Appendix C3

 General Description of the ULTRA MINIRES 


The MiniRes geophysical resistivity meter is a highly accurate and precise portable scientific instrument designed to provide years of rugged use while maintaining laboratory levels of stability and accuracy. The instrument is also very easy to use. 

 

The MiniRes is not reliant on a digital microprocessor for operation. This has several advantages over typical microprocessor-based systems. A microprocessor-based system generally relies on a menu where the operator has to work his way up or down the menu system to get where he can control the function of interest. In the MiniRes, every function of the instrument corresponds to an individual switch or indicator on the front panel. Therefore, access to a particular function is instantaneous. Another disadvantage of microprocessor-based systems is that they generate electromagnetic noise. Also, they are susceptible to electromagnetic interference (EMI) and electrostatic discharge (ESD). The MiniRes has a slow (but accurate) system clock that generates very little noise. Since it is synchronous with all other functions of the instrument, it does not interfere with the operation of the instrument.

 

A key characteristic of the MiniRes is the receiver architecture. The receiver utilizes a technique called "synchronous detection". This is an extremely powerful theoretical concept that allows stable and accurate readings to be taken in noisy environments with a minimum of transmitter power. Thus, the MiniRes is able to acquire hundreds of measurements with 4 standard "D" cell alkaline batteries as its sole power source.

 

The "synchronous detection" method has the intrinsic ability to filter out vast amounts of noise, allowing stable readings to be taken in extremely noisy environments. Other electronic architectures use a "brute force" method of generating large amounts of transmitter power. This brute force approach is effective, but suffers two disadvantages. First, the brute force method requires large, heavy batteries that lose power with each measurement and, second, the large transmitter currents demand excellent, low resistivity transmitter electrode "plants". Operators of that equipment regularly carry extra water and salt to douse the transmitter electrodes and, thereby, reduce their resistances to the point that the heavy transmitter currents can pass. The MiniRes would require this only in the most extreme of circumstances. 

 

The MiniRes utilizes some of the latest, most sophisticated solid state electronic components available. Many other resistivity meters use mechanical relays for switching the transmitter and receiver circuitry. The MiniRes has no mechanical relays. It utilizes rugged high voltage solid state components for all receiver and transmitter functions. These provide reliable and stable readings year after year.

 

The power supply for the MiniRes consists of four standard alkaline "D" cells hooked in series providing approximately 6 volts. A set of batteries provides hundreds of readings. There is no need for a battery charger. A "BATTERY LOW" indicator light comes on when the batteries get close to needing replacement. Typically, a few hours of data can be taken even after the "BATTERY LOW" LED illuminates. Also, the operation can be further extended if the "RANGE" switch is set to the HIGH (1.9999 Kohms) Ohm position, since the 1.9999 Kohms position requires less current and less power.

 

The transmitter circuit can generate up to 720 volts, peak-to-peak. This allows the instrument to work in very resistive environments with long transmitter electrode spacing.  

 

Units come standard with a frequency of 2.50 Hertz and there is excellent rejection of both 50 and 60 Hertz AC power noise.  A new 3 position range switch offers settings of 1.9999 Kohms, 19.999 Ohms, and 1999.9 milliOhms.

 

Considerable effort was expended in the design of the MiniRes to provide continuous feedback to the operator of every critical function of the instrument. Hence, an "OVERRANGE" detection indicator light assures the operator that the instrument is working properly within its linear bounds. The "BATTERY LOW" indicator provides an early warning for battery replacement. The "LINE OPEN" indicator signals to the operator that the transmitter, transmitter cables or electrodes are not operating properly. All of these quality control indicators assure consistent and robust measurements.                                        


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