A temperature sensor assembly for monitoring a temperature at a measurement point within a subsea installation, the temperature sensor assembly comprising: a fixture portion operable to be installed within the subsea installation, the fixture portion comprising: a constant volume container operable to be positioned at the measurement point, the constant volume container filled with a fixed volume of a temperature responsive fluid such that an increase in a temperature at the measurement point induces an increase in a pressure of the temperature responsive fluid; and a fixture connector operable to be positioned at an accessible location on the subsea installation, the fixture connector including a fixture connector reservoir bounded by a first ambient diaphragm and a first seating surface, the fixture connector reservoir hydraulically coupled to the constant volume container such that an increase in a pressure of the temperature responsive fluid urges the first ambient diaphragm away from the first seating surface; and the temperature sensor assembly further comprising: a retrievable portion operable to selectively couple to the fixture portion, the retrievable portion comprising: a retrievable connector operable to couple the retrievable portion to the fixture portion, the retrievable connector including a retrievable connector reservoir bounded by a second ambient diaphragm, the second ambient diaphragm operably engaged with the first ambient diaphragm such that the second ambient diaphragm is urged away from the first seating surface by the first ambient diaphragm when the first ambient diaphragm is urged away from the first seating surface by an increase in the pressure of the temperature responsive fluid, and wherein a pressure within the retrievable connector reservoir increases in response to movement of the second ambient diaphragm away from the first seating surface; and a pressure sensor operable to detect and measure the pressure within the retrievable connector reservoir. a constant volume container operable to be positioned at the measurement point, the constant volume container filled with a fixed volume of a temperature responsive fluid such that an increase in a temperature at the measurement point induces an increase in a pressure of the temperature responsive fluid; and a fixture connector operable to be positioned at an accessible location on the subsea installation, the fixture connector including a fixture connector reservoir bounded by a first ambient diaphragm and a first seating surface, the fixture connector reservoir hydraulically coupled to the constant volume container such that an increase in a pressure of the temperature responsive fluid urges the first ambient diaphragm away from the first seating surface; and the temperature sensor assembly further comprising: a retrievable portion operable to selectively couple to the fixture portion, the retrievable portion comprising: a retrievable connector operable to couple the retrievable portion to the fixture portion, the retrievable connector including a retrievable connector reservoir bounded by a second ambient diaphragm, the second ambient diaphragm operably engaged with the first ambient diaphragm such that the second ambient diaphragm is urged away from the first seating surface by the first ambient diaphragm when the first ambient diaphragm is urged away from the first seating surface by an increase in the pressure of the temperature responsive fluid, and wherein a pressure within the retrievable connector reservoir increases in response to movement of the second ambient diaphragm away from the first seating surface; and a pressure sensor operable to detect and measure the pressure within the retrievable connector reservoir. a retrievable connector operable to couple the retrievable portion to the fixture portion, the retrievable connector including a retrievable connector reservoir bounded by a second ambient diaphragm, the second ambient diaphragm operably engaged with the first ambient diaphragm such that the second ambient diaphragm is urged away from the first seating surface by the first ambient diaphragm when the first ambient diaphragm is urged away from the first seating surface by an increase in the pressure of the temperature responsive fluid, and wherein a pressure within the retrievable connector reservoir increases in response to movement of the second ambient diaphragm away from the first seating surface; and a pressure sensor operable to detect and measure the pressure within the retrievable connector reservoir.