I drank the koolaid!

[Leejnd]

Yes, dimensional analysis is always my sanity check in physics. If the units cancel, you've probably done the calculations right!

Okay, now I'm starting to sweat! Give me a dangling participle or a split infinitive, I've got you covered. But dimensional analysis?? Hell, I stopped helping my kids with their math homework in the 5th grade!

I'm actually doing okay...for now. Not done yet. I knew the math would be the hardest part. I just haven't had a need to do complicated math formulas in so many years, my brain seems to have atrophied in that region. But Steve reminded me to just keep telling myself I CAN do it, and then go do it. I'm just taking it one step at a time. I'm through basic Gas Management, moving on to Gas Management with Dissimilar Tanks.

I do have another question: I bought wet notes, and there are things I need to write in them (GUE EDGE etc.) before Saturday. Steve mentioned something about a special pencil - do I need to buy a special pencil? Or can I write on them with any pencil?

 

[Blackwood]

Okay, now I'm starting to sweat! Give me a dangling participle or a split infinitive, I've got you covered. But dimensional analysis?? Hell, I stopped helping my kids with their math homework in the 5th grade!

Sounds a lot scarier than it is. Dimensions are things we measure. In construction, length, width and height may be common dimensions. In diving, it's pressures and volumes and depths (etc).

What Lynne is getting as is this:

Say I want to know the volume in a tank at a given pressure.

My dimensions are CF and PSI. I also happen to know the tank factor, which is the ratio of rated volume to rated pressure (sometimes *100). So tank factor has the units of CF/PSI (or CF/100PSI if you like to do it that way).

So if I know PSI and I know CF/PSI, just by analyzing the dimensions, I can figure out how to get CF.

CF/PSI * PSI = CF.

But the real power is in finding mistakes. Sure, I can say 2600/.026=100000, but that's asking for trouble. Better to say 2600PSI / (.026CF/PSI) = 100000PSI^2/CF, because now I know I did something wrong. I should have CF, not PSI^2/CF. Oh, I should be multiplying TF and pressure, not dividing pressure by TF. That would give me CF. A'ha.

That's dimensional analysis. Nothing with the space-time continuum.




(if you're working with metric units, it's L/BAR rather than CF/PSI, but the math steps are the same)

 

[Rainer]

Okay, now I'm starting to sweat! Give me a dangling participle or a split infinitive, I've got you covered. But dimensional analysis?? Hell, I stopped helping my kids with their math homework in the 5th grade!

I'm actually doing okay...for now. Not done yet. I knew the math would be the hardest part. I just haven't had a need to do complicated math formulas in so many years, my brain seems to have atrophied in that region. But Steve reminded me to just keep telling myself I CAN do it, and then go do it. I'm just taking it one step at a time. I'm through basic Gas Management, moving on to Gas Management with Dissimilar Tanks.

I do have another question: I bought wet notes, and there are things I need to write in them (GUE EDGE etc.) before Saturday. Steve mentioned something about a special pencil - do I need to buy a special pencil? Or can I write on them with any pencil?

The math gets very easy with practice (there are only so many things to calculate before a typical dive). Just stick with it.

You can basically use any pencil with your wetnotes. Ideally, you don't want anything that has metal, which will rust over time. Pencils that must be sharpened are also dispreferred, since it's unlikely you'll be bringing a sharpener along for the dive. My personal favorite is the Pilot Croquis, but they're a pain to find (I just order them online). Having a spare isn't a bad idea in case (i.e. when) you eventually drop one.

 

[TSandM]

Use a soft lead -- #2 pencils seem to be too hard to make good, thick lines on wetnote paper.

For things you are going to use over and over again, use permanent marker. I have tank factors for all the common tanks in a table in my wetnotes in permanent marker.

 

[Leejnd]

That's dimensional analysis. Nothing with the space-time continuum.

OH NO...at what point are they going to introduce the space-time continuum calculations????

Just kidding of course. :blinking:Thanks for the above - I'm going to take some time to study it to see if it sinks in.

Having a spare isn't a bad idea in case (i.e. when) you eventually drop one.

Now THAT's funny - and I'm sure completely true!

