Page 136 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed
P. 136
Chapter 4: Conventional Survey 115
a
Air inlet/exhaust
Air reservoir
Meniscus mark
Capillary tube
b
Figure 4.15 The Martin depth-measuring device. A schematic diagram of this device is shown in (a) and a working version in (b). The device has a reservoir of air and a capillary tube. It is filled with air at depth so that the meniscus sits in the capillary tube. The device can be moved around at the same level to measure distances to objects.
point. From this mark, depth measurements can be made to points on the site. Obviously, great care is required not to raise the instrument too far above the datum plane in which case air will bubble out of the tube and the calibration will be lost. The instrument provides a constant datum plane from which measurements down to the site can be made. If the capillary tube is narrow and the reservoir large, then small variations in the height of the instrument produce large movements of the meniscus in the tube. If
