SI unit: Temperature and humidity

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This article is part of a series detailing the SI units and our calibration services. This article centers around temperature and humidity.

Unit of thermodynamic temperature: kelvin (K)

​The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10−23 when expressed in the unit J K−1, which is equal to kg m2 s−2 K−1, where the kilogram, metre and second are defined in terms of h, c and ∆νCs.

Information on the practical realisation of the definition of the kelvin can be found in the ​Mise en pratique for the definition of the kelvin in the SI.

Humidity

​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

Calibration of radiation thermometers

Blackbody radiators are used in calibration of radiation thermometers. The operation range of MIKES radiators is -40 °C ... 1500 °C. 

In MIKES radiation thermometers are calibrated by using either a calibrated reference pyrometer or reference radiators.

The size of the radiation source (size-of-source effect, SSE) affects the calibration results of a radiation thermometer. On demand, the size-of-source-effect is measured at VTT MIKES.

Fixed point calibration of platinum resistance thermometer

Standard platinum resistance thermometers (SPRT) of good quality (i.e. stable) are calibrated at the fixed points of the ITS-90 temperature scale. A fixed point cell usually contains pure metal, e.g. tin, zinc, aluminium or silver sealed in a crucible of purified graphite. The graphite crucible is enclosed in a fused quartz tube.

The fixed point cell is placed in a vertical tube furnace and the temperature is slowly raised until the melting is complete. At this stage the furnace temperature is reduced to a value slightly below the melt temperature in order to start solidification. When the metal is in a supercooled state, the thermometer to be calibrated is carefully inserted into the cell. The thermometer is coupled to a resistance bridge using four wire coupling. The resistance bridge is used to measure the electrical resistance of the thermometer during the solidification state. The thermometers are usually calibrated using three or five different fixed points.

Calibration of hygrometers

Most dew-point meters are calibrated using a dewpoint generator. The measurement standards of humidity laboratory at MIKES cover the dew-point temperature range -80 °C to +84 °C. Dew-point calibrations are also carried out as comparison calibrations in calibrators, for instance for capacitive dew-point meters.

Most relative humidity sensors are calibrated in a climatic chamber. The relative humidity is calculated from measured temperature and dew-point temperature. If the achievable uncertainty is not sufficient or the temperature range extends to below +10 °C, the calibration is performed using a humidity generator. Relative humidity sensors are calibrated in the range 10 %rh to 95 %rh at temperatures between -20 °C and +85 °C.

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