3 replies to this topic

[MrAnderson]

Hi Guys,

So I was running through the NCEES practice problems and one of them asked to use the "critical oxygen deficit" equation for the Streeter-Phelps Dissolved oxygen sag curve. I was using Goswami's equation:

t_c = (1/k_r-k_d) x ln[(k_dL_o-k_rD_o+k_dD_o)/k_dL_o) x (k_r/k_d)].

I ended up getting the wrong answer, as it turns out (from the solution, google, and other sources) the equation they used was:

t_c = (1/k_r-k_d) x ln[(1-(D_o(k_r-k_d)/k_dL_o) x (k_r/k_d)]

This is very frustrating, there are so many little mistakes in the Goswami book that I find myself wasting my time cross checking it's validity. Which equation should I use when it comes time for the exam?

Edited by MrAnderson, 21 October 2011 - 09:33 PM.

[civilized_naah]

How are those 2 equations different?

[civilized_naah]

MrAnderson,
I would really like to know which one of those equations is the correct one. I can't spot the difference!

[thebettersmith]

Mr Anderson.. the 2nd equation simply factors the 1st equation, sir.

Mr. Smith