Fall of potential - how to calculate reference distance manually

Prototyp_V1.0

Advanced Member level 2
Hi.

When doing measurements of earth resistance using fall of potential method, the normal procedure is to feed all results into an app. This app will generate a curve, based on the measurements and then it will output a reference distance in meters and also an estimated resistance for that particular distance, and it's supposed to also be actually measured at the reference distance (the measured results are normally very close to the estimated).

This is all good. But the problem is : Is it possible to manually calculate the reference distance, either by hand or by using a spread sheet ?

I have being reading on Megger web site (that is the instrument my company uses) but none of the online lectures does explain this in depth.

I have taken the time of making a PDF where I have two actual measurements. Then I've put onto some txt and arrows to explain.

Attachments

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Super Moderator
Staff member
Do you wish to find a formula? Something like meters x 0.3 to the power of 5 equals so many ohms? I use that as an example because your graphs appear to have an exponential relationship of distance to ohms.

Or do you wish to create an equation which takes into account the geology of the terrain... water table, rock structure, proximity to a river, compactness of soil, how long ago since rain fell, etc?

Prototyp_V1.0

Advanced Member level 2
The distance in meters are the unknown. That is what the app is magically calculating of itself, but the methods are unknown.

Have you looked in the atached pdf file?

Super Moderator
Staff member
Yes, your pdf has two graphs.

Are you asking how to interpolate an XY position on the straight line between two data points where you marked it?

High school math is able to derive it. Find the proportion of its distance between the two X values, then calculate the number which is the same proportion between the Y values. (Or else start with the Y position to find the X position).

Prototyp_V1.0

Advanced Member level 2
Are you asking how to interpolate an XY position on the straight line between two data points where you marked it?
The X position, yes - that is the value I want to find. Once I know that, the Y is easy calculated.

wwfeldman

Advanced Member level 3
what is the criteria for picking the distance you want to calculate?

since the goal seems to be to find the distance from the starting point, (malepunkft),
what characteristic of the resistance or the shape of the curve, or ...? are you looking for?

Prototyp_V1.0

Advanced Member level 2
what is the criteria for picking the distance you want to calculate?
From what I can remember (this is like 2 or 3 years ago, so I should probably try to ask again) - it is to find the flattest point on the curve - and also the bending on the curve must meet certain criteria. But I've never get that explained in depth. The explanation I get from those that is having course is "u just use the app".