Excel - Semi Log Plot

[PetroGalZ]

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 + m?. 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

 

[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IRstuff]

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

 

[GregLocock]

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

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IDS]

=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/

 

[GregLocock]

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


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[joerd]

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 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Ah, that's it. Caught me out once.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.