# Thread: Need help with math problem

1. Thanks again Andy. I got it now. The reason for my questions was that I was trying to figure out how the different ductulator formulas work so I could use my scientific calculator to do them, or plug them into an Excel spreadsheet application. Somebody else here was asking about the formulas a while back, but nobody knew them. I found a formula that calculates the pressure drop per 100' when the cfm and duct diameter are known:

pd = (.109136 x CFM^1.9)/dia^5.02

But I wanted to know how to find the diameter when CFM and pd are known. With your help, I've got it now. My \$8.99 Casio scientific calculator won't allow me to use a decimal point in a fractional exponent, so I couldn't do ^1/5.02. So I just changed it from a fraction to a decimal equivalent of .1992

dia = (.109136 x CFM^1.9/pd)^.1992

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I guarantee you this will seperate the Dems from the Repubs right quick. We will always have the thinkers vs. the doers.

3. Originally posted by midhvac
But I wanted to know how to find the diameter when CFM and pd are known. With your help, I've got it now. My \$8.99 Casio scientific calculator won't allow me to use a decimal point in a fractional exponent, so I couldn't do ^1/5.02. So I just changed it from a fraction to a decimal equivalent of .1992

dia = (.109136 x CFM^1.9/pd)^.1992

You've got the idea.

If I'm reading your equation correctly (I really should look it up), you can get precise answers doing the following on your Casio:

calculate A:

A = CFM^1.9

next B:

B = .109136 x A /pd

next C:

C = LN(B) this is the natural log of B (I'm assuming the Casio has log functions)

next D:

D = C / 5.02

finally E:

E = exp(D) this is the e^x function which is inverse to the natural log

4. Originally posted by glennwith2ns
Originally posted by mattm
You big bunch of math nerds.
Yea , bet they got pocket protectors too>)
And only the best pocket protectors... We're still trying to figure out where to put that Parker name on them...

5. Originally posted by Andy Schoen
Originally posted by glennwith2ns
Originally posted by mattm
You big bunch of math nerds.
Yea , bet they got pocket protectors too>)
And only the best pocket protectors... We're still trying to figure out where to put that Parker name on them...
Do me a favor-------------DON'T put it on there.

6. Originally posted by Steve Wiggins
I guarantee you this will seperate the Dems from the Repubs right quick. We will always have the thinkers vs. the doers.
Are you lost again, buttcheek?

7. Originally posted by ac rookie
I think I am going into another field. I will never figure this stuff out!!!
It's not hard. Actually, to accomplish what Mid wants to accomplish, you spend fifty bucks on a ductulator that does this FOR you...

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