The Short Answer
Use light, consistent gauging pressure, record the temperature, and take readings across at least three different axes. PTFE compresses under normal micrometer force and expands roughly ten times as much as steel per degree, so both pressure and temperature change the number you get.
Why Steel Method Fails on PTFE
Three properties break the usual approach:
- PTFE is soft. Shore D 50–65. A micrometer tightened with normal feel deforms the surface and reads undersize — and different operators tighten differently, so the same ball produces different numbers.
- PTFE expands with heat. A coefficient of roughly 100–160 × 10⁻⁶ /°C against steel's ~12. A ball measured in a 35°C workshop and the same ball in a 20°C lab are genuinely different sizes.
- PTFE has a transition near 19°C. The crystalline structure shifts around that point and dimensions change measurably with it. Measurements taken either side of 19°C will not agree, and neither is wrong.
Correct Method
- Stabilise the ball. Let it reach room temperature — at least a few hours if it arrived from a cold or hot environment. Measuring a ball straight out of a delivery van in winter is meaningless.
- Record the temperature. Ideally measure near 23°C. Whatever you use, write it on the inspection record. A tolerance without a temperature is not a specification.
- Use a ratcheting micrometer or constant-force gauge. The ratchet exists precisely to remove operator feel from the reading. Use it every time, and do not override it.
- Take at least three readings on different axes. Rotate the ball roughly 90° between readings. The spread between them is your roundness indication; the average is your diameter.
- Do not clamp and leave it. PTFE creeps under sustained load, so a ball left in a closed micrometer reads progressively smaller. Take the reading and release.
Separating Diameter From Roundness
This is the part most incoming inspections miss. A single micrometer reading gives you a number that could describe a perfect sphere or a noticeably oval one — you cannot tell which from one measurement.
For check valve duty this matters more than diameter accuracy, because sealing depends on the ball contacting its seat consistently as it rotates. Take your three or more readings, and look at the spread. A tight spread means a round ball. A wide spread means an oval one, regardless of how good the average looks.
Checking for Voids
Internal voids are the failure mode you cannot see. The practical non-destructive screen is density: weigh the ball, measure its volume by displacement, and compare against the 2.14–2.20 g/cm³ range for virgin PTFE. A result measurably below that range indicates internal porosity or incomplete sintering.
This is worth doing on incoming consignments from a new supplier, particularly for larger diameters where voids are hardest to avoid and most consequential.
What to Put on Your Drawing
A specification that can actually be met and verified states:
- Nominal diameter and tolerance band
- Sphericity or roundness requirement, separately
- Surface finish, if the application seals
- Measurement temperature
- Resin grade — virgin or filled, and which filler
Leaving temperature off is the most common omission, and it is the one that produces disputes between a supplier's inspection record and a customer's.