Okay...back to the math...:loopy:

 

[Leejnd]

FYI, re the pencil - Steve recommended this:

Pilot Croquis 6B
<http://www.jetpens.com/product_info.php/products_id/3352>

I just ordered a few.

Lynn, I like your idea of using the permanent marker for the things I'm going to want to leave in there. I'll do that.

 

[Leejnd]

Okay, sorry for the multiple posts...

I'm going to ask for some help now because you guys have all be AWESOME and frankly I'm stumped. And Tom is too, so we're both asking for help.

In the Gas Management worksheet, the last two exercises read as follows:

4. If a diver must return to the ascent line, what is the minimum pressure that his/her twin LP104's need to be filled on order to do a 20 minute dive (20 minutes out and 20 minutes to return) at an average depth of 90ft? [Assume a SCR of .75 ft3/min.]

5. If a diver prefers to return to the ascent line, what is the minimum pressure that his/her twin AL80' need to be filled in order to do a 25 minute dive (25 minutes out and 25 minutes to return) at an average depth of 70 ft? [Assume a SCR of .75 ft3/min.]

Tom and I are stumped. Note that both say a total duration of the dive (20 for the first, 25 for the second), but they use that same number for *out* and *back*, which would mean a 40 minute dive, and a 50 minute dive!

I have to assume that this is another typo - but which way should we go? Should it be a 20 minute dive, or a 40 minute dive?

 

[ScubaFeenD]

Okay, sorry for the multiple posts...

I'm going to ask for some help now because you guys have all be AWESOME and frankly I'm stumped. And Tom is too, so we're both asking for help.

In the Gas Management worksheet, the last two exercises read as follows:

4. If a diver must return to the ascent line, what is the minimum pressure that his/her twin LP104's need to be filled on order to do a 20 minute dive (20 minutes out and 20 minutes to return) at an average depth of 90ft? [Assume a SCR of .75 ft3/min.]

5. If a diver prefers to return to the ascent line, what is the minimum pressure that his/her twin AL80' need to be filled in order to do a 25 minute dive (25 minutes out and 25 minutes to return) at an average depth of 70 ft? [Assume a SCR of .75 ft3/min.]

Tom and I are stumped. Note that both say a total duration of the dive (20 for the first, 25 for the second), but they use that same number for *out* and *back*, which would mean a 40 minute dive, and a 50 minute dive!

I have to assume that this is another typo - but which way should we go? Should it be a 20 minute dive, or a 40 minute dive?

Not that we cant help you here, but why dont you ask your instructor? It seems you are paying him a handsom sum for education, and these worksheets are kind of important for your education. Plus, then he can help address deficiencies in your understanding by posing even more questions for you to think through.

 

[Leejnd]

Tom did already. He wasn't sure, and said he'd get back to us, which I'm sure he will.

We were just hoping somebody in here has gone through the class recently and might have encountered this, and knew the answer off the cuff.

I can leave these two for now and move on until I get that cleared up.

 

[Rainer]

I read them as 40 and 50 minute (total) dives, respectively.

Why not just run the calculations for both (as practice) and then put down whichever answer the instructor wants come class time?

Okay, sorry for the multiple posts...

I'm going to ask for some help now because you guys have all be AWESOME and frankly I'm stumped. And Tom is too, so we're both asking for help.

In the Gas Management worksheet, the last two exercises read as follows:

4. If a diver must return to the ascent line, what is the minimum pressure that his/her twin LP104's need to be filled on order to do a 20 minute dive (20 minutes out and 20 minutes to return) at an average depth of 90ft? [Assume a SCR of .75 ft3/min.]

5. If a diver prefers to return to the ascent line, what is the minimum pressure that his/her twin AL80' need to be filled in order to do a 25 minute dive (25 minutes out and 25 minutes to return) at an average depth of 70 ft? [Assume a SCR of .75 ft3/min.]

Tom and I are stumped. Note that both say a total duration of the dive (20 for the first, 25 for the second), but they use that same number for *out* and *back*, which would mean a 40 minute dive, and a 50 minute dive!

I have to assume that this is another typo - but which way should we go? Should it be a 20 minute dive, or a 40 minute dive?