Checking the instrument for straightness

We have from time to time found it necessary to straighten an instrument. The jacking forces required to penetrate stiff formations can, if the instrument is not supported by the ground over its entire length, bend the instrument.

We have a jig which holds the instrument and allows a dial gauge to plot the departure from true as the instrument is rotated – see Figure 26-1. However such an arrangement is easily contrived, and it may be equally convenient to support the instrument in a lathe between centres.

Figure 26-2 shows a typical diagram which we produce at the time the instrument is supplied to record the total runout at defined locations along the instrument length. If the instrument is being checked for straightness then this diagram should be referred to for details of the instrument's condition when it was supplied.

Figure 26-2 Typical completed straightness diagram

Figure 26-2 Typical completed straightness diagram

A template of the straightness check diagram is attached to this web page for download.

Measuring the total indicator reading is not easy because there are few places where the instrument is perfectly smooth, or where it can be rotated through 360°.

Note that a measurement is taken on the instrument centre tube as close as is practicable to where the drill string connects. Scraps of membrane have been placed under the bag clamp rings so that they sit 'true'.

Pressuremeter references: