Hi Ed,
Thanks very much for replying to my questions. Your answers
cleared up all my confusion. The method you are using is so
different from the method we normally use that the great variance
between your findings about conductivity of concrete and what we
normally measure is easily explained.
While your measurements are not going to correlate with ones made
using more rigorous methods, they are nonetheless interesting.
And they will show what can be expected when using a VOM to make
this measurement. Please keep posting your results untill there
don't seem to be any changes.
Please see specific comments interspersed in the previous
message(s) below.
To: <towertalk@contesting.com>
>Date: Thu, 15 Oct 1998 00:27:31 -0400
>From: "Edward W. Sleight" <k4sb@mindspring.com>
>
>Eric Gustafson wrote:
>
>>Hi Ed,
>>
>>Your post raises a number of questions for me. There isn't
>>enough information to make sense of your findings. So that we
>>can interperet your results, can you answer the following
>>questions for us?
>>
>> 1. How did you make this measurement?
>
>The measurement was made by placing one lead in the center of
>the concrete block, with the other connected to an 8' ground rod
>6' away.
>
>
>
>> 2. What kind of measuring device was used?
>
>A Fluke Model 7 Volt\Ohm meter.
This was the main source of difference. A DC based measurement
of the conductivity of a fairly complex electrolyte system (one
electrolyte solid, another in series with it semi-solid) will
yield results which are time variant, and not very repeatable.
You should notice that the measurement changes over the entire
time you leave the VOM (or DVM) connected. Until the system
becomes fully polarized. There may be enough leakage to prevent
complete polarization. But there are a number of variables at
work here and the bottom line is that measurements taken this way
will not be particularly repeatable and will not bear much
relation to the true value of the conductivity of the system for
AC currents or very short transient DC currents. Reversing the
DC measurement polarity and doing an average will not improve the
repeatability or accuracy of this method.
The meter we normally use for this and ground electrode
resistance measurements generates its own AC signal. The
frequency is fairly high - up around 800 Hz I think - and chosen
to not be an integer multiple of either 50 or 60 Hz. Anyow, the
use of AC prevents polarizing the electrolyte. It is made by
Sperry. I have seen others by Biddle and Honeywell. They all
work essentially the same way. Moderate frequency AC for
electrolyte conductivity measurements and three terminal fall of
potential method to get the resistance of the earth terminal
connection to earth (also an AC measurement). Most industrial
equipment rental places will rent them for reasonable rates.
>
>
>
>> 3. How was the connection to the center of the base made?
>
>Pressure applied to the lead.
This is unlikely to produce a low enough impedance connection
into the concrete to permit evaluation of the bulk conductivity
of the concrete. The right way to make this connection is to
connect to a conductor that was immersed in the concrete when it
was poured. Is there a J-bolt or pier pin or tower section
sticking out of the base that you could connect to? This would
give you a much better connection into the solid electrolyte.
In fact, to evaluate the conductivity of the concrete alone would
require two such electrodes that were in the pour but not
attached to any conductive reinforcing material. Two free
floating J-bolts would do nicely for this.
>
>
>
>> 4. What is the measured electrode connection resistance to
>> earth for the rod?
>
>Not sure what you're asking here, but from the earth to the rod
>is about .2 ohms
I think that the combination of the earth electrolyte and the
differing material of the ground rod and your meter probe is
making a weak battery that is causing your DC meter to lie to you
about this one. It is extremely unlikely that the resistance to
earth of a single 8 foot rod is much less than 10 ohms unless the
soil is very fine grained, uniform to a depth of 8 feet and
_highly_ conductive. If all that is true, you are indeed a lucky
man. Particularly if these conditions hold for more than a few
wavelength radius around the tower.
Does the resistance measure the same when the meter probes are
reversed?
>
>
>
>> 5. Does the concrete contact the earth directly or does the
>> hole have some kind of liner (wood form, plastic sheet,
>> etc.)?
>
>Concrete connects to direct earth, however, a wood form of 2x10s
>is still in place. Thinking about removing it tomorrow to expose
>the sides. One exception, I poured 4 inches of concrete into the
>bottom before the main pour to make sure the rebar did not touch
>earth.
The form is probably not a big factor.
>
>
>
>> 6. What kind of earth are we talking about?
>
>Let me put it this way, solid rock hard clay. As a matter of
>fact, it took 3 hours for a 14" bucket backhoe to dig the first 3
>feet. It finally became a matter of simply using the teeth and
>scratching. As an aside, I had to dig a slanted hole for one of
>the guys to a 25G. I used a pickax, post hole digger, 1" wood bit
>in an 1/2" drill, and a piece of steel rod to dig it. The clay is
>so hard it would stall the drill. It took me 4 hours to dig a 4'
>hole in this way. The hollow steel pipe was the best, drive it
>into the ground for a few inches with a full sized sledge hammer,
>beat it loose, and then drive the dirt out of the center with a
>piece of aluminum tubing. There is very little doubt in my mind
>that if I could have screwed the base bolts into the ground,
>concrete would not have been necessary. You can swing a pick ax
>as hard as you can and it will penetrate about 3".
Sounds like you had quite a lot of fun. Our earth is just as
much of a pain in the @#$ to work with but for different reasons
(rocks and caliche clay).
>
>
>
>
>> 7. Did you make a similar measurement between two of the
>> (otherwise unconnected) ground rods?
>
I was trying to get you to measure the resistance between two
rods of identical material. It should avoid the effects of the
differing position in the electrochemical series of the metal in
the probes and the metal in the rods. But it should still show
time variance due to electrolyte polarization.
>Not yesterday, but did today. Another test at 6pm this evening
>(concrete now 54 hours old, made an error on time in the first
>post , the concrete was 30 hours old, not 36) and resistance
>today varied between 950K to 970K between a common center point
>and all 4 ground rods. That's a reduction in resistance of about
>500K ohms in one day. Weird...
>
>More tomorrow, but please remember this is merely an observation
>over time and no opinion is implied.
As I said above, keep 'em coming. I'm interested to see how long
it takes for the readings to stop changing.
73, Eric N7CL
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|