Page 134 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 4: Conventional Survey 113
Air supply
Miniscus
Ruler
Datum
Figure 4.13 Active bubble tube has air slowly bubbling through the tube. The point at which the bubbles stop coming out of one end and start coming out of the other end means the ends of the tube are level.
measurement is taken from the air–water meniscus at the other end of the tube. This point lies in a plane with the reference point where the air is bub- bling out. Great care has to be taken to ensure that the hydrodynamic effects of pathways taken by the air do not produce anomalous effects. It has been noted that, if air is introduced at the top of the loop, there is a tendency, when air starts to come out of one end of the tube, for it to con- tinue to exit from this end. This happens even when the other end is sig- nificantly higher, whereas, according to theory, the air should be coming out at the higher end.
In the preferred static technique (Figure 4.14), a fixed volume of air is introduced into a clear plastic tube about 10 mm in diameter. The reference end of the tube is attached to a stake and the working end of the tube is placed alongside it. Enough air should be introduced into the tube so that there is a reasonable length of water filling the tubing at either end. Thus should the tube be inadvertently moved above or below the datum, air will not be spilled from the tube and upset its calibration. Alternatively, a large reservoir can be mounted at the static end of the tube to ensure that there is no overflow of air from the tube. Once the air is fixed in the tube and the static end is fixed, the meniscus at the mobile end of the tube will always remain at the same level provided the tube is not raised too high (or too low) so that there is air loss. If the tube is not distorted, the mobile end of