The Miller-Casella thermometer was used for most of the water temperature readings during the Challenger expedition. Its U-shaped tube held mercury and two floating markers that recorded the highest and lowest water temperature through which the thermometer traveled.

Scientists spaced these thermometers at specific distances along a line and lowered it over the side. Metal cases protected the instruments from bumps and bangs. When the rope was hauled back in, scientists read the thermometers and recorded the temperature for the depth that matched the length of the rope.

Their method assumed that the coldest temperature was measured at the greatest depth and that the warmest water was on the surface. But soon Challenger scientists asked "What if the thermometer had gone through a cold layer of water on its way up?" They needed better thermometers that would reliably record temperature at known depths.