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Three-arm Caliper

The three-arm caliper probe provides a single continuous log of borehole diameter as recorded by three mechanically coupled arms in contact with the borehole wall. 38mm and 60mm models are available to suit a range of well diameters. The caliper is a useful first log to determine the borehole conditions before running more costly probes or those containing radioactive sources.

Principle of Measurement:

Opening and closing of the motor-driver caliper arms is by surface command, allowing the probe to run into the borehole with the arms retracted. Once opened, the spring-loaded arms respond to borehole diameter variations as the probe is raised up the borehole.


  • Small diameter for slim-hole operation
  • Extension arms available for 38mm probe
  • Optional natural-gamma measurement


  • Caliper
  • Natural Gamma (optional)
  • Borehole volume


  • Minerals/Water/Engineering
  • Location of borehole collapse or obstructions
  • Cement volume calculations for grouting
  • Identification of hard and soft lithology
  • Location of cracks, fissures, caving, faulting, casing breaks
  • Correction of other logs affected by borehole diameter


  • Temperature: 0-70⁰C (extended ranges available)
  • Max. pressure: 20MPa

Standard three-arm caliper probe

  • Diameter: 38mm
  • Length: 2.13m
  • Weight: 9.2kg
  • Range: 40-300mm and 40-710mm

Long-arm caliper probe

  • Diameter: 60mm
  • Length: 2.50m
  • Weight: 13kg
  • Range: 65-1000mm

Operating Conditions

  • Borehole type: open/cased; water/air-filled
  • Centralisation: recommended in large holes

Sales Information

I002035 Standard caliper probe with arm extension kit and calibrator
I002037 – including natural gamma
I002041 Long-arm caliper probe with calibrator

Download Three-arm Caliper sheet


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Robertson Geologging Ltd., 2017

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