Page 135 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

114 Maritime Archaeology: A Technical Handbook, Second Edition
mark
mark
Fixed end
Moving end
Height below datum
Figure 4.14 Static bubble tube has a fixed volume of air in tube, and one end (the fixed end) is brought to a fixed datum point and the meniscus point marked on the working end. The measuring end can then be used to measure from meniscus to object.
the tube can be moved around the site and heights can be measured from meniscus to objects. These heights should correspond to the height of the reference mark. At this point, the mark lies in the same horizontal plane as the original calibration. Vertical measurements can then be made from the reference mark up or down to the point of interest.
An interesting, pressure-based system which was proposed a number of years ago by Colin Martin consists of an air reservoir with a fine capillary tube (Figure 4.15). A volume of air is introduced into the reservoir and an adjustable marker is set to locate the position of the meniscus at a partic- ular reference point on the site. The instrument can then be moved over the site and, provided the meniscus remains at the datum mark on the cap- illary tube, the mark will lie in the same horizontal plane as the reference