Please excuse me if this question is misplaced and also for my narrow understanding of physics.

Firstly, I'm asking simply because the robot I've now been set to work with uses an air pneumatics system. By way of a small vacuum it can move about objects from point A to point B. This vacuum is somehow created from a small attachment that is connected to a steady supply of pressurised air.

This crude illustration is made on MS Paint shows how the vacuum is made. I'm still puzzled as to why the input of air doesn't simply flow out of both of the other openings.

crude illustration


I can intuit that the volume of air from the exhaust is the sum of the input and the additional air taken from the vacuum but what is the force of the vacuum and how is it measured?

(Side note)

Does the surrounding environment (atmospheric presure) have any impact on the vacuum?

Any response is much appreciated.

  • 1
    $\begingroup$ Hopefully someone with more knowledge can provide a better answer but I suspect it is to do with relative diameters of the pipes requires a drop in pressure at the vacuum tube to maintain the correct flow through the device. $\endgroup$
    – nivag
    Jun 18, 2014 at 15:49
  • $\begingroup$ Thanks for the reply. If this were true the mechanism I have would only show this internally. My mechanism has three openings of all the same size. $\endgroup$ Jun 18, 2014 at 15:54
  • $\begingroup$ Are you sure it is a T-valve and not a 3-way ball valve? $\endgroup$
    – Kyle Kanos
    Jun 18, 2014 at 15:56
  • $\begingroup$ My only certainty is that it looks like I have depicted it in my question above. I'm new to pneumatics and can make no presumtions. It does not look like the ball valve you've shown. $\endgroup$ Jun 18, 2014 at 16:07
  • 1
    $\begingroup$ The vacuum is created by Bernoulli's principle. The high velocity of the air passing though the horizontal tube lowers the pressure in the side branch. I have never used a pump of this sort that worked with an air stream, but I've many times used this sort of pump that worked using a water stream. $\endgroup$ Jun 18, 2014 at 16:46

2 Answers 2


A fairly good schematic of an air-powered Venturi pomp:


The geometry (alignment of nozzle and mixer) is crucial, it is inertia that directs the compressed air toward the mixer.

The difference between the pressure of the compressed air and the ambient pressure is of course important, as it is driving the flow. In turn, this flow is the cause of the suction that can generate vacuum — in your case, though you do not describe in which conditions it is used, the object seems simply to be moved by entrainment in the air flow.

  • $\begingroup$ This diagram has helped greatly in my understanding. Now, is there a formula where the strength of the vacuum is relative to the strength of the compressed air? $\endgroup$ Jun 19, 2014 at 14:47
  • $\begingroup$ No, there is no general formula, you'd need to solve the Navier-Stokes equations in the relevant geometry in order to get quantitative results. $\endgroup$
    – Joce
    Jun 19, 2014 at 15:27

It is called a Venturi pump. Free expansion of compressed gas along the top leg results in a jet of gas with net momentum from input to exhaust. Any gas entering from below that collides with this jet gets momentum directing it towards the exhaust. It is a neat way of pulling a (not great) vacuum without needing electricity to run a pump.

  • $\begingroup$ How does this explain why the air does not flow out of both openings? $\endgroup$ Jun 19, 2014 at 7:12
  • 1
    $\begingroup$ Robert - the trick is you need a high enough gas pressure and an inlet nozzle. Again, the free expansion of the high pressure gas results in a large directed momentum. As an analogy, a water sprinkler can send jets (sprays) of water that don't just go every which way when leaving the nozzle. $\endgroup$
    – Jon Custer
    Jun 19, 2014 at 10:53

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