Annex A

(normative)

Calibration and corrections

A.1 Measuring devices

All the control and the measuring devices shall be periodically checked and calibrated to show that they provide reliable and accurate measurements.

A copy of the latest calibration test report shall be available at the job site.

A.2 Membrane stiffness

The pressure loss p_m due to the stiffness of the membrane shall be obtained from an inflation test according to the procedures described in A.2.1 and A.2.2.

The calibrations described below shall be carried out as follows:

• at each change of flexible dilatometer membrane;
• otherwise at intervals appropriate to the use the probe has received but at least once a year.

A.2.1 Preparation of flexible dilatometer for membrane pressure loss test

The flexible dilatometer probe shall be connected by a short line (less than 2 m) to a pressure source. The membrane shall then be inflated at least three times by injecting fluid up to the maximum deformation.

For this operation the pressure control unit shall be fitted with a pressure measuring device accurate to better than 10 kPa.

A.2.2 Membrane stiffness correction test

The probe is placed upright in the open air. The probe shall be inflated as if it was in the ground, using pressure increments small enough to properly define the complete range of diameters of the membrane.

If a liquid is used to pressurize the flexible dilatometer, the difference in elevation between the probe and the measuring unit shall be taken into account.

The pressure versus probe diameter curve shall be plotted as in Figure A.1 showing the increase of diameter with increase of pressure. For the correction for membrane stiffness of the pressure p_r of an actual test as read at the measuring unit, the procedure shall be as follows. The pressure given in Figure A.1 corresponding to the increase in diameter measured during the actual test shall be subtracted from the test pressure reading to give the pressure acting on the ground.

A.3 Membrane compression coefficient for variant B dilatometer

The calibrations described below shall be carried out as follows:

• at each change of flexible dilatometer membrane;
• otherwise at intervals appropriate to the use the probe has received but at least once a year. The probe shall be placed into the compression calibration cylinder (see 4.5.4).

The probe shall be pressurized by increments Δp_r initially of 100 kPa and then by pressure steps appropriate to define the curve up to the full pressure rating of the instrument. For each pressure hold, the probe diameter shall be recorded (see Figure A.2).

In Figure A.2 the difference between the upper parts of curves 1 and 2, each approximated by a straight line, shall be used in correcting for the effect of membrane compression. The procedure to obtain the membrane compression coefficient a between two pressure holds p₁ and p₂ is as follows.

On the calibration graph corresponding to Figure A.2, two horizontal lines at ordinates p₁ and p₂ shall be drawn and the intersection points with curves 1 and 2 shall be reported as:

• d₁₁ and d₁₂ on the d axis for pressure p₁;
• d₂₁ and d₂₂ on the d axis for pressure p₂.

The membrane compression coefficient between the pressure holds p₁ and p₂ is given by

This coefficient shall be calculated once for all, as long as the two curves exhibit straight line portions in the part where they are used.

For instruments with multiple displacement transducers, the mean value of a shall be used in the calculation above.

Key

• Curve 1 diameter d_r as read
• Curve 2 calibration cylinder diameter d_c expansion under pressure, to be obtained either by calculation from elastic properties of the material or by direct measurement