10 Apr, 2026

Dairy Tube vs Schedule 10 vs Schedule 40: Which Pipe Purge Kit Do I Need?

Grab the wrong pipe purge kit for the job, and you’ll know about it the moment you strike an arc. 

A poor seal means argon leaks out, oxygen creeps in, and the back of your weld ends up sugared – that grainy, oxidised mess that no inspector will pass and no dairy plant will tolerate.

The fix is simple: match your purge kit discs to the actual internal diameter of the pipe or tube you’re welding. But here’s where it gets tricky. A 2″ dairy tube and a 2″ schedule 40 pipe don’t have the same internal diameter. Not even close. Choose the wrong specification and your discs won’t seat properly, your purge won’t hold, and you’ll burn through argon trying to compensate.

This guide breaks down the differences between dairy tube, schedule 10, and schedule 40 pipe purge kits so you can order the right one the first time.

Quick Answer

• Dairy tube purge discs are sized to the actual outside diameter (OD) of the tube, which is also very close to the internal diameter. A dairy tube has thin walls and tight tolerances – common in food, beverage, and pharmaceutical work.
• Schedule 10 purge discs suit pipes with thinner walls and a larger internal bore. Typical in low-to-medium pressure stainless steel applications.
• Schedule 40 purge discs suit pipes with thicker walls and a smaller internal bore. Used in higher-pressure industrial piping.
• The pipe size stamped on the label (e.g. 2″) doesn’t tell you the internal diameter. You need to know the specification – dairy tube, SCH10, or SCH40 – to get discs that actually fit.

Why the Specification Matters for Purge Disc Sizing

This is the part most welders don’t think about until they’ve already wasted half a bottle of argon.

Tubes and pipes are dimensioned differently. A dairy tube (also called a sanitary tube) is measured by its actual outside diameter and a specific wall thickness – typically 1.6 mm for standard dairy applications. The OD is what it says on the label. A 2″ dairy tube has a 2″ (50.8 mm) outside diameter.

Pipes, on the other hand, use Nominal Pipe Size (NPS). And NPS is not a direct measurement of anything – it’s a naming convention. A 2″ NPS pipe actually has an outside diameter of 60.3 mm. The internal diameter then depends on the schedule (wall thickness). A 2″ schedule 10 pipe has a thinner wall than a 2″ schedule 40 pipe, which means the schedule 10 has a larger bore.

So for a 2″ nominal size, you’re looking at three completely different internal diameters depending on whether it’s the dairy tube, SCH10, or SCH40. Three different purge disc sizes.

If your disc is too small, it won’t seal against the pipe wall. Gas escapes, oxygen gets in, and you get sugaring on the root pass. If it’s too large, you can’t get it into the pipe at all – or it jams and won’t pull through. Learn more about common purge mistakes that cause sugaring (and how to fix them) here. 

Dairy Tube Purge Kit: When to Use It

Dairy tube (sometimes called sanitary tube or hygienic tube) is the standard in food processing, dairy, beverage, pharmaceutical, and biotech facilities. It’s manufactured to ASTM A270 or equivalent standards, with a polished internal surface finish (typically Ra ≤ 0.8 µm) to prevent bacterial growth.

The key characteristics that affect purge kit selection:

• Thin wall thickness – typically 1.6 mm for standard sizes
• Measured by actual OD – a 2″ tube genuinely has a 50.8 mm outside diameter
• Larger bore relative to its nominal size compared to schedule pipe
• Tight manufacturing tolerances – meaning purge discs need to be precise

A dairy tube pipe purge kit uses white food-grade nitrile discs (not black industrial nitrile) to avoid contamination risk during sanitary welding. The discs are sized specifically for the internal diameter of the dairy tube at each size.

If you’re welding stainless steel tubes in a food plant, brewery, dairy facility, or pharmaceutical cleanroom, this is the specification you need.

Schedule 10 Purge Kit: When to Use It

Schedule 10 pipe has a thinner wall than the schedule 40, but thicker than a dairy tube. It’s common in low-to-medium pressure stainless steel piping systems – think process lines, water treatment, HVAC, and some chemical applications.

