---
title: "Siphon, Pressure Limit & Cavitation Interactive Lab | Sandbox Physics"
description: "How high can a primed siphon lift the liquid? Only until the absolute pressure at the crest falls to the liquid's vapor pressure."
canonical_url: "https://sandboxphysics.com/labs/siphon-pressure/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/siphon-pressure/"
last_modified: "2026-10-27"
---

M095 · Fluid mechanics

# Siphon, Pressure Limit & Cavitation

**How high can a primed siphon lift the liquid?** Only until the absolute pressure at the crest falls to the liquid's vapor pressure. The drop to the outlet drives the flow. Raising the crest adds no drive; it only lowers the pressure up there, and friction lowers it further. At the limit the liquid boils at the top and the column breaks. For water at sea level, the crest can be at most about 10 metres above the surface it draws from.

Raise the siphon crest, change how far the outlet drops, and prime or break the liquid column. Follow the absolute pressure along the tube, see when the vapor-pressure limit rejects the column, and estimate the flow resistance from timed collection.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

The source level is fixed, the outlet is open to the atmosphere and the losses are prescribed. Priming is set directly, and the vapor pressure serves only as a screening limit. Bubble growth, startup and two-phase flow are not simulated.

Physics tutorial

## A siphon needs both energy and a liquid column

**Background** Liquid flows steadily through a pipe under constant gravity, and the flow is treated as incompressible and one-dimensional. Centreline heights are measured upward from the laboratory datum.

**Why it matters** Before trusting a fitted curve, see how the shape of the pipe, the energy carried by the liquid and the simulated sensor readings relate to each other.

Start with the essentials

- **Focus question**: Why does raising the crest put the flow at risk without adding any driving head?
- **One-sentence intuition**: The drop to the outlet supplies the energy, while the crest height and the losses along the way lower the absolute pressure.

### Core mathematical model

#### The outlet drop sets the speed

$$
\begin{aligned}v&=\sqrt{\frac{2g(z_S-z_o)}{1+f_D L/D+K_\Sigma}}\\Q&=Av\end{aligned}
$$

The term for the kinetic energy leaving the outlet stays in the formula, even in the no-loss comparison.

#### Absolute pressure along the tube

$$
\begin{aligned}p_{\mathrm{abs}}(s)&=p_a+\rho g\left[z_S-z(s)\right]\\&\quad-\frac{\rho v^2}{2}-\rho g h_L(s)\end{aligned}
$$

With friction and bend losses, the lowest pressure can sit just after the crest.

#### Three conditions for flow

$$
\begin{gathered}\text{primed},\quad z_S>z_o,\\\min_s p_{\mathrm{abs}}(s)>p_v\end{gathered}
$$

All three must hold. A rejected solution is still drawn as a diagnostic curve, but no volume is collected.

#### Flow from timed collection

$$
V_i=b+\widehat Q t_i
$$

A free intercept absorbs any sensor tare offset, and only the acquired readings enter the fit.

#### Only the total resistance can be found

$$
\widehat R=\frac{2g(z_S-z_o)}{(\widehat Q/A)^2}-1
$$

Collected volumes cannot separate Darcy friction from minor losses.

### Common difficulties

#### Gauge and absolute pressure

**Typical misconception** A negative gauge pressure always means the liquid cannot flow.

**Better mental model** Add the ambient pressure first, then compare the result with the specified vapor pressure. The check covers the whole path.

#### The fitted resistance is an estimate

**Typical misconception** The fitted coefficient is just the reference slider under another name.

**Better mental model** Clear the record and the estimate disappears. Noise can even give an unphysical negative coefficient: the fit is unconstrained, so the result is not clipped, and it is not a safety certificate.

### Run the experiment

1. 01

   #### Prime the column

   Choose Unprimed tube, then press Prime the tube.

   **What to observe:** An unprimed tube carries no flow, even when the available energy would allow it.

2. 02

   #### Move the outlet

   Lower the outlet and compare with No-loss comparison.

   **What to observe:** The outlet drop sets the speed. Raising the crest alone does not drive the ideal flow.

3. 03

   #### Find the pressure limit

   Raise the crest or lower the ambient pressure, and look at the marked station with the lowest pressure.

   **What to observe:** Once the pressure there reaches the specified vapor pressure, the column is rejected.

4. 04

   #### Collect liquid and estimate

   Return to Primed siphon, press Acquire timed volumes, then press Export raw readings.

   **What to observe:** The slope of volume against time gives the flow and the total resistance, but it cannot separate the different loss mechanisms.

Physical conclusion

Pressure can recover downstream even when energy has been lost. The liquid must also stay above its vapor threshold along the whole pipe for the solution to hold.

Motion & dynamics · Starter path

## From a falling ball to an orbit

Start by measuring gravity and splitting motion into components. Then add contact forces and collisions, and follow the same laws all the way to an orbit.

[Start here → **Free Fall & Measuring Gravity** Capture a few drops and fit the camera positions to estimate gravitational acceleration. Then add air damping and watch the fit drift from the truth. Observe · 14 min](https://sandboxphysics.com/labs/free-fall/)

1. [1 Free Fall & Measuring Gravity](https://sandboxphysics.com/labs/free-fall/)
2. [2 Projectile Flight Designer](https://sandboxphysics.com/labs/projectile-flight/)
3. [3 Inclined Plane & Friction](https://sandboxphysics.com/labs/inclined-plane/)
4. [4 One-Dimensional Collision Bench](https://sandboxphysics.com/labs/collision-bench/)
5. [5 Mass–Spring Oscillator](https://sandboxphysics.com/labs/mass-spring/)
6. [6 How Orbits Take Shape](https://sandboxphysics.com/labs/orbit/)

[View the full path →](https://sandboxphysics.com/topics/mechanics/#starter-path)