Unit of thermodynamic temperature: kelvin (K)
The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
This definition refers to water having the isotopic composition defined exactly
by the following amount of substance ratios:
- 0.000 155 76 mole of 2H per mole of 1H,
- 0.000 379 9 mole of 17O per mole of 16O,
- 0.002 005 2 mole of 18O per mole of 16O.
Practical realization of the definition of the kelvin
Triple-point-of-water cells provide a convenient realization of this definition. For temperatures other than the triple point of water, direct measurements of thermodynamic temperature is in practice difficult, time consuming, and not a practical means of disseminating the kelvin. As an alternative, the International Temperature Scale (ITS-90) provides an internationally accepted recipe for realizing temperature in a practical way.
Humidity is the amount of water vapor present in the air (or any other gas). There are three main ways the express humidity: relative, absolute and and specific of which relative humidity is the most commonly used.
- Relative humidity is the amount of water vapor in the air, expressed as a percentage of the maximum amount that the air could hold at the given temperature; the ratio of the actual water vapor pressure to the saturation vapor pressure.
- Absolute humidity is the total mass of water vapor present in a given volume (g/m3) or mass of air (g/kg).
- Specific humidity (or moisture content) is the ratio of the mass of water vapor to the total mass of the moist air parcel.
Temperature and humidity calibration services