RM85 RESISTANCE METER SYSTEM
The RM85 Resistance Meter forms the heart of a versatile measuring system for rapid area or vertical profile measurements. Targets can be archaeological, environmental, forensic, utility, geological or military. There are two models available: BASIC and ADVANCED. Both models can be used in Probe Mode where conventional probes are inserted into the ground for area mapping or vertical profiling. An optional internal multiplexer card is available that allows the RM85 to control multi-probe arrays such as multi-depth Twin.
The ADVANCED model has a wider range of currents, operating frequencies and higher output voltage to allow operation in more demanding situations such as very dry conditions, noisy locations or where extra sensitivity is required. The half-current option allows the user to optimise signal to noise ratio against probe contact resistance compliance. The ADVANCED model can also be used in Wheel Mode where it is mounted on an MSP25 or MSP40 Mobile Sensor Platform (with spiked wheels in place of the probes) for fast, detailed resistance mapping and, optionally, simultaneous magnetic surveys with the FM256. A real time resistance reading output is available for integration with external wheeled systems. A GPS logging option for the ADVANCED model records GPS position with each reading and provides real time monitoring of signal quality and DOP.
The functionality of the RM85 Resistance Meter can be expanded to include logging of fluxgate gradiometer data by the addition of the FAB1 (Fluxgate Adapter Box 1) and a SENSYS FGM650 fluxgate gradiometer. Measurements can be made with a handheld system or MSP25 cart system. The RM85 then becomes a 3-in-1 instrument able to offer both resistance (probe and wheel mode) and magnetic measurements.
Flash memory is used to store readings: 2745600 for the BASIC model, 5491200 for the ADVANCED model or 164,000 readings with GPS location (sufficient for 2 ha at 0.25m sample interval, alpha and beta measurements made with the MSP25 or MSP40). Data can be downloaded using either a USB or RS232 connection at up to 115200 baud. An expansion port can connect and communicate with external modules. The BASIC model can be upgraded to the ADVANCED model at a later date.
Probe Mode and Multiplexing
When used with PA20 or PA3 probe arrays, the RM85 can log data with arrays such as Twin, Pole-Pole, Double-Dipole, Wenner, Schlumberger, Gradient etc. The optional internal multiplexer card allows the RM85 to automatically configure and log data from multi-probe arrays--the number of measurement lines increases from the standard 4 up to 8. Eight different programs can be defined, each consisting of up to 16 configurations. Readings are logged manually, or automatically by a special current sense circuit - insertion of the probes triggers a measurement or multiplex sequence.
The RM85 has improved noise rejection capability whilst providing much faster speeds compared to an RM15 - survey time can be almost halved for Twin arrays, especially when multiplexed. This is due to changes to the multi-pole measurement filters, a wider range of operating frequencies, a wider range of Auto-Log delay times and the addition of Speed Boost and Insertion Delay settings. As the reading settles Speed Boost logs data at an earlier but predictable part of the waveform. Insertion Delay allows the user to set a time to get all the probes correctly inserted into the ground but then use a fast Auto-Log Delay time for the multiplex steps; this can be useful in dry conditions.
When mounted on an MSP25 or MSP40 Mobile Sensor Platform, resistance data can be collected much faster and at higher spatial resolution than is practical with conventional probes arrays. For example, when the MSP25 is configured as a square array, alpha, beta and gamma and GPS data can be collected at a sample interval of 0.25m at a rate of better than 0.6s/m; alpha, beta and GPS can be collected at 0.3s/m. Wheel mode multiplexing can also be programmed by the user, including Twin measurements. If an FM256 is added to an MSP25 or MSP40, simultaneous gradiometer data can be collected at 1, 2 or 4 times the RM85 sample interval, triggered by the RM85. Logging of readings can be triggered either under timer control or by distance encoder pulses from an encoder wheel. GPS position can also be logged in the RM85 for each reading. When downloaded into Geoplot (new version pending), the GPS referenced data can be converted to local co-ordinate systems, and then sampled and re-gridded for further processing and presentation.
Readings are stored in flash memory using square or rectangular grids with dimensions of 10, 20, 30, 40, 50 or 100m. Sample interval can be set to 1, 2, 4, 8 or 16 readings per metre. The data logger keeps track of, and displays, survey position for zig-zag or parallel traverses. Incorrect readings may be deleted singly, or as complete lines, with one keystroke. A dummy reading or line of dummies can be inserted in place of physical obstacles, again with one keystroke. A mirror image dummy line completion key is also provided for zig-zag surveys when part of the standard grid cannot be surveyed.