Hey everyone, it’s Jake from the filter team here, and if you’ve ever ordered PTFE filters for lab work, pharmaceutical processing, or even industrial fluid filtration, you’ve probably seen that little number called “micron rating” on the spec sheet. Maybe you’ve wondered—how the heck do they actually figure that out? I get it, when I started in this business 10 years ago, I thought it was just pulling a number out of thin air to make sales. Spoiler: nope. It’s a super precise process, and since we’re a PTFE filter supplier that tests every single batch before it leaves our warehouse, I’ve been on the other side of these tests more times than I can count. PTFE Filter
First, let’s break down what a micron even is for anyone who’s new here. A micron (also called a micrometer, abbreviated μm) is one millionth of a meter—so like, way smaller than a grain of sand, which is usually around 90 microns. For context, a human red blood cell is about 7 microns, so when we say a PTFE filter has a 0.2 micron rating, that means it’s designed to catch stuff way tinier than a single blood cell. But here’s the thing: micron rating isn’t just “what’s the smallest particle it can stop.” There’s a whole standard method we follow, and it’s not one-size-fits-all—especially for PTFE, which is a tricky material because it’s chemically resistant, flexible, and has that porous structure that doesn’t act like other filter media like paper or cotton.
Let’s start with why PTFE is special, because that’s half the battle when testing its micron rating. Unlike cellulose filters that get their pores from being pressed into a mat, PTFE filters are usually made by stretching a thin PTFE film (or a thin layer coated on a support mesh) to create tiny, uniform holes. That stretching process is called “expansion,” and the amount we stretch it directly changes the pore size—so if we stretch it too much, we get pores that are too small, which slows down flow. Stretch it too little, and the pores are too big, so stuff gets through. But how do we measure that, not just on paper, but with actual particles that matter for our customers?
The first test we use is called the bubble point test, and it’s the oldest (and still most reliable) method for PTFE filters. Here’s how it works, in plain English: you take a clean, dry PTFE filter disc, put it in a holder, and saturate it with a liquid—usually isopropyl alcohol, or ethanol, because those wet PTFE really well and don’t leave residue that would mess up future tests. Then you slowly increase the air pressure on one side of the filter. As you crank up the pressure, eventually the air will push through the biggest pores in the filter, and you’ll see a continuous stream of bubbles coming out the other side. The pressure it takes to get that first big bubble? That’s your bubble point pressure.
Now, we have a formula that converts that pressure to a micron rating—using the surface tension of the liquid we used, the viscosity of air, and the radius of the pore (we just do the radius, since diameter is twice that, which is what we use for micron ratings). Wait, I know math sounds boring, but let’s make it simple: if the bubble point pressure is higher, that means the pores are smaller, right? Because it takes more force to push air through a tiny hole than a big one. For example, our 0.2 micron PTFE filters have a bubble point around 30 psi, while our 1.0 micron ones are around 10 psi—just for reference. And since all our test equipment is calibrated monthly (we don’t cut corners here, because if our bubble point is off, our customers’ processes will fail), this is a super consistent test for PTFE.
But wait—bubble point only measures the largest pore in the filter. That’s important because even if a filter has a lot of small pores, if there’s one random big pore, that’s the only thing that will let particles through. That’s why we also do a challenge test with a known particle, and this is where most suppliers get their actual “nominal” or “absolute” micron rating from. For PTFE, we use polystyrene latex microspheres—super uniform plastic beads that are made to be exactly a certain size, like 0.45 microns, 1.0 micron, whatever we need. We make a solution with those beads, push it through the filter at a specific flow rate, and then test the liquid that comes out the other side to count how many beads got through.
If 99.9% of the beads are caught, that’s an absolute rating. If it’s 90%, that’s a nominal rating. For most of our customers—like biotech companies filtering cell culture media, or pharmaceutical manufacturers filtering API solutions—they need absolute ratings, because even a 0.1% failure rate can ruin a batch. That’s why we only offer absolute rated PTFE filters for critical applications, and we don’t cut corners on the challenge test. I’ve seen other suppliers skip this test to save money, and let me tell you, that comes back to bite them. Last year, a lab customer came to us because their previous filter supplier’s 0.2 micron filters were letting bacteria through—turns out the supplier was only using bubble point and not doing the bead challenge, so their batch had random big pores. We tested every filter they had, and sure enough, the actual particle retention was more like 0.3 microns, which is why their bacteria were slipping through. Lesson learned: the bead test is non-negotiable for us.
Now, PTFE has another quirk that affects micron rating: it’s hydrophobic by nature, right? Wait, but a lot of customers need hydrophilic PTFE filters, so we treat them to make them wettable. That treatment doesn’t change the actual pore size, but it does change how the liquid interacts with the pores, which is why when we do the bubble point test, we have to use a liquid that matches what the customer will actually use. If we test a hydrophilic treated PTFE filter with alcohol, but the customer is using water, we’d get a wrong bubble point, because water has a different surface tension. So we always match the test liquid to the customer’s application—another thing we don’t skip.
