Discontinued. Replaced by RM85


The RM15-D Resistance Meter forms the heart of a versatile measuring system for rapid area or vertical profile measurements. Used with a PA20 probe array (or earlier PA5) the RM15-D is ideally suited for use with conventional single arrays such as the Twin, Pole-Pole, Double-Dipole, Wenner, Schlumberger, Gradient etc. An expansion port allows the RM15-D to control multi-probe systems via the MPX15 Multiplexer module. This allows multiplexing multi-depth Twin and other arrays that can be configured using the PA20 / PA5 probe array. The RM15-D is an integral part of the MSP40 Mobile Sensor Platform, a wheeled resistance array that will provide rapid and detailed resistance surveys. 

The RM15-D is available in two versions, BASIC and ADVANCED, both having built-in data loggers with memory capacities of 3600, 15000 or 30000 readings. The logger provides powerful functions for fast and efficient surveys, permitting one man surveys, and keeps track of the survey position, giving both audible and visual indication of current survey position.

Readings may be logged manually, or automatically by a special current sense circuit. The modular plug-in card design allows easy upgrade from BASIC to ADVANCED meter and increase of memory capacity. An analogue output is provided for interfacing to external devices.

It is possible to upgrade from the earlier RM15 to the RM15-D which has an internal detector that can distinguish between the special NiMH battery pack provided and the alternative alkaline battery holder, thus preventing accidental charging of alkaline cells.


The BASIC RM15-D is ideal for general use by amateurs or students. It is designed primarily for use with the Twin array, though it can be used with other arrays such as the Wenner etc. with reasonable accuracy providing the baseline is small. It provides current ranges of 1mA and 0.1mA at an output voltage of 40V, giving a maximum logged resolution of 0.005 ohms. Synchronous detection and multi-pole filtering help reduce noise effects due to natural and power line earth currents, a problem which can be particularly serious on urban sites. Since it is not possible to have narrow bandwidth and fast settling at the same time, a choice of three filter time constants (measurement time 0.25s to 1s) are provided which you can select, depending on severity of the interference. AC operation overcomes polarization and contact voltage effects between the probes and earth. The frequency of 137 Hz is carefully chosen to avoid power line operating frequencies and harmonics, thus giving maximum rejection of power line earth currents. The RM15-D BASIC can be easily upgraded to the RM15-D ADVANCED model.

Integral data logger, 3600-30000 readings
Basic and Advanced versions
One man survey capability
Auto-Log mode for rapid surveys
Survey position tracking
Range 20470 ohms to 0.0005 ohms
Constant current 0.1mA, 1mA, 10mA
Output voltage 40 / 100V
Measurement times : 0.25s, 0.5s and 1s
Frequency 137Hz, 85Hz, or 35Hz
Expansion port for control of multiplexed probe systems
Compact - 200 x 120 x 90 mm.
Lightweight - 1.5 Kg including batteries
Low power - 44.5 hours from single charge

Recommended Systems


The ADVANCED RM15-D is designed for professional or research purposes use where greater accuracy is required, and includes all the features of the RM15 BASIC plus many more. The RM15-D ADVANCED provides an additional current range of 10mA and 100 V output, giving a maximum logged resolution of 0.0005 ohm. The 10 mA range can be used to further improve the noise rejection of the BASIC instrument and also provide improved accuracy with wider probe array spacings--this is because the system gain does not need to be so high, and hence measurement errors will be reduced. The 100 V output gives extra compliance, especially useful in dry conditions at higher currents, when it will improve the reliability of auto-logging and enable readings to be taken which may not otherwise be possible with a BASIC instrument. There is a choice of operating frequencies: 35 Hz, 85 Hz or 137 Hz, with the lower frequency providing the greatest accuracy for conventional four probe arrays, due to reduced capacitive and inductive errors.


Readings are stored in a non-volatile memory which may be partitioned into grid sizes of 10m, 20m or 30m, with reading interval being either 1m or 0.5m. The data logger keeps track of survey position for both zig-zag and parallel traverses and displays current grid, line number and line position. An additional survey tracking parameter is introduced when using the MPX15 multiplexer. Incorrect readings may be deleted singly, or as a complete line, with one keystroke. A dummy reading can be inserted if a physical obstacle prevents a true reading being taken or a line may be completed with dummy readings, again with one keystroke. A mirror image "complete" line key is also provided for zig-zag surveys when part of the standard grid cannot be surveyed. An ENABLE LOG key controls data logger functions thus preventing accidental loss of data.


Due to a special current sense circuit in both the BASIC and ADVANCED RM15-D logging can be automatic with any probe configuration that has a current probe mounted on the mobile frame, for example the Twin array. The operator simply inserts the probes of the mobile frame into the ground and the RM15-D will determine when the reading has settled, log it, and then give an audible beep to tell the operator to move onto the next reading. This mode of operation greatly reduces errors and operator fatigue, leading to a considerable increase in area survey speed - typically 400 readings may be taken in about 12 minutes. Auto-logging with the MPX15 Multiplexer enables large data sets to be acquired very rapidly.


The RM15-D requires a probe array to be able to take measurements and software to process and display the data. Accessories presently available include the PA3 Probe Array, the PA20 Probe Array, MPX15 multiplexer and MSP40 Mobile Sensor Platform. The PA20 and MPX15 are especially recommended for detailed and rapid area surveys. An anti-vibration mounting kit is also required if you wish to mount the RM15-D on a PA20 frame. Program Geoplot 3.0 is recommended for data transfer, processing and presentation of the data. See Recommended RM15 Systems for further details. The RM15-D is supplied as standard with an instruction manual, battery charger, data dump cable and a robust executive style carry case.

Recommended Systems
Upgrading an RM15 to and RM15-D            Back to top of page

Example resistance survey showing the walls of a medieval cemetery and buildings (associated with a hospital complex), Blues squares measure 20m x 20m. Shade plot range +1.5 to -1.5 SD.

PA20 Probe Array

MSP40 Mobile
Sensor Platform

PA3 Probe Array

MPX15 Multiplexer

Carrying Case