Temperature is measured with thermometers that may be calibrated to a variety of temperature scales. The pressure sensor should be calibrated periodically to maintain accuracy. Invasive PP measurements can be calibrated or uncalibrated. These indirect measurements must be calibrated via a direct measurement, most commonly a McLeod gauge. Absolute thermometers are calibrated numerically by the thermodynamic absolute temperature scale. Measuring devices are calibrated relative to some known unit. The application equipment must be correctly calibrated and properly used. Then, the measurement is calibrated in some way. They are calibrated to ISO volumetric A grade standards. The calorimeter can be calibrated by adiabatically doing externally determined work on it. Most thermometers are originally calibrated to a constant-volume gas thermometer. The axial input stress must first be calibrated against the output torque. For this reason each probe must be calibrated individually. The scale can be calibrated in Brix or refractive index. Secondary standards are calibrated with reference to a primary standard.