You may be wondering why I have been blogging so much lately. I have been studying a lot more (I am currently at 82.25 hours), and my creative outlets – this blog and my projects – have kept me sane. During my first test cycle last April, I would stay up late studying, then crochet until midnight while watching Forensic Files. I’m trying to have healthier outlets this time around.
The biggest challenge of this exam is not being thrown off by extraneous information or slightly unconventional units given to you in the problem statement. (Note I did not say tricky, because there are furious discussions on engineering boards about how the test isn’t TRICKY if you’re a good ENGINEER and you know how to DETERMINE what information is USEFUL, so stop WHINING about the DIFFICULTY. Can it not be tricky BUT fairly testing your skills as an engineer? I digress). For example –
(and if you’re like SHUT UP ABOUT YOUR TEST ALREADY, I DON’T CARE, you can ~skip it, skip it~)
I am studying about air stripping, which is a technique used to remove volatile organic compounds in groundwater or wastewater. By using this technique, you can transfer volatiles from the water into an air stream. An equation representing the relationship between the flow rate of water with the flow rate of air is:
Qw = QaH’, in which Qw = the flow rate of water, Qa = the flow rate of air, and H’ is the dimensionless Henry’s Law constant (the governing principle behind this technique).
I like this equation because it’s simple and has just a few variables. I was working on a problem set on Friday where I was supposed to find the flow rate of water, given the flow rate of air and the Henry’s Law constant. Easy. I plugged everything in, but paused at the units. The Henry’s Law constant I was given was not dimensionless, so my answer had units of cubic meters per second times atm*L/mol. Hmmm. This didn’t seem right.
I peeked at the answer and saw this equation used in the solution:
Qw = QaH’/RT
WHERE THE HELL DID THIS COME FROM? R is the universal gas constant and T is temperature…was I supposed to be using the Ideal Gas Law with this solution? BUT WHY?
Cue thirty minutes of furiously going through all my references to figure out where this equation came from. Finally, I found this:
Oh – so to get a dimensionless constant for my formula, I just need to divide by R and T to make it unitless. WHY DIDN’T YOU SAY SO?
And this highlights the most frustrating aspect of studying for this exam – going down rabbit holes and figuring out why certain equations are used in the place of others. Now that I know WHY this equation is used, I know I won’t miss a similar problem on the exam. But had I not studied this, I would have stared at this problem on the exam for 15 minutes. Then I would have run out of time and would have been forced to circle “B” for the last ten problems. (You think I’m joking, but this happened on both exam attempts. Hence why I’m on my third try YAYYYYYY).
Ok, I should probably go study for real.
Categories: PE Exam Grumpiness