Unprotected thermometer

From Glossary of Meteorology

unprotected thermometer

A reversing thermometer (for seawater temperature) that is not protected against hydrostatic pressure.

The mercury bulb is therefore squeezed, and the amount of mercury broken off on reversal is a function both of temperature and of hydrostatic pressure. When compared with the simultaneous reading of a protected thermometer, which is affected by temperature only, the unprotected thermometer reading can be converted to pressure, and then, by applying the mean density of the water, to depth.