Absence of B-reading or A – Remedy

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The absence of the B-reading is a serious inconvenience. Without B the operator keeps inflating beyond 1.1 m, blowing the membrane.

It is therefore imperative to correct this defect.

There can be two reasons of the missing B:

 

  • Electrical reason of missing B
    Dirt (or grains or piece of paper or cloth or else) in one of the 4 points evidenced by arrows in the picture. All these points must be perfectly clean to insure electrical continuity. You may use a solvent, a cloth, or fine sand paper. The final result must be that the 4 points are perfectly clean.
 
  • Mechanical reason of missing B
    The quartz cylinder must slide absolutely free in the 2.05 mm diameter hole of the Sensing Disk. Hence the 2.05 mm diameter hole must be perfectly clean. This can be done by rotating (manually) a 2.05 mm drill point, then inserting in the 2.05 mm hole and rotating a tapered wooden tooth-pick on the bottom face of the disk.
 

During all above operations use abundant compressed air with an air pistol.

Specific Bibliography

The operator should learn how to disassemble the blade for a proper cleaning by reading page 29 and following pages of Marchetti 2001 Bali course maintenance.

Additional Info

Section 7.1.1 of TC16 DMT 2001

Check the presence of the B-reading

After cleaning, always check (before applying the membrane) the systematic presence of B.

Check B without membrane (Fig. A). The continuity tester must be applied the same as in Fig. B. Using a cylindrical “dowel” push down and then release up the quartz cylinder 100 times. All signal inversions (On and Off) must be sharp and prompt. If not, clean again.

Check B with membrane (Fig. B). When the membrane has been applied, push center of membrane. Buzzer should sound.

 

NOTE. There is no need to do such complete cleaning periodically. Cleaning has to be done every time a membrane is broken, unless, despite the broken membrane, no dirt has entered the blade.

If a blade has worked regularly during an investigation, there is no need of a complete cleaning before going to the next site. However, before going to the next site, you may still wish to be sure of the presence of the B-reading. This can be easily checked (without opening the blade) by performing many calibrations in the usual way. In other words, in the calibration configuration, push-pull the piston of the syringe (say 20 times) and make sure that all signal inversions (On and Off) are sharp and prompt. If not, open the blade and do a complete cleaning.

Fig. A Without membrane
Fig. B With membrane

Example of dirt covering metal parts that should be clean

Submerge disk, spring, cylinder in a solvent (acetone or 3-chloro-ethilene)

Absence of A

In general the reason why there is no sound in the A position is that the soil pressure is not sufficient to close the membrane. This may happen in soft soils at shallow depths. In this case apply suction using the syringe in order to retract the membrane, and record as A-reading the suction needed to activate the buzzer (a negative pressure).

 

Correlated information

When executing by SDMT a seismic measurement, the membrane must be closed. Otherwise the system will not acquire seismograms. Therefore in soft soils at shallow depths use the syringe if not sure that the soil pressure is sufficient to close the membrane.