessary to balance "precision - cost" in combination with gas purity.

2. Distinguish between "inner wall" and "outer wall" roughness

Suppliers may confuse "inner wall" and "outer wall" data (the outer wall is easier to polish, and the Ra value is usually lower). It is necessary to clearly mark "inner wall roughness Ra value" in the inspection report to avoid using the outer wall data to replace the inner wall.

3. Clean the inner wall after testing

After professional testing (especially contact type), the inner wall should be blown with high-purity nitrogen gas or wiped with alcohol to remove any probe wear debris that may have been left over during the testing process (to avoid contaminating the subsequent gas). 

Summary: Simplification of the judgment process

During procurement: Check the "Surface Roughness Test Report" (check Ra value, qualification, sampling method);

After receiving the goods: Visual inspection + touch + endoscope preliminary screening (exclude obviously inferior pipes);

For high-demand scenarios: Use contact-type / laser roughness instrument for detection (precisely verify Ra value and retain detection records).

Through this process, 100% guarantee that the inner wall roughness of the stainless steel pipe meets the cleanliness requirements of laboratory gas pipelines, avoiding gas contamination, valve blockage or deviation in experimental data due to inner wall roughness.