# Balloons, gloves, inner tubes and silicone sheet as diaphragms

> What is the best diaphragm material for a Stirling engine?

- Category: Hardware › Found & household materials
- Kind: Standard part
- Basis: Measured thicknesses of common latex and silicone sheet sources; stretch limits from material data
- Page: https://xemstudio.com/objects/balloons-and-diaphragms/ · JSON: https://xemstudio.com/api/geometry?object=balloons-and-diaphragms · Plain text: https://xemstudio.com/objects/balloons-and-diaphragms/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- Party balloon (latex, 12 in): 0.15–0.25 mm wall, stretches 600%; cut the neck off and use the flat 'equator' zone. Nitrile glove: 0.10 mm (palm), low stretch, best for low-pressure diaphragms under Ø40. Bicycle inner tube (butyl): 0.9–1.1 mm, stiff, fine for Ø80+ with 5 mm travel. Silicone sheet: 0.25 / 0.5 / 1.0 mm, survives 230 °C, the only one for a hot-side diaphragm.
- Sizing: a diaphragm's effective area is about the mean of the clamp bore and the centre disc; for the 12 oz can engine (Ø66 can, 10 mL syringe swept 2 mL) a Ø50 clamp bore with a Ø25 centre disc and 3 mm of travel gives about 4.4 cm² × 0.3 cm = 1.3 mL per direction. The clamp rings in the can-engine kit are Ø40, Ø50 and Ø60 bore.
- Clamp the sheet flat and slack (not pre-stretched) between two printed rings on six M3 screws; a smear of silicone grease on the lower ring seals it. Balloon rubber dies in sunlight in a few weeks; silicone lasts years.

## Sizes

| material | thickness_mm | max_temp_c | stretch | use |
|---|---|---|---|---|
| 12 in latex balloon | 0.15–0.25 | 60 | 600% | cold-side diaphragm, Ø30–60 |
| nitrile glove | 0.1 | 80 | 300% | small low-pressure diaphragm |
| bicycle inner tube (butyl) | 0.9–1.1 | 100 | 200% | large diaphragm, long travel |
| silicone sheet | 0.25 / 0.5 / 1.0 | 230 | 400% | hot-side or long-life diaphragm |
| PE film (freezer bag) | 0.05 | 70 | low | displacer-rod seal only |

## Where the files are

- Wikipedia: [Diaphragm (mechanical device)](https://en.wikipedia.org/wiki/Diaphragm_(mechanical_device)) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Toy balloon](https://en.wikipedia.org/wiki/Toy_balloon) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds (videos and write-ups)](https://longearthsociety.com/engineering/) — web, video — LES site

## Questions

**What can I use as a Stirling engine diaphragm?**

A latex balloon for the classic cold-side can engine, silicone sheet (0.5 mm) for anything hot or long-lived, nitrile glove for tiny engines, inner tube for big slow ones.

**How thick is a balloon?**

0.15–0.25 mm uninflated at the equator; it thins to under 0.1 mm inflated. Use it flat and slack.

**What size diaphragm for a soda can Stirling?**

Ø50 mm clamp bore with a Ø25 centre disc moves about 1.3 mL over 3 mm of travel, matched to a 12 oz can displacer.

## Fits with

- [Can-engine printed kit: can caps, diaphragm clamps, displacer guides, crank stands, CD/platter hubs, syringe cradles](https://xemstudio.com/objects/can-engine-printed-kit/)
- [Plastic syringes 1–60 mL as pistons and cylinders](https://xemstudio.com/objects/plastic-syringes/)
- [O-rings, common metric sizes (ISO 3601)](https://xemstudio.com/objects/o-rings-metric/)
- [Aluminium beverage cans: 12 oz standard, 16 oz, 330/500 mL, sleek, slim, mini](https://xemstudio.com/objects/beverage-cans/)