(I didn't really have success in getting this answered on the Engineering-side so I'll try here)

I'm not very familiar with vacuum systems so this became a bit of an issue for me when I'm currently looking at some simulation values from a functionality someone (much more skilled than me) has programmed.

The result is P_tot = -253 Pa and P_static = -289 Pa

As P_tot = P_static + P_dynamic I would have imagined it to be the same for negative pressure system, as it is just a relative value, in other words that in those cases P_tot would be the most negative (the sum of the negative P_static and P_dynamic values).

I'm then currently wondering if I'm looking at a bug or there is something truly lacking in my understanding of these types of pressure systems and could someone clarify this for me in that case?


1 Answer 1


Dynamic pressure is $\frac{1}{2}\rho v^2$ which is positive definite. So the "total pressure" is always higher than the static pressure. Also, I assume you are referring to gauge pressure here, rather than absolute pressure (which is always positive).

  • $\begingroup$ Hi Chester, yes it will be a reference to gauge pressure and this is a suction system calculating the flow over such a system and the total, static and dynamic pressure losses. Sorry I will have to clarify if this "higher" means that the "total pressure" is always "more positive" than the static pressure (meaning that the P_tot = -253 Pa and P_static = -289 Pa, mentioned above, are correct for a suction system) or if one takes the absolute value of both that the abs(P_tot) should be higher than the abs(P_static)? $\endgroup$ Mar 17, 2018 at 12:08
  • $\begingroup$ You realize that there is a difference mathematically between being "more positive" and having a "higher absolute value," correct? You also realize that there is a difference between the "absolute value of the gauge pressure" and the "absolute pressure," correct? $\endgroup$ Mar 17, 2018 at 12:13
  • $\begingroup$ Yes, I'd think so but I'll repeat just in case I've misunderstood something. With "more positive" I meant that -253 is larger than -289 and with the absolute values I meant that abs(-253) = 253 which is less than abs(-289) = 289. What I'm essentially trying to figure out is that if he has modeled it in the wrong way. Right now the program apparently calculates the total static and dynamic pressure losses and then sums them to give the P_tot, eg P_tot = -289 + 36 = -253, while I think it should have been P_tot = -289 -36 = -325 $\endgroup$ Mar 17, 2018 at 13:01
  • $\begingroup$ And I'll add just in case: Absoute pressure is zero in perfect vacuum and that is its reference point, gauge pressure is zero at ambient pressure $\endgroup$ Mar 17, 2018 at 13:18
  • $\begingroup$ Like I said, static pressure is positive definite. So it should be -289+36, not -289-36. He's right, and that must mean that you're ....? $\endgroup$ Mar 17, 2018 at 13:59

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