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Suppose that I have a syringe piston, connected to a closed sample cell through a 3 mm diameter stainless tube (SS). Its like syringe, ss tube, sample cell; all are connected in line with fittings. The whole system is filled with water. when I push the syringe, water is pressurized and it flows through ss tube, then to the sample cell. My question is instead of a 3 mm diameter ss tube if I use a 6 mm diameter SS tube what will be the change? The tube length is kept constant, say for example 10 cm and I push the syringe by 1 cm (all other conditions remain the same). I am thinking in this school of thought. Here tube length is the same. So when the diameter of the tube is increased, the volume of water inside the tube will increase. When I push the syringe by 1 cm, more volume of water is present inside the 6 mm diameter tube, large amount of water is compressed, so the pressure created will be large and because of this high pressure, flow occurs from the piston, tube to sample cell, Note flow will be more in this case as pressure is high. In contrast, in the 3 mm diameter tube, the volume of water inside the tube is less as diameter is less. When syringe is pushed by 1 cm, the less volume fo water present in tube is pressurized and flow occurs. As water volume is less, pressure is less and flow is less. is this correct? I would like to get answer for my doubt.

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What happens to water pressure if diameter of the tube is increased?

The answer depends upon whether you are referring to the static pressure, after everything has equalized, or the pressure while water is flowing through the small diameter tube.

In the case of static pressure, if we ignore the effects of gravity, the pressure in the water will be the same everywhere. With gravity, it will be slightly more closer closer to the earth.

In the case of dynamic pressure, the pressure will be higher in the syringe that is pumping, lower in the small diameter tube, and then higher at the other end, but not as high as in the syringe. This is due to the Venturi effect.

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