# Capsule Core (DD-478)

> Most automatic fire sprinkler heads are held closed by a glass bulb filled with a heat-sensitive liquid, or by a fusible metal link. When the surrounding air reaches the rated temperature, typically around 57-74 °C (135-165 °F), the liquid expands and shatters the bulb, or the link melts, releasing water only from the heads that are hot. The footage, credited to xplainingtheworld, shows a red sprinkler bulb bursting and water spraying.

- Department: Tech
- First published: 2025-02-04
- Last updated: 2025-02-04
- Permalink: https://detailenjoyer.com/case/capsule-core/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## Quick facts

- **DD-478-F01**: Glass bulb or fusible link. _(Write-up · Exhibit A (Trigger))_
- **DD-478-F02**: Expands with heat. _(Write-up · Exhibit B (Liquid))_

## Write-up

Each sprinkler head is held shut by a tiny glass bulb. Heat makes the liquid inside burst it.

1. **Trigger**: Glass bulb or fusible link.
2. **Liquid**: Expands with heat.
3. **Rating**: About 57-74 °C.
4. **Release**: Bulb shatters.
5. **Local**: Only hot heads open.

## Reel vs. record

- **Note**: None needed.

## Video: Capsule Core: Each Sprinkler Head Is Held Shut by a Tiny Glass Bulb

- Duration: 24.0 s · Published: 2025-02-04
- File: https://detailenjoyer.com/assets/videos/DFpvL7pKJaI.mp4

### Transcript

[...] fire sprinklers [...] or fusible element [...] high temperatures. [...] typically around [...] or 57°C - 74°C, [...] the glass capsule [...] link melts. [...] water to spray [...] NOW YOU KNOW!

## Questions & answers

**Q: What holds the head shut?**  
A: Capsule. _(Source: Write-up · Exhibit A (Trigger) · Wikipedia: Fire sprinkler system)_

**Q: What breaks it?**  
A: Heat. _(Source: Write-up · Exhibit B (Liquid) · Wikipedia: Fire sprinkler system)_

**Q: Typical trigger?**  
A: Rating. _(Source: Write-up · Exhibit C (Rating) · Wikipedia: Fire sprinkler system)_

**Q: Alternative element?**  
A: Melts. _(Source: Write-up · Exhibit D (Release) · Wikipedia: Fire sprinkler system)_

**Q: Do all heads open?**  
A: Local. _(Source: Write-up · Exhibit E (Local) · Wikipedia: Fire sprinkler system)_

## Sources

- [Wikipedia: Fire sprinkler system](https://en.wikipedia.org/wiki/Fire_sprinkler_system)

## Cite this case file

Easterling, Marcus. "Capsule Core." The Detail Department (DetailEnjoyer), case file DD-478, last updated Feb 4, 2025. https://detailenjoyer.com/case/capsule-core/