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Medical physics applies physics to quantitative problems in medical imaging, nuclear medicine and radiation therapy. It connects an instrument's signal to a physical quantity, tests the model behind that inference, and establishes its measurement uncertainty. Examples include reconstructing an image from radiation or acoustic measurements and determining the absorbed dose delivered by an ionizing radiation beam.

The sealed-water calorimeter of Steve R. Domen gives a concrete measurement example. Domen's 1994 study placed sealed water in a glass cylinder, 110 mm long and 33 mm in diameter, inside a 30 cm water-filled acrylic cube. Two calibrated thermistors formed opposite arms of a Wheatstone bridge; their change in electrical resistance measured the small temperature rise. The glass also limited convection.^[ref]

In the ideal water calorimetry balance, deposited energy becomes heat, givingwhere $D$ is absorbed dose and $c_p$ specific heat capacity. Using a rounded illustrative $c_p=4180\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$, depositing $1\mathrm{J}$ in $1\mathrm{kg}$ corresponds to one gray and onlyThis small signal makes thermal control and measurement calibration central to the experiment.

Domen also investigated the heat defect: radiation-induced chemistry can release or store energy, changing the heat readout. Approximately 3700 measurements varied dissolved-gas conditions and compared the response with other calorimeters. The test examined whether a measured temperature rise represented the intended absorbed dose, rather than assuming the equivalence.^[ref]

Metrological traceability connects such measurements through documented calibrations and their measurement uncertainties. In the IAEA/WHO network, national laboratories and secondary-standard laboratories link reference dosimeters to the instruments used in practice. A primary measurement standard supplies the reference realization; the calibration chain transfers it to other instruments.^[ref]

The sealed-water calorimeter of Steve R. Domen is an apparatus for measuring absorbed dose through the radiation-induced temperature rise of a controlled water volume. Domen's 1994 design enclosed the inner water in a thin-walled cylindrical glass vessel about 110 mm long and 33 mm in diameter, immersed in a larger water bath. The vessel also acted as a barrier against convection.^[ref]

Two calibrated thermistors served as temperature sensors in opposite arms of a Wheatstone bridge, increasing the electrical response to their common temperature rise. The apparatus therefore connected a small thermal change to a measurable resistance change, rather than directly counting radiation events.^[ref]

A central experiment compared water fillings with different dissolved-gas conditions. The temperature response could change because radiation also drove chemical reactions, motivating investigation of the heat defect. This comparison illustrates a useful apparatus principle: controlling the geometry and reading the thermometer carefully does not, by itself, establish that every joule deposited in the water becomes measurable heat.^[ref]

Steve R. Domen developed physical water calorimetry methods at the National Institute of Standards and Technology. His 1994 sealed-water instrument measured absorbed dose from a minute radiation-induced temperature rise, while investigating how dissolved gases change the fraction of deposited energy that appears as heat.^[ref]

The apparatus enclosed water in a thin glass vessel immersed in a larger water tank. Two calibrated thermistors in glass capillaries formed part of a Wheatstone bridge that detected the temperature response. The vessel also limited convection. Comparing different dissolved-gas conditions tested the heat defect, rather than assuming that all absorbed energy immediately becomes heat.^[ref]

For an original ideal calculation, neglect the heat defect and heat transfer during a reading. With water specific heat capacity $c_p=4180\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$ and measured rise $\Delta T=0.240\mathrm{mK}$,This illustrates why sub-millikelvin thermometry matters. It is not a reported run: the real measurement model must include the instrument's corrections and measurement uncertainty.