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Capsule Core

Short answer: 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.

VideoWrite-up1 sources
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What should you know about Capsule Core?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. Glass bulb or fusible link.

    DD-478-F01 · Write-up · Exhibit A (Trigger)

  2. Expands with heat.

    DD-478-F02 · Write-up · Exhibit B (Liquid)

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

[...] 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!

Burned-in captions, read frame by frame (see transcript.md) · 24.0s Reel ·

Video coming soonCapsule Core · Instagram Reel
Capsule Core: Each Sprinkler Head Is Held Shut by a Tiny Glass Bulb Instagram Reel · · 24.0s

The write-up

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

  1. Exhibit A

    Trigger

    Glass bulb or fusible link.

  2. Exhibit B

    Liquid

    Expands with heat.

  3. Exhibit C

    Rating

    About 57-74 °C.

  4. Exhibit D

    Release

    Bulb shatters.

  5. Exhibit E

    Local

    Only hot heads open.

Questions people ask about Capsule Core

What holds the head shut?

Capsule.

Source: Write-up · Exhibit A (Trigger) · Wikipedia: Fire sprinkler system

What breaks it?

Heat.

Source: Write-up · Exhibit B (Liquid) · Wikipedia: Fire sprinkler system

Typical trigger?

Rating.

Source: Write-up · Exhibit C (Rating) · Wikipedia: Fire sprinkler system

Alternative element?

Melts.

Source: Write-up · Exhibit D (Release) · Wikipedia: Fire sprinkler system

Do all heads open?

Local.

Source: Write-up · Exhibit E (Local) · Wikipedia: Fire sprinkler system

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. What holds the head shut?

    1. A glass bulb
    2. A magnet
    3. A rubber band
    4. Ice
    Show the answer

    Answer: A. A glass bulb Capsule. Source: Write-up · Exhibit A (Trigger) · Wikipedia: Fire sprinkler system

  2. What breaks it?

    1. Expanding liquid
    2. Smoke
    3. Sound
    4. Water pressure
    Show the answer

    Answer: A. Expanding liquid Heat. Source: Write-up · Exhibit B (Liquid) · Wikipedia: Fire sprinkler system

  3. Typical trigger?

    1. 57-74 °C
    2. 20 °C
    3. 300 °C
    4. 0 °C
    Show the answer

    Answer: A. 57-74 °C Rating. Source: Write-up · Exhibit C (Rating) · Wikipedia: Fire sprinkler system

  4. Alternative element?

    1. Fusible link
    2. Wire
    3. Spring only
    4. Battery
    Show the answer

    Answer: A. Fusible link Melts. Source: Write-up · Exhibit D (Release) · Wikipedia: Fire sprinkler system

  5. Do all heads open?

    1. Only hot ones
    2. All at once
    3. None
    4. Random
    Show the answer

    Answer: A. Only hot ones Local. Source: Write-up · Exhibit E (Local) · Wikipedia: 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/

Permalink: https://detailenjoyer.com/case/capsule-core/ · Last updated

Markdown version

Cite a single fact by its ID, for example DD-478-F01 → https://detailenjoyer.com/case/capsule-core/#DD-478-F01