# Units to use in this formula?

Should standard SI units be used in this formula or are other units supposed to be used for the formula for stress in a thin-walled spherical pressure vessel?

$$\sigma_{yield}=\frac{P_g r_i}{2t}$$

where $\sigma_{yield}$ is the yield strength of the material, $P_g$ is the gauge pressure, $r_i$ is the inner radius of the sphere, and $t$ is the thickness of the sphere wall.

You can use any consistent set of units. That includes SI.

But if you try to use a pressure in ATM, a sphere radius in feet, a wall thickness in mils, and want the stress in tons per square inch, you will probably get the wrong answer that you deserve!

• Unless you use the disgusting conversion factor that would go along with it!
– JMac
Jan 18, 2017 at 17:33

Yield should be in MPa, Pressure Pa, and R in mm t is thickness therefor mm

• Any justification or sources for why those units must be used? I'm quite confident that just because t is thickness it doesn't need to be mm.
– JMac
Jan 19, 2017 at 11:55