While the DS2438 datasheet lists an operating temperature of -40C to +85C, on page 4 of the same datasheet there is a statement that indicates that the measurable temperature range is from -55C to +125C. What the statement on page 4 is trying to describe is that the temperature reading in the registers has enough bits to represent temperatures from -55ºC to +125ºC. This is an important detail to communicate to the user, so that the user does not assume the DS2438 stops reporting temperature values should the temperature exceed the -40C to +85C range.