BOF-EX Retrievable Borehole Extensometer
The BOF-EX is a retrievable, multiple-point single-tube borehole extensometer from Roctest that measures ground displacement at several depths along a single borehole using screw-actuated mechanical anchors and a spring-loaded displacement transducer.
- Range
- 25-50 mm (vibrating wire); 50 mm (potentiometer); 38-100 mm (LVDT)
- Accuracy
- ±0.25% F.S. vibrating wire (±0.1% option); ±0.5% F.S. potentiometer/LVDT
- Resolution
- 0.02% F.S. (vibrating wire); readout-dependent (potentiometer/LVDT)
Overview
In detail
How it works
BOF-EX is a multiple-point single-tube extensometer built from four components: a mechanical anchor, a measurement module, extension tubing and centralizers. An installation strings several measurement modules in series down a borehole, each spanning a lower and upper anchor, so displacement is measured in sections distributed along the hole. The mechanical anchor's three contacting shoes, spaced at 120 degrees, are screw-actuated from the collar of the hole until they grip the borehole wall; the shoes flex to accommodate small borehole deformations while holding anchoring force. The measurement module is a watertight capsule housing a spring-loaded linear displacement transducer, whose moving spindle contacts the lower anchor of each section.
Installation
BOF-EX installs in a standard 76mm ('N' size) borehole using an installation rod, with 1 to 10 measurement modules per string and a minimum anchor spacing of 30cm when using a 25mm-range transducer. Because every component is fully retrievable, the whole system can be installed and later withdrawn from the hole; it can also be fully grouted in place, or, in poor-quality rock, installed inside a telescoping plastic casing pre-grouted in the hole.
Sensor options
Three transducer types are available: vibrating wire (25-50mm range, +/-0.25% F.S. accuracy, 0.02% F.S. resolution, with an optional +/-0.1% F.S. accuracy and a 3kOhm thermistor), potentiometer (50mm range, +/-0.5% F.S. accuracy) and DC LVDT (38-100mm range, +/-0.5% F.S. accuracy). All three operate from -20C to +80C and read out through the SENSLOG data-acquisition system over IRC-41A cable.
Specifications
Technical specifications
- Range
- 25-50 mm (vibrating wire); 50 mm (potentiometer); 38-100 mm (LVDT)
- Accuracy
- ±0.25% F.S. vibrating wire (±0.1% option); ±0.5% F.S. potentiometer/LVDT
- Resolution
- 0.02% F.S. (vibrating wire); readout-dependent (potentiometer/LVDT)
- Operating temperature
- -20°C to +80°C (all transducer types)
- Borehole diameter
- 76 mm standard
- Measuring modules
- 1 to 10 per installation
- Minimum anchor spacing
- 30 cm (with 25mm-range transducer)
- Data acquisition
- SENSLOG
- Cable
- IRC-41A
Related
Often specified alongside
ESS Earth
Resistance Wire Extensometer RWE
- Application
- Underground hard rock and coal mine displacement monitoring
- Monitoring points
- Pillars, stope walls, roof
Geosystems
JM Series Vibrating Wire Jointmeter
- Range
- 25, 50, 100, 150, 200, 250, 300 mm (200/250/300 mm not available for JM-E)
- Accuracy
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