The caliper tool measures the variation in bore hole diameter as it is withdrawn from the bottom of the hole, indicating any cave-ins or swelling in the hole.
Typically, the caliper arms are mechanically connected to a linear or rotary potentiometer such that a change in the angle of the arms causes changes in resistance. These changes in resistance are proportional to average borehole diameter. In some probes, the voltage changes are converted to a varying pulse rate or digitized down hole to eliminate or minimize cable transmission noise. Different arm length can be used to optimize sensitivity for the borehole diameter range expected. Vertical resolution of caliper measurements is a function of the size of the contact surface (arm tip or pad); the response of the mechanical and electronic components; and digitizing interval used.
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| APPLICATIONS & BENEFITS |
- Economical.
- Serves as early warning of cave-ins or swelling prior to casting of the in-situ piles
- Used to determine borehole diameter and shape and wall roughness
- In advanced cases can be used to find the volume of the drilled hole
- Can be used to serve as an indication on the stability of the walls of the drilled holes
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| LIMITATIONS |
- Selection of arm lengths and angle, and tip diameter will affect sensitivity
- Shorter arms generally will provide more detail of the shape of the borehole wall than longer arms. However, size of caliper probe and borehole diameter may also determine arm lengths used
- Effected by the drilling fluids kept in the hole
- Continuous maintenance of the arms is required to enable easy movement of the arms
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