Appendix G

Appendix  G     Accuracy of Resistivity Measurements


The following is a test performed on MiniRes Serial Number 11.


Various precision resistors were used to derive an estimate of the accuracy of the MINIRES measurements.  Some of the resistors used were 0.01% accuracy and, therefore, are more accurate than the Fluke is able to measure. 


The last resistor measured, a 16.5 Kohm 0.1%, read 16507 on the Fluke, but read 16468 on the MINIRES.  The input impedance of the MINIRES has a magnitude at 30.0 Hertz of about 6.7 Megohms, even though the input resistance is 20 Megohms.  The reason that the MINIRES is reading abnornally low on the higher values is because of the loading effect of the 6.7 Megohm input impedance.  The calculation of the parallel combination of 16507 ohms and 6.7 Megohms is 16466 which clearly explains the reason for the small error on the higher resistance measurements.  Every earth resistivity meter has this loading effect, most have an input resistance of 10 Megohms.  Since the MINIRES has 20 Megohms of input resistance it is very likely that the MINIRES is less effected by this loading effect than other earth resistivity meters. (Serial No. 15 and above have 200 Megohms input resistance.)



Type of                        MINIRES          MINIRES          IP                     Fluke               "TRUE"

Resistor                      READING        READING        READING        READING        VRS.

                                    200 Ohm         2 Kohm                                                           MINIRES

                                    Range             Range                                                             READING


200 Ohm .1%            199.92            200                 -.03                 199.9              0.06%


100 Ohm .1%            99.99              100                 .00                   99.98              0.01%


5 Ohm 1.0%                 4.98              5                      .01                   4.95                0.6%


300 Ohm .01%            N/A                300                 0 to +1            300.0              0%


1000 Ohm .01%          N/A                999                 +2                    1000.1            0.1%


2000 Ohm .01%          N/A                1998               +3 to +4          2000               0.1%


16.5 Kohm .1%           N/A                16468             +22                 16507             0.2% see note above  


All of these results fall within the accuracy specification of the MINIRES.  The large error on the IP illustrates a problem that naturally arises when IP data is acquired in areas of high ground resistivity.  Much more attention to field procedure details should accompany IP surveys with any IP instrument compared with basic resistivity measurements.


                                                                                                                        27 Sept  98  -  BTA

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