Soil Resistivity

http://en.wikipedia.org/wiki/Resistivity

Depends on many factors the major being the water content. Years ago i had to set up a system for water content measuring for balsa wood in a boat manufacturing shop. It was very involved.

What more conductivity? Pee on it.

Ohmmeter on the cube? That brings up some complicated cubistic images, with no clear resolution.

No, we measure, more or less linearly between 2 stakes (4, including the exciting voltage's), then use math to figure, how a meter cube would behave.

The sheet or bulk resistance was defined over a hundred years ago. they are solid.

Reciprocal of resistivity. 1/R like.

That's right

Resistance is in fact measured for surface area or cross section and length of the conductor. Greater surface area decreases the resistance and greater length increases the resistance. You can call it proportional to the surface area and inversely proportional to the length.

Once you have a cure, which means volume and it defines uniform dimensions of easy definition. Cube has all sides equal and it needs only one arm to be defined. 1mx1mx1m or 1 cubic meter is for standard value and that does not mean you need to actually have 1 cubic meter to be measured. You may use 1 cubic cm or even 1 cubic mm.

Soil resistivity is not all that simple and stable parameter. it varies with applied voltage as it is ionic conductivity that you measure and whether you do AC or DC measurement and at what voltage and at what frequency, all that matters.

Learn about ionic conductivity first to know soil resistivity.

What I can not get is why there is a .m (meter) in this unit. I can understand if it is only ohm since we might accept that a cubic meter soil has 100 ohm then. I could not understand what you proposed...

i always foud C·cm-2 confusing (*) . . . here we have electric charge density (qubicssss)
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^{* -- as to transfer to Max Amp's rating for cbles -- too long ago (maybe it'll make sense now)}

. . . "continue what you where doing"

Suppose you have a cubic metre which measures 100 Ohms from one face to the opposite face. Now stack up 8 cubes in a 2x2x2 cube and measure the resistance: it has doubled because the distance has doubled, but, it has divided by 4 because the area is four times as great. The result 50 Ohms.

Chop the original cube into 27 cubes and each has a resistance of 300 Ohms.

If you were talking about a bit of wire with consistent cross section you would say it had a resistance of x Ohms PER metre, but, when you're talking about a solid with variable cross sectional area and length you have to describe the resistance as x Ohm metres: the resistance is proportional to the length divided by the area.

Try calculating the resistance of a block of soil which is NxMxL metres.

Randall:

Good analogy!

However it not so simple when it comes to bulk volume which is interconnected and there is surface leakages. Like touch screen and then similar small area probes inserted in soil that make it somewhat difficult.

How soil resistivity is actually measured by different types of probes and finally computed to near accurate value using some formula can also be projected here being useful information. Soil moisture, soil density are also some valuable parameters.

The answer lies within the Wikipedia page #1↑.

There is such a device to measure soil resistivity for grounding....

http://www.esgroundingsolutions.com/about-electrical-grounding/what-is-soil-resistivity-testing.php

I've used this same type of testing for generator grounds in the desert.

Is there a language barrier here? There is more than one definition of meter in the English language. One is a measurement and one is an instrument.

One is spelt metre the other meter.

Perhaps that's the problem. Australia and America differ and if The spelling is not the same in every english speaking country. How would someone from a non english country know?

Hi jinxnao,

if you take a cubic sample of some material and press it between two square metal plates (same size as one face), the electrical resistance you measure between plates will be directly proportional to the distance between plates (say L) and inversely proportional to the face area (in this case L x L). So one L is cancelled and the proportionality constant (which is resistivity in our case) will be ohms times meters (not square meters or cubic meters).

You can perhaps cut the material to size or join the blocks to increase size but can't compress as that will change the material itself and its electrical properties also.

Do you agree the formula for Resistance (i.e.) R = Rho * (L/A)?

From the above formula, can you deduce that Rho = {(R * A) / L}?

Do you agree that Unit of Resistance is Ohm, that of Area (A) is in sq.m and that of Length (L) in m?

Now, do unit balancing in the above formula for Rho:

Rho = (Ohm * sq.m / m)

or

Rho = Ohm-m.

Got it?

From that old For-What-Its-Worth department
Shyam hit the nail on the proverbial head, and, corbinstein's link to the Wenner 4-Pin method of testing (soil resistivity) was an excellent "Pointer".

Additionally, the concept of soil (or, electrolyte) resistivity measurement comes into play (as a very important factor) in the engineering of Cathodic Protection Systems (e.g. for buried or submerged pipelines).

In this field , such measurements are usually stated in ohm-cm (ohm-centimeters); as in NACE's TM0497 (Standard Test Method...Underground or Submerged Metallic Piping Systems) or RP0176 (Standard Recommended Practice: Corrosion Control of Steel Fixed Offshore Platforms Associated with Petroleum Production) ... standard seawater as in Gulf of Mexico given as 20-25 ohm-cm.

Besides the Wenner method, there are "Soil Resistivity Boxes", whose dimensions are such that (when used properly), can provide a "direct-conversion" (correlation) between resistance as read on an ohm-meter / multimeter and (the soil's) resistivity.

<link>

Interesting one. I will like to try these for education and training as they look very easy to construct and easy to use by the students. However, it does require filling of soil and that will change the density of the soil.

"Absolutely correct" (with regard to 'compaction' issue).... Also note that when using to measure seawater, etc, the liquid in the box should be kept at the same temperature as the water from which the sample was drawn (for 'absolute' precision/accuracy, "good science"...)

_{Click on the image to see a supplier...}

Temperature and Pressure both unless you measure at sea level. Sea layers have different salinity and temperature profiles and perhaps it is good to directly measure in the sea itself. It is not difficult to confine water in a standard cavity using specially designed probe.

I think this idea may be worth trying to design a special probe for sea layers measurement in real time and transmission by wireless for data collection, perhaps from a floating Buoy. http://en.wikipedia.org/wiki/Buoy

I have seen somewhere that soil juice (chemical seepage) is collected from drill hole into a ceramic cup for analysis. I liked this idea as it is worth knowing the cause of conductivity or resistivity rather than just a physical parameter.

I am working on capacitive sensor and new results are loaded here on this CR4 link. These results are interesting because I am measuring AC current of kHz range (no DC) through capacitor where dielectric media is water.

http://cr4.globalspec.com/thread/84531#comment952180