Quick Start

Quick Start  -  Taking a Typical Field Reading

 

Four electrodes are used for field readings. They are placed along a straight line, usually at uniform intervals. The instrument is placed near the middle of the line. The far two electrodes are used to put electric current into the earth. The middle two electrodes are used to measure the voltage in the earth. The far electrodes are connected to the MiniRes with cables. The cables are connected to the C+ and the C- binding posts on the right side of the MiniRes. The middle two electrodes are connected to the P+ and the P- binding posts on the left side of the MiniRes. Be sure the C+ and P+ cables go to the electrodes on the same side of the MiniRes.

 

Field operations with cables can be hazardous. Please read the warnings in Appendix A

 

Selection of cables, placement of cables and care of cables is very important to obtaining good field data. Please read Appendix E for details about cables and electrodes.

 

To take a reading of earth resistivity, be sure that people are clear of the cables and electrodes then press the RED power button located in the middle of the faceplate. There is no “OFF” switch. The power will remain on for about a half-minute and then automatically turn off. The reading will appear on the digital meter. Record the meter reading, together with the spacing of the electrodes. If the electrodes were spaced at uniform intervals (a) along the line, then the apparent earth resistivity is calculated as follows:

 

                            R(apparent)  =   Ra   =  2   p   a   R  =  6.283  a   R

 

where  R  is the resistance read on the MiniRes. If the units of are feet, then the apparent resistivity is in Ohm-Feet. If the units of a are in meters, then the apparent reisistivity is in Ohm-Meters.   

 

If the LINE OPEN LED is flashing the instrument does not have sufficient voltage to emit a constant current, the best solution is to reduce the transmitter electrode contact resistance by wetting the ground around them or inserting them deeper. If that fails, the RANGE switch can be moved to the HIGH position. On the Super MiniRes this reduces the output current from 10 milliamps to 1 milliamp but increases the available voltage from about 400 to about 730. On the Standard MiniRes it reduces the current from 5 to 0.5 milliamps

 

If the LINE OPEN LED is on steady, it indicates a very high resistance in the transmitter cables or electrodes. Check for a loose connection or broken cable.

 

THE INTELLIGENT RED "ON" BUTTON - All first generation (Standard) MiniRes units have a red ON button near the middle of the faceplate.  When it is pressed the meter turns on for about 45 seconds.  If more time is needed, the button can be pressed again and the meter will be on for another 45 seconds.  Some users commented that 45 seconds was too long to wait if they were obtaining a solid reading in 25 or even 20 or 15 seconds.

 

On the second and subsequent generations (Super and ULTRA) MiniResunits the function of the ON button has been redesigned.  If it is pressed the meter is on for about 45 seconds as before.  However, if you are finished before 45 seconds, press the button again and the unit turns off.  If you know you will need more than 45 seconds, press the button twice.

 

The CRI Circuit (Contact Resistance Indicator)

 

Starting partly through the third generation (ULTRA) MiniRes, the "CRI" circuit was added to the receiver of the MiniRes.  It is somewhat analagous to the "LINE OPEN" circuit for the transmitter cables and electrodes.  It consists of a two-colored LED located near the P+ and P- binding posts.

 

When one of the transmitter (current) cables is broken or one of the transmitter electrodes is not making contact with the earth, the "LINE OPEN" LED comes on.  When one of the receiver (potential) electrodes or the receiver cables is not in contact with the earth, the red LED comes on.  If a little contact is made then the LED turns green.  Finally, if good contact is made with the earth the LED goes out.

 

In quantitative terms:

 

            Red LED                    50K Ohms        to        Infinite resistance

            

            Green LED                 5K Ohms          to        50K Ohms

 

            LED out                      no resistance    to        5K Ohms

 

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For details on the various field methods, read the next sections of the manual. For details on the care of the MiniRes, read that section of this manual. Appendix C has a detailed description of the MiniRes. Examples of resistivity surveys are presented in some of the appendices; for example Appendix M.





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