[TowerTalk] Fwd: Tower base/Ufer ground

[Roger (K8RI) on TT]

On 5/18/2014 12:44 AM, Jim Lux wrote:
> 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..

Not if you do the math right.

>
> Resistivity is in ohm-m, conductivity in 1/(ohm-m) or S/m  (since S is 
> 1/ohms)
Resistivity is not ohms per m, it's usually given as ohms per cubic 
centimeter.

Resistance usually is dimensionless, unless it's bulk and then it's so 
many ohms per unit length

You are confusing resistance with resistivity. You are losing a unit 
somewhere because resistivity is ohms per cubic meter or more often ohms 
per cubic centimeter.  Look up the dictionary definition. Resistance is 
between two points. Resistivity takes into account the volume as the 
units are .
Resistivity is in ohms per cubic centimeter. I worked with developing 
standards for liquids and circular solids With a Silicon production 
company for many years.
  The dictionary definition: for resistivity takes into account both 
area and length. IE: volume as in resistivity ohms ^3 cm.  Had a devil 
of a time getting the techs to stop confusing the two.  The easiest was 
to give them a probe consisting of 2, 1cm square plates, 1 cm apart, 
then thy had no conversions.



> Resistance = Length * resistivity/cross sectional area.

No. Again that is resistivity

> 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
>
>

Conductivity is the reciprocal of resistivity, not resistance. Please 
stop working the formulas and just look up the definitions

73

Roger  (K8RI)