Because the wall is thinner than SCH40, the internal bore is larger for any given pipe size. A schedule 10 purge kit accounts for this with appropriately sized discs.

You’ll typically see SCH10 stainless steel pipe in:

• Process piping where pressures are moderate
• Water and wastewater treatment facilities
• General industrial installations where the full wall thickness of SCH40 isn’t warranted
• Applications where weight and material cost matter

A schedule 10 purge kit uses black nitrile rubber discs – the same hard-wearing material, just sized for the SCH10 bore.

Schedule 40 Purge Kit: When to Use It

Schedule 40 is the heavy-duty option. Thicker walls, smaller bore, higher pressure ratings. You’ll find SCH40 pipe in oil and gas, petrochemical plants, high-pressure steam lines, and any application where the pipe needs to handle serious operating pressures.

Because the wall is thicker, the internal diameter is smaller at every pipe size. A schedule 40 pipe purge kit uses discs machined to match that smaller bore – and using SCH10 discs in a SCH40 pipe means they’ll be too large to fit.

SCH40 is common in:

• Oil and gas processing
• High-pressure industrial piping
• Steam and condensate lines
• Carbon steel and stainless steel applications in heavier-duty environments

What Happens If You Use the Wrong Purge Kit?

We see this more often than you’d think. A welder grabs whichever purge kit is in the van, sets up, and starts welding – only to find the root pass has oxidised because the discs aren’t sealing properly.

Here’s what goes wrong:

• Discs too small (e.g. SCH40 discs in a SCH10 pipe): The disc doesn’t contact the pipe wall. Argon flows around the disc instead of being contained between the two discs at the weld zone. You get a weak purge, excessive gas consumption, and sugaring on the inside of the weld. On sanitary work, that’s an automatic fail.
• Discs too large (e.g. SCH10 discs in a SCH40 pipe): The disc won’t slide into the pipe. You either can’t set up the purge at all, or you force it in and the disc deforms – which means it still won’t seal properly.
• Dairy discs in schedule pipe (or vice versa): Same problem, different reason. The dimensions simply don’t match.

The cost of a purge kit is tiny compared to cutting out a failed weld, re-prepping the joint, and welding it again. Getting the right discs from the start saves time, gas, and frustration.

How to Figure Out Which Kit You Need

Before ordering a pipe purge kit, confirm these three things:

1. Is it a tube or pipe? If you’re working in food, dairy, beverage, or pharma with polished stainless tubes – it’s a dairy tube. If it’s industrial piping with schedule ratings – it’s pipe.
2. What’s the schedule? Check the pipe markings, the job spec, or the project drawings. SCH10 and SCH40 are the most common, but other schedules exist.
3. What size range do you need? Most pipe purge kits cover 1½” to 6″ or 1½” to 12″. Pick the range that covers your job – or both, if your work varies.

If you’re running a tube purge kit alongside a pull-through setup, make sure both are specced for the same pipe or tube type. Learn more about tube purge kit vs pull-through purge kits and when to use each here. 

Not sure what you’ve got? Measure the internal diameter with a set of callipers and compare it against a pipe dimension chart. Or just give your supplier a call – it takes two minutes to confirm.

Quick Selection Table

Specification	Wall Thickness	Internal Bore	Typical Applications	Disc Type
Dairy Tube	Thin (~1.6 mm)	Largest	Food, dairy, beverage, pharma	White food-grade nitrile
Schedule 10	Medium	Mid-range	Process piping, water treatment, HVAC	Black nitrile
Schedule 40	Thick	Smallest	Oil & gas, high-pressure, steam	Black nitrile

For the same nominal size, the dairy tube will always have the largest bore and SCH40 the smallest.

Get the Right Pipe Purge Kit from Industrial Experts

Ordering the correct purge kit shouldn’t be guesswork. At Industrial Experts, our pipe purge kits are available in dairy tube, schedule 10, and schedule 40 specifications – in both 1½”–6″ and 1½”–12″ size ranges. We also supply custom-sized replacement discs if your application falls outside the standard range.

Tell us the pipe type, the schedule, and the size range – we’ll make sure you get the right kit for the job. Contact our team or browse the range online.