Schoolyard Barometer

 

If I lift or tilt one sealed end of a pipe or water hose that is full of water, while the other end is submerged in a pan of water (to prevent air from entering the pipe), the weight of the water in the pipe will cause a negative pressure at about 23' elevation, analogous to the pressure difference between the air inside a high-flying aircraft and the outside air (with some adjustment,, depending on my altitude, and on the fact that an airplane interior pressure is not set at sea-level -- 14.7 psi).

Yes, your analogy is physically accurate. When you lift a sealed, water-filled pipe while keeping the open end submerged, the weight of the water pulling downward creates a partial vacuum (negative gauge pressure) at the top of the pipe.

This is exactly how a water barometer works. The physics behind your scenario breaks down into a few key points:

The 23-Foot Pressure Drop

At sea level, atmospheric pressure is 14.7 psi (pounds per square inch).

A column of water exerts a pressure of about 0.433 psi per foot of elevation.
If you lift the sealed end to 23 feet, the water column exerts a downward pressure of roughly \(23 \times 0.433 \approx \mathbf{10\text{ psi}}\).
Because the bottom is open to the atmosphere (14.7 psi) and the top is sealed, the pressure at the very top drops to about 4.7 psi absolute (\(14.7 - 10\)).

The Airplane Analogy

Your comparison to a high-flying aircraft is spot on.

Commercial airplanes typically maintain a cabin pressure equivalent to an altitude of 6,000 to 8,000 feet, which translates to roughly 11 to 12 psi absolute.
If you raise your pipe to about 6 to 8 feet at sea level, the absolute pressure at the top of the pipe (\(14.7 - (8 \times 0.433) \approx 11.2\text{ psi}\)) perfectly mirrors the comfortable air pressure inside a cruising airliner.
Raising it to your mentioned 23 feet drops the internal pressure to ~4.7 psi, which actually mirrors the brutal outside atmospheric pressure at around 25,000 feet in the sky.

The Ultimate Limit: 34 Feet

You can only tilt or lift the pipe so far before the physics change. At sea level, standard atmospheric pressure can only support a maximum water column of about 34 feet (14.7 psi / 0.433 psi/ft).

If you lift the sealed end higher than 34 feet, the absolute pressure at the top hits 0 psi (a perfect vacuum).
At this point, the water will actually boil at room temperature (vapor pressure), and the water level will refuse to rise any higher, leaving an empty vapor space at the top of the pipe.

To explore this further, let me know:

Are you designing a siphon system or a DIY flow loop?
What is your local altitude above sea level?
Do you need to calculate the exact flow rate or pipe diameter needed for your project?

The water barometer seems like a good project for a middle-school or high school physics class, if they had the vertical range to manage it. (Most schools may not have that, but many could make due with half of the elevation just by exploiting a sloping landscape near the building.)

Using a sloping landscape is a brilliant, practical workaround for the height constraint. It transforms a logistical hurdle into an interactive lesson on vertical vs. linear distance in fluid dynamics.

Why the Sloping Landscape Works

Hydrostatic pressure depends strictly on vertical height (head), not the length of the pipe.

• If a classroom uses a 46-foot long hose laid along a hill with a 30-degree slope, the total vertical rise is exactly 23 feet (\(\sin(30^\circ) \times 46\)).
• The pressure drop at the top will be exactly the same as if the hose were hanging straight down from a 23-foot balcony.

Practical Tips for a School Project

If a teacher were to set this up, a few practical adjustments make it much easier to manage than a traditional vertical setup:

• Clear Vinyl Tubing: Using a clear, flexible PVC hose allows students to see the water column. They can measure the exact point where the water column stops rising if they push past the ~34-foot vertical limit.
• Easy Priming: Instead of trying to lift a heavy, water-filled pipe, students can submerge the entire hose in a reservoir at the top of the hill, seal one end with a heavy-duty valve, and walk the open end down the slope into a lower basin.
• Vacuum Gauge Integration: Tapping a cheap mechanical vacuum gauge into the sealed top end gives students a direct, visual readout of the "airplane cabin" negative pressure as the hose is moved down the hill.

