How long is a normal sized straw

Kitchen experiments - super straw

THE ATTEMPT

You can stick a couple of extra-long straws into each other for this experiment. But I recommend the following structure:

  • a thin clear plastic tube, 5-10m long;
  • a carafe of water or juice.

Down in the garden I put one end of the hose into the carafe, and I hold the other end up on the balcony. I put a colored mark on the hose after every meter, so I can easily see that it's a good six meters in altitude from the garden to the balcony. So it's a pretty ambitious drinking project: sucking up juice six meters up. The hose has an inside diameter of 3 mm. So you can easily calculate that just 4.2 cl, i.e. two shot glasses full of juice, fit into six meters of hose. Sucking them up can't be that difficult. Or?

THE RESULT

At first it's very easy. For the first one or two meters, the juice rises quickly in the hose. But then sucking becomes harder and harder. I can already see how the juice is approaching, but shortly after the four-meter mark, nothing works anymore. Since I can suck as I want. I can't get the juice any higher. Six meters are not possible.

THE EXPLANATION

In fact, there is a maximum limit height up to which a liquid can be sucked up: the so-called maximum geodetic suction height. This is not a technical limitation that could be overcome with better materials, but a fundamental physical limit. So not only thirsty children fail to suck up a liquid from a greater depth, but also the most powerful water pump. The maximum geodetic suction height is 10.33 meters.

To understand this limitation, one has to consider what actually happens when a pump draws a liquid. Sucking up means: The pressure is greater from below than from above. At the bottom, the normal air pressure presses on the surface of the water and thus also pushes the liquid from below into the pipe. Before the pump is applied, the pressure conditions at the upper end of the pipe are no different: the normal air pressure also presses on the water in the pipe from above. The result is a standstill: the water does not move up or down. If you now switch on the pump, it reduces the air pressure at the upper end and the water rises up the pipe. But the column of water sucked in has weight. And at some point it will be pulled down more by gravity than it will be sucked up by the difference in air pressure. Then the maximum suction height is reached.

Theoretically, how high this suction height is depends only on the air pressure and the water temperature and the height above sea level at which the pump is located. For normal pressure (that is 1013.25 millibars, earlier this pressure was called "one atmosphere"), four degrees cold water and zero meters above sea level, the maximum suction height is 10 meters 33. At higher altitudes, i.e. at lower air pressure, the water can be pumped up over an even lower height. However, this value of 10.33 meters is only theoretically valid. In practical pump operation, there are also various factors that significantly reduce the maximum height: the friction of the liquid on the pipe walls, the vapor pressure of the water or the residual pressure that remains in the pump, as no pump can create a really perfect vacuum. Overall, the pump engineers have to be satisfied with a maximum suction height of seven to eight meters.

CONCLUSION

Sucking up has narrow limits. Nevertheless, it is quite possible to bring liquid to the surface of the earth even from great depths. Neither mining nor most of the water wells could otherwise operate. In this case, however, it is not sucked but pressed. An overpressure of almost any size can be generated in a pressure pump. Bringing water up from a depth of several hundred meters is no problem. And the insurmountable physical suction limit loses its meaning.

Editorial staff:
Peter Ehmer

Status: 14.06.2016, 5:00 p.m.