Oceanography

Reversing thermometers

In the earliest, temperature measurements at some depth below the surface where made by bringing a water sample up to the deck of a ship in an insulated bucket and measuring the sample temperature with a mercury thermometer. Although these measurements were not accurate, they gave the first evidence that below the top 1000m the ocean is cold even in the tropics. They also showed that highly accurate measurements are required to resolve the small temperature differences between different ocean regions at those depths.

The first instrument that (through the use of multiple sampling and averaging) achived the required accuracy of 0.001⁰C was the reversing thermometer. It consists of a mercury filled glas pipe with a 360⁰ coil. The pipe is restricted to capillary width in the coil, where it has a capillary appendix. The instrument is lowered to the desired depth. Mercury from a reservoir at the bottom rises in proportion to the outside temperature. When the desired depth is reached the thermometer is turned upside down (reversed), but the flow of mercury is now interrupted at the capillary appendix, and only the mercury that was above the break point is collected in the lower part of the glass pipe. This part carries a clibratd gradation that allows the temperature to be read when the thermometer is returned to the surface.

To eliminate the effect of pressure, which compresss the pipe and causes more mercury to rise above the break point during the lowering of instrument, the thermometer is enclosed in a pressure resistant glass housing. If such a “ protected reversing thermometer” is used in conjunction with an “unprotected reversing thermometer” (a thermometer exposed to the effect of pressure), the difference between the two temperature reading can be used to determine the pressure and thus the depth at which the reading were taken. The reversing thermometer is thus also an instrument to measure depth.

Reversing thermometers require a research vessel as platform and are used in conjunction with Nansen or Niskin bottles or on multi-sample devices.