Since the ancient Egyptians, we have used siphons to transfer liquid between containers or over barriers. A siphon is a tube which carries liquid with no air and the rule for a successful siphon is that the water level at the outlet side must be lower than the water level at the inlet. What happens in between and specifically how high the tube is raised is not important.
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00:11 What is a Siphon?
00:29 Siphon Assumption
00:45 400 Years Ago Engineers Noticed This!
01:00 Why Do Siphons Have A Maximum Height?
01:38 Torricelli's (Galileo's Student) Brainwave
02:12 Torricelli's Did This To Study The Effects & How Mercury Barometer Was Born?
03:07 Pascal Climbed High Altitudes With Torricelli's Barometer To Prove This
03:43 Why Use Mechanical Bordon Gauges?
For a long time, the assumption held that siphons worked because the weight of water on the outlet side of the siphon is higher than the weight of the water on the inlet side, meaning that the heavier side pulled the water like a rope or a chain over the bend.
400 years ago engineers noticed something interesting. They saw that when a siphon was used at just over 10 meters the water would stall and a gap would appear at the top of the tube. The question that no one could answer was - why do siphons have a maximum height?
Galileo, the father of modern physics even got involved. He suggested that the gap was vacuum and that it was caused by the weight of water overcoming the pull of vacuum as if the imaginary rope was stretching under the weight. He reasoned that vacuum had only a limited power to lift and after a certain height it would run out of strength.
One of the Galileo's students named Torricelli agreed that what they saw was vacuum but he couldn't see the logic of vacuum acting as a force of attraction and he tried to explain things differently. His brainwave was that the atmosphere surrounding the basin at the bottom of the tube has weight and that weight pushes down on the liquid exerting pressure on the pool at the lower end of the siphon, in turn, pushing water up the tube. If the height and therefore the weight of the water in the tube becomes large enough, it balances the force of air pressure at the bottom and the water stalls. The gap at the top then is indeed a vacuum but it's not pulling on the liquid. It can't because there is nothing there.
Instead of a siphon Torricelli filled a tube with water, plugged it in one end and inverted it into a basin of water to study the effects. Say we measured a height of 10.32 meters of water with a density of a thousand kilograms per meter cubed under Earth's gravity force of 9.81 Newton's per kilogram, the equation for water pressure gives us 101298 Newton's per meter square or Pascals of pressure at the bottom. That's 1,013 millibars absolute.
Torricelli's realized that by using a heavier liquid like mercury he could achieve the same way with a much shorter column and so the mercury barometer was born. With measurements in lengths of mercury and still today much of the vacuum world measures pressure in millimeters of mercury or as we call it now Torr.
Soon after Blaise Pascal took ideas further by realizing that if atmospheric pressure has weight then the pressure should change according to the altitude. He used Torricelli's barometer to prove that. By climbing to higher altitudes he measured that the atmospheric pressure would reduce. The barometer is still a very important instrument. Not least because the only pressure acting on it is that which is being measured. It measures to a good accuracy the absolute pressure of a gas irrelevant to any other atmospheric conditions. Clearly, a mercury barometer requiring nearly a meter in height has its practical limitations.
So today mechanical Bourdon gauges are the most common inline pressure measuring device that we use in industrial vacuum. The basic principle is something like a party horn or a party blower, that is, a flattened cylinder will tend to straighten if the pressure inside it raises relative to the outside pressure. What that means is Bourdon tubes are always measuring pressure relative to the surroundings.
If well constructed and well-calibrated regularly Bourdon gauges are a compact and cost-effective way to provide clear, accurate and consistent vacuum measurements.