# Tri-clamp bubbler gravity caps: measure specific gravity from the lid with two dip tubes

> How can I measure gravity in a stainless fermenter without a hydrometer inside?

- Category: Hardware › Sanitary clamp fittings
- Kind: Hardware
- Basis: Xem Studio design: the standard 1½ in solid cap with two Ø4.76 × Ø2.4 tubes whose open ends are a fixed Δh apart, each with a 4 mm hose barb; the method is hydrostatics, Δp = ρ g Δh
- Page: https://xemstudio.com/objects/tri-clamp-bubbler-gravity-caps/ · JSON: https://xemstudio.com/api/geometry?object=tri-clamp-bubbler-gravity-caps · Plain text: https://xemstudio.com/objects/tri-clamp-bubbler-gravity-caps/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- Push a slow trickle of gas (the same gas as the headspace, CO₂) down both tubes; each tube's back-pressure equals the liquid head above its tip. The difference between the two is ρ g Δh, so specific gravity = Δp / (997 × 9.81 × Δh). The fill level and the headspace pressure cancel out, as long as both tips stay under the liquid.
- Read the difference with one differential pressure sensor across the two barbs. With Δh 200 mm, water gives 1956 Pa and every 0.001 of gravity adds about 2 Pa, so pick a sensor that resolves 1 Pa or better; a longer Δh gives more signal.
- Nothing goes in the vessel but two thin tubes: no floating hydrometer, no radio through stainless steel.
- Every clamp face is the ISO 2852 profile used across this site: 2.85 mm rim, flat face with the gasket groove (K 1.6 deep, R3 1.2 bottom, 23° walls, P2 0.8 edges) on C2, 20° back cone. Flange C3 / groove C2 / tube OD × wall, mm: ½–¾ in 25.0 / 20.4 (derived) / 12.7–19.05 × 1.65; 1–1½ in 50.5 / 43.5 / 25.4–38.1 × 1.65; 2 in 64.0 / 56.5 / 50.8 × 1.65; 2½ in 77.5 / 70.5 / 63.5 × 1.65; 3 in 91.0 / 83.5 / 76.2 × 1.65; 4 in 119.0 / 110.0 / 101.6 × 2.11; 6 in 166.9 (6.570 in) / 156.5 / 152.4 × 2.77; 8 in 217.7 (8.570 in) / 207.0 / 203.2 × 2.77; 10 in 268.5 / 261.0 / 254.0 × 3.40; 12 in 319.3 / 311.8 / 304.8 × 3.40.

## Sizes

| deep_tube_in | dh_mm | dp_water_pa | dp_sg_1050_pa | pa_per_0.001_sg |
|---|---|---|---|---|
| 12 | 100 | 978 | 1027 | 1 |
| 12 | 150 | 1467 | 1540 | 1 |
| 12 | 200 | 1956 | 2054 | 2 |
| 16 | 100 | 978 | 1027 | 1 |
| 16 | 150 | 1467 | 1540 | 1 |
| 16 | 200 | 1956 | 2054 | 2 |

## Where the files are

- physics: [Hydrostatic pressure: Δp = ρ g Δh (two bubbler tubes at a height difference Δh give the density of the liquid between them)](https://en.wikipedia.org/wiki/Hydrostatics) — web — physics
- GW Kent: [Tri-clamp fittings guide: tube OD, flange OD and ID per size to 12 in; solid cap thickness](https://www.gwkent.com/triclamp-guide) — web — publisher's guide; dimensions cited
- Rubber Fab: [Tri-Clamp gasket dimensional chart: gasket ID, OD and bead diameter per size (2 in bead 2.22 in / 56.50 mm, 1.5 in 1.71 / 43.50, 3 in 3.29 / 83.50)](https://shop.andersonprocess.com/media/assets/product/documents/rubber-fab-tri-clamp-dimensions.pdf) — PDF — maker chart; dimensions are facts (verify at link)

## Free STEP/STL (6 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/index.json

- bubbler-cap-1.5in-12in-dh100: [STEP](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-12in-dh100.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-12in-dh100.stl) — 1½ in cap with two Ø4.76 bubbler tubes, 304.8 and 204.8 below the face (Δh 100), 4 mm hose barbs on top
- bubbler-cap-1.5in-12in-dh150: [STEP](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-12in-dh150.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-12in-dh150.stl) — 1½ in cap with two Ø4.76 bubbler tubes, 304.8 and 154.8 below the face (Δh 150), 4 mm hose barbs on top
- bubbler-cap-1.5in-12in-dh200: [STEP](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-12in-dh200.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-12in-dh200.stl) — 1½ in cap with two Ø4.76 bubbler tubes, 304.8 and 104.8 below the face (Δh 200), 4 mm hose barbs on top
- bubbler-cap-1.5in-16in-dh100: [STEP](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-16in-dh100.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-16in-dh100.stl) — 1½ in cap with two Ø4.76 bubbler tubes, 406.4 and 306.4 below the face (Δh 100), 4 mm hose barbs on top
- bubbler-cap-1.5in-16in-dh150: [STEP](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-16in-dh150.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-16in-dh150.stl) — 1½ in cap with two Ø4.76 bubbler tubes, 406.4 and 256.4 below the face (Δh 150), 4 mm hose barbs on top
- bubbler-cap-1.5in-16in-dh200: [STEP](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-16in-dh200.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-bubbler-gravity-caps/bubbler-cap-1.5in-16in-dh200.stl) — 1½ in cap with two Ø4.76 bubbler tubes, 406.4 and 206.4 below the face (Δh 200), 4 mm hose barbs on top

## Questions

**Why two tubes and not one?**

One tube measures depth × density, so a level change looks like a gravity change; two tubes a fixed distance apart measure density alone.

**Which length for a 6 × 18 in vessel?**

The 16 in cap: the deep tip sits just above the dished bottom, the shallow tip 100–200 mm higher, both under a normal fill.

## Fits with

- [Tri-clamp brewing sensor lids: 4, 6 and 8 in caps with every port a fermenter needs](https://xemstudio.com/objects/tri-clamp-brewing-sensor-lids/)
- [Tri-clamp dip-tube caps: 1½ and 2 in, ⅜ in tube 6 to 24 in long](https://xemstudio.com/objects/tri-clamp-dip-tube-caps/)
- [Tri-clamp vessels with a dished or round welded bottom: 4, 6 and 8 in, 12 to 24 in deep](https://xemstudio.com/objects/tri-clamp-dished-bottom-vessels/)