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Excel - Semi Log Plot

Excel - Semi Log Plot

Excel - Semi Log Plot

(OP)
I am having some difficulty plotting permeability and porosity data in excel to find the slope and intercept as a common not a natural log. Where do I find the equation for this common log?  Log(k) = b + mǿ. How do I plot a trendline for a common log? The slope should be in the form %¯¹ (1/percent). The data is given below.

Porosity (%)    Permeability (mD)
2             0.13
4.1             0.87
4.2             0.6
4.9             0.23
5.2             0.17
6             0.44
6             0.27
6             0.3
6.1             0.5
6.1             0.77
6.5             0.8
6.7             2.01
7.3             3.05
7.7                 1.11
7.8             1.47
8.3             0.8
9.2             3.62
9.5             2.71
10             4.55
11.1             15.2
11.7             11.19
12.5             11.08
13.4             77.09
13.9             14.67
14.3             21.1
15             80.44
15.3             61.52
15.7             58.2
15.9             38.88
16.3             157.26
16.5             64.73
16.8             223.71
17.2             96.75
17.5             230.07
17.8             567.03
18.3             117.04
18.8             141.93
20.1             174.85
21.2             233.48
21.7             3209.84
21.9             526.18
 

RE: Excel - Semi Log Plot

use LOG, to take the log base e, of the y axis measurement and then treat it as a first order fit.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Excel - Semi Log Plot

To convert from ln to log, you simply divide ln(x) by ln(10):

exp(ln(x) = 10^log(x) = exp(ln(10)^log(x) = exp(ln(10)*log(x))

--> ln(x) = ln(10)*log(x)
--> log(x) = ln(x)/ln(10)

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Excel - Semi Log Plot

LOG allows you to specify the base as the second argument - cute but practically useless

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Excel - Semi Log Plot

=LOG(x)
=LOG(x,10)
and
=LOG10(x)

will all return the same number (log x to the base 10)

I don't see why allowing the base to be entered as the second argument is practically useless.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
 

RE: Excel - Semi Log Plot

Long story short - because I didn't know that LOG(x) defaulted to LOG10(x)!
  

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Excel - Semi Log Plot

To add to this: if you ever decide to try VBA, be aware that LOG is the natural logarithm in Visual Basic.

Cheers,
Joerd

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

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