[TowerTalk] Fwd: Tower base/Ufer ground

[Jim Lux]

On 5/17/14, 9:13 PM, Roger (K8RI) on TT wrote:
> On 5/17/2014 11:27 PM, Jim Lux wrote:
>> On 5/17/14, 6:11 PM, Drax Felton wrote:
>>> What is the conductance per inch of concrete in mho's ?
>>>
>>>
>> http://www.k1ttt.net/technote/concrete.html
>> 10-50 mS/m
>
> Shouldn't the medium be in cubic units?


No.. it works out right if you do the dimensional analysis..

Resistivity is in ohm-m, conductivity in 1/(ohm-m) or S/m  (since S is 
1/ohms)

Resistance = Length * resistivity/cross sectional area
so to get ohms as resistance, you need resistivity to be ohms * length 
(because Length/area is 1/length)

Likewise, conductivity is = Area * conductivity/length, so it needs to 
be Siemens/meter



>
> Resistivity, the inverse of conductivity is in cubic units.

nope.. see above.. A bar of something that is twice as long has twice 
the resistance.  The resistance is inversely proportional to the cross 
sectional area.

> Unfortunately the "new" Si units are no longer meaningful and require
> conversions.

Siemens is just the new name for mho, which has been around for a long time.

Yes, converting various units gets tricky when there's lengths 
involved.. Trying to convert a resistivity given in Ohm-cm to a 
conductivity in mS/m for instance, you have to make sure you get the 
right powers of ten.


It's like surface resistivity, which is given in ohms per square, 
because it turns out the length unit is unimportant.

A sheet of material that is 1x1 cm has exactly the same resistance as a 
sheet that is 1 m x 1m (it's 100 times as long, but also 100 times wider)



> Typically Resistivity is measured in CC as would Siemens(mho's), but
> much depends on the size of the samples. Measuring CC is a whale of a
> lot easier than cubic meters (the probes are a whole lot easier to make
> and use
>
> 73
>
> Roger (K8RI)
>
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