Resistance Meter  RM4

The RM4 Resistance Meter and accessories have been designed specially for the archaeologist who wishes to carry out his own resistivity surveys but has no prior knowledge of the techniques involved. The instrument is ideally suited to the popular Twin probe array, though it can be used with other arrays such as the Wenner etc. with reasonable accuracy providing the baseline is small.

The RM4 uses only a low voltage (40 V) and low current (1 mA) for measuring earth resistance to a resolution of 0.01 ohms. Its low power consumption gives it long battery life and makes it one of the most compact and lightweight designs available. A 3 1/2 digit display provides all the resolution required without the need for back-off or balancing controls, making readings much faster and easier to take. Once a range has been selected, there will normally be no need to change it during the course of a survey.

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 two time constants (measurement time 1s or 2s) are provided which you can switch between depending on severity of the interference. AC operation overcomes polarisation and contact voltage effects between the probes and earth. The frequency of 137 Hz is carefully chosen to avoid power line operating frequencies, harmonics and sub-harmonics and thus provides maximum rejection of power line earth currents.

A major source of error in resistivity surveying can be the high contact resistance which often occurs between probes and ground. However the RM4's combination of constant current transmitter and high input impedance receiver renders these effects negligible, giving reliable results in all but the very driest of conditions. If conditions are very dry and readings cannot be made with the normal 1mA constant current range, then the RM4 can be switched over to the High Contact Resistance mode. This reduces the constant current by approximately one third, whilst still maintaining the same output voltage, resulting in a three fold increase in the constant current compliance.

An analogue output is provided for driving a chart recorder or for data logging purposes. In the latter case, a range status output is also available which not only tells the data logger which range is in use, but also indicates if probes are out of the ground. This may be used to provide semi-automatic data logging.

Where built-in data-logging facilities, greatly improved specifications and control of multiplexed probe arrays is required, the
RM15-D Resistance Meter should be selected in preference to the RM4. Please note that the DL10 Data-Logger (designed for use with the RM4) is no longer available.


The RM4 requires a probe array to be able to take measurements, and software to process and display the data - see Recommended RM4 System for further details.


Waterproof components are used throughout, including all connectors, instrument housing, switches, display, leads etc. The polycarbonate case of the RM4 has exceptionally high impact strength, making it extremely durable but at the same time lightweight.

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