If you are thinking about adapting this for a class or a specific demonstration, let me know:

• Would you like a step-by-step lesson plan template or a materials list?
• Should we calculate the exact hose lengths needed for a specific hill angle or slope?
• Do you want to include how barometric weather changes affect the water level day-to-day?

The schoolyard garden hose can be used for 3/4 or 9/10 of the total length needed, to reduce cost, if hose clamps or other fasteners are available. And we should aim for the vacuum height, and include clear tubing, to enable reading of the barometer.

That is a highly cost-effective way to build it. By splicing a standard, opaque garden hose for the long slope with a shorter piece of clear vinyl tubing at the top, you keep costs low while keeping the most critical part—the water line and barometer reading—fully visible.

Here is how to structure this hybrid setup to safely and accurately reach the maximum vacuum height.

📐 Sizing the System for Full Vacuum

To reach the ultimate barometric limit where a full vacuum forms, your setup needs a minimum vertical rise of 34 feet (at sea level).

Using your 9/10 hybrid ratio, a standard 50-foot garden hose paired with a 6-foot clear tube gives you a 56-foot total length. This length is perfect for managing various hill slopes:

• At a 45° slope: A 50-foot total length achieves ~35 feet of vertical rise (just hitting full vacuum).
• At a 30° slope: You would need roughly 68 feet of total length (e.g., a 60-foot garden hose + 8-foot clear tube) to hit that 34-foot vertical height.

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🛠️ The Anatomy of the Hybrid Barometer

To prevent air leaks—which will instantly ruin the vacuum—the connections must be airtight.

[Sealed Top Valve] 
       │
[Clear Vinyl Tubing]  <-- The "Gauge" (Water level drops into here)
       │
[Barbed Adapter + Hose Clamps]
       │
[Standard Garden Hose] <-- The "Body" (Stays completely full of water)
       │
[Open End Submerged in Basin]

1. The Sealed Top: Secure a plastic or brass ball valve to the top of the clear tubing. This allows you to open it to fill the hose with water, then lock it airtight.
2. The Splice (The Critical Junction): Use a barbed hose adapter (male garden hose thread to barb, or a double-sided barb coupler) to bridge the garden hose and the clear vinyl tubing. Secure it firmly with screw-tight hose clamps (worm-gear clamps). Wrap the threads in Teflon tape to ensure an absolute seal.
3. The Clear Tube Length: Make the clear section 5 to 8 feet long. This ensures that when the water column drops from the absolute top to its natural equilibrium point (~30 to 34 feet vertically, depending on your local weather and altitude), the meniscus lands right inside the clear window for the students to measure.

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🏃‍♂️ Classroom Execution: The "No-Lift" Priming Method

Filling a 60-foot hose and walking it around without introducing air bubbles can be tough for students. This method makes it seamless:

1. Submerge and Connect: Place the bottom basin at the base of the hill. Connect your garden hose and clear tubing on the slope.
2. Fill from the Bottom (Best Method): Hook the top valve up to a school outdoor spigot using a temporary adapter. Flush water down the system until it fills the bottom basin and all air bubbles are forced out.
3. Seal and Disconnect: Close the top ball valve completely to trap the water. Disconnect it from the spigot.
4. Watch it Drop: As long as the bottom end stays submerged in the basin, the water at the top of the clear tube will immediately drop down a few feet, leaving a vapor vacuum space at the very top. Students can then use a tape measure to find the exact vertical distance from the basin water level to the clear tube water level.

To nail down the exact setup, let me know:

• What is the estimated slope angle or vertical drop of the hill you have in mind?
• Would you like the exact hardware store shopping list (thread sizes, barb diameters) to make sure everything clamps together perfectly?

  John Champagne    @TallPhilosopher     @TallPhilosopher on X