SprinklerClear

How does a water sprinkler work?
The tick-tick-tick sprinkler has no motor: its own jet throws an arm aside, a spring swings it back, and every tap turns the head three degrees. Watch that happen in slow motion, see why sprinklers must overlap, and find out why a fire sprinkler never sets off its neighbours.

▶ Play with it  ·  Read the 60-second explainer  ·  Watch the 40-second video

In 60 seconds

1. No motor, just water. An impact sprinkler is a riser pipe, a bearing the head turns in, a nozzle tilted up at about 25 degrees, and a spring-loaded arm with a spoon that sits in the jet. Water pressure also squeezes a stack of washers in the bearing, which seals it and gives the friction that holds the head still between taps.
2. Pressure becomes speed, and air decides the throw. The nozzle turns pressure into speed: v = 0.96 × √(2P/ρ), about 23.6 m/s at 3 bar, the push of a tank 31 m up. With no air that jet would land 47 m away. Drag on the drops cuts it to about 14 m, and makes the best angle about 33 degrees, not 45.
3. Tick: the arm strikes the body. The jet hits the spoon and flings the arm aside. A spring swings it back and it strikes the anvil on the body, sharing its momentum with the head, which steps round about 3 degrees. That happens about 2.5 times a second. At the end of the arc a trip lever raises a tab, and blows on the tab chatter the head quickly back.
4. One sprinkler waters badly, many water well. A single sprinkler puts down most of its water close in and little at the rim. Set on a grid about half a wetted width apart, neighbours fill each other in. Christiansen's 1942 coefficient of uniformity scores the evenness: 12 m spacing at 3 bar gives about 90% in this model, and 1 bar leaves dry-centred doughnuts.
5. Flood, sprinkler or drip. To get the same water to the roots you pump need ÷ efficiency. FAO's guide figures are 60% for flood, 75% for sprinklers and 90% for drip; India's ICAR puts ordinary flood irrigation at only 30 to 50%. For a crop using 5 mm a day on one hectare, that is 83,000 litres by flood against 56,000 by drip.
6. The fire sprinkler is a different machine. Each fire sprinkler head is a valve held shut by a glass bulb of liquid that shatters at its rated temperature, 68 °C for a red bulb. Every head feels only its own heat, so only those over the fire open: one head was enough in 77% of the fires NFPA counted where sprinklers operated.

Words worth knowing

Term	Meaning
Impact sprinkler	A sprinkler turned by a spring-loaded arm that its own jet throws aside and that strikes the body on its return.
Nozzle	A smooth narrowing that turns water pressure into speed: v = C × √(2P/ρ), with C about 0.96.
Pressure head	Pressure written as a height of water: about 0.1 bar for every metre.
Radius of throw	How far from the sprinkler its farthest drops land. Air drag on the drops sets it.
Trip lever and collars	The lever that flips at each end of the arc and makes a part-circle sprinkler turn back.
Coefficient of uniformity (CU)	Christiansen's score for even watering: 100 × (1 − average miss ÷ average depth). 100% is perfectly even.
Application rate	How fast water is put on the ground, in millimetres an hour. One millimetre is one litre on every square metre.
Field application efficiency	The share of the water put on a field that ends up where roots can use it.
Glass bulb	The liquid-filled capsule that props a fire sprinkler shut and shatters at a set temperature.

A short history

2,000 years from a steam toy that spun in Alexandria to circles of green you can see from an aeroplane.

• 60 · A ball that spins on its own steam (Hero of Alexandria, Alexandria, Egypt)
• 1871 · A patent for watering the lawn (Joseph Lessler, Buffalo, New York, USA)
• 1874 · A sprinkler that opens by itself (Henry S. Parmelee, New Haven, Connecticut, USA)
• 1881 · Grinnell makes it dependable (Frederick Grinnell, Providence, Rhode Island, USA)
• 1933 · The sprinkler that goes tick, tick, tick (Orton Englehart, Glendora, California, USA)
• 1942 · A number for evenness (J. E. Christiansen, University of California, USA)
• 1952 · A pipe that walks in a circle (Frank Zybach, Strasburg, Colorado, USA)
• 2015 · Per Drop More Crop (Government of India, India)

The full story, with 30 moments, charts, people and 46 sources: glassbox.how/e/sprinklerclear/history. The data lives in history.json.

Video and slides

Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.

File	What	Size
glassbox/reel.mp4	Reel / Short, with captions and soundtrack	1080×1920
glassbox/video.mp4	YouTube video, with captions and soundtrack	1920×1080
glassbox/slide-1…10.jpg	Instagram carousel	1080×1350
glassbox/thumb.jpg	YouTube thumbnail	1280×720
glassbox/cover.jpg	Share card and repo social preview	1200×630
glassbox/history-reel.mp4	“History in 10 moments” Reel / Short	1080×1920
glassbox/history-slide-*.jpg	History carousel	1080×1350
glassbox/post.json	Post copy and schedule used by the publish kit

Privacy

This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.

It remembers a few things in your own browser only, and never sends them anywhere:

Browser storage key	What it holds
sprinklerclear.v1	Which chapters you have opened, your best quiz scores, and sound on or off.

Exactly what each one sees is at glassbox.how/privacy.

Licences

• Code: MIT. Use it, change it, ship it.
• Explanations, text, images and videos (glassbox.json, glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/sprinklerclear”.
• Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
• The Glassbox name and logo aren't covered by either licence. See the terms.

Found a mistake? Open an issue. Corrections happen in public.

Run it

It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.

python3 -m http.server 8000

Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.

Then open http://localhost:8000.

How it's built

File	What
index.html, css/app.css	The page and its styles
js/app.js, js/stage.js, js/ui.js, js/kit.js	The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director
js/sprinkler.js	The physics, with no drawing in it: nozzle, drops flying through air, one sprinkler's footprint, the field map and Christiansen's CU, the impact-drive simulation, the water budget and the fire-sprinkler heat model
js/parts.js	Shared drawing kit: the impact sprinkler model, chart boards, turned parts, flying drops
js/chapters/*.js	One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes
glassbox.json	Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how
reel in each chapter	The storyboard the Glassbox studio records into short videos
glassbox/	The published video, slides, thumbnail and post copy
fonts/, vendor/three/	Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT)