Thermometric Depth Gage

(unprotected deep-sea reversing thermometer), an instrument for measuring the depth to which oceanographic devices are lowered. It is a deep-sea reversing thermometer whose bulb is not protected from hydrostatic pressure.

When a thermometric depth gage is lowered into the sea, the bulb is compressed by water and some of the mercury is forced into the bore. Thus, the length of the column of mercury in the bore is determined not only by the water temperature but also by the hydrostatic pressure, which is proportional to the depth reached by the gage. A thermometric depth gage is lowered into the water together with a protected deep-sea reversing thermometer, and the depth is calculated from the difference in the readings of the two instruments.