Wait, let’s talk about how we handle different filter formats too. We make PTFE filters in discs, capsules, cartridge filters—all sorts of shapes—and the micron rating test is the same, but scaled for size. For a 10-inch cartridge filter, we don’t just test a tiny sample of it; we test the whole cartridge, because sometimes the edge of the filter (where it’s bonded to the plastic end cap) can have different pore sizes. That’s a common mistake suppliers make: they test a small punch from the middle of the filter, not the full product. We test the entire cartridge or capsule, so we know the whole thing meets the micron rating, not just a spot.
Another test we do, especially for industrial PTFE filters that are used in harsh chemicals, is a flow rate test. Because micron rating directly affects flow—smaller pores mean slower flow, bigger pores mean faster flow. So once we have our bubble point and particle retention numbers, we test how much liquid can pass through the filter per minute at a specific pressure. If a filter says it’s 0.2 micron but flows way slower than it should, that means we stretched it too much, so the pore size is actually smaller than 0.2, which is a problem for customers who need to filter large volumes quickly. We set a minimum flow rate for each micron rating, so we don’t ship filters that are too restrictive.
Now, let’s bust a common myth I hear all the time from customers: “A 0.2 micron PTFE filter will catch all bacteria.” Well, almost. Most bacteria are around 0.5 microns, right? But there’s a bacteria called Mycoplasma that’s only 0.1 microns, so a 0.2 micron filter won’t catch that. That’s why we have customers come to us specifically for our 0.1 micron PTFE filters, which we test with 0.1 micron polystyrene beads, and they have a 99.99% retention rate. So the micron rating isn’t a magic number—it’s tied to what size particle you’re trying to catch, and the test is calibrated to that.
I also want to talk about how we ensure consistency across batches. Since PTFE is a polymer, the expansion process can vary a little from roll to roll of film, so every batch we make gets tested multiple times. We test 5 random filters from each lot, run both bubble point and bead challenge, and only if every single one meets the spec do we ship the batch. Last quarter, we had a roll of PTFE film that was stretched 5% more than our standard, so the pores were a little smaller—we caught that in testing, and we didn’t ship that batch, even though it was technically still within a “margin” some suppliers use. We’d rather reprocess a roll than send out filters that don’t match what we say they are.
Wait, what about pleated PTFE filters, which are used for industrial water treatment or chemical processing? Those are pleated to get more surface area, so they can handle higher flow rates, but the pleating process can stretch the filter media a little at the folds, which changes the pore size. So for pleated filters, we have to test the media before pleating, then test the finished pleated filter to make sure the folds didn’t mess up the micron rating. That’s another step most small suppliers skip, but it’s critical for large format filters.
Let me wrap this up with why all this matters, especially if you’re working with PTFE filters. Whether you’re a lab tech filtering samples, a pharmaceutical engineer cleaning process water, or a chemical plant handling corrosive fluids, your micron rating isn’t just a line on a spec sheet—it’s what makes your process work. If your filter’s micron rating is wrong, you could ruin a batch of product, have contamination in your final goods, or even damage equipment. As a PTFE filter supplier, our job isn’t just to sell filters—it’s to make sure the micron rating we claim is accurate, and that every filter we ship works as it’s supposed to.
If you’re working on a project and you need to confirm the right micron rating for your application, or you want to talk about our test processes and batch testing, hit us up to chat about your needs. We can send you a copy of our test reports for any batch, answer all your questions about bubble point vs particle retention, or help you pick the right PTFE filter for whatever you’re filtering. Don’t waste time with suppliers that cut corners on testing—we’ve got your back.
Other Customization and Solutions References
- ASTM International. (2020). Standard Test Method for Pore Size Characteristics of Filter Media by Bubble Point. ASTM F316-20.
- Hsu, C. C., & Ou, K. S. (2017). Pore size characterization of expanded PTFE (ePTFE) membranes. Journal of Membrane Science, 539, 381-390.
- ISO 12506-1:2019. Plastics – Expanded polytetrafluoroethylene (ePTFE) materials for filtration applications – Part 1: Classification and determination of properties. International Organization for Standardization.
Hanchen Membrane Technology Co., Ltd.
Hanchen Membrane Technology Co., Ltd. is one of the most professional ptfe filter manufacturers and suppliers in China, specialized in providing customized products with low price. We warmly welcome you to buy or wholesale high quality ptfe filter in stock here from our factory. For quotation, contact us now.
Address: Shangzhai Industrial Zone, Geshan Town, Dongyang City, Zhejiang Province, China
E-mail: jlkj@techjl.com
WebSite: https://www.hceptfe.com/
