National humidity standard at atmospheric pressure
Name of standard: National humidity standard at atmospheric pressure
Designation: ECM 341-1/15-060
Year of publication: 2015
Year of extension: 2020
Workplace: dep. 6036 CMI RI Brno
Guarantor: Ing. Jiří Bílek
Range of dew point temperature of frost point temperature | Uncertainty (k = 2) |
(-75 až -65) °C | 0,24 °C |
(-65 až -50) °C | 0,16 °C |
(-50 až -30) °C | 0,10 °C |
(-30 až 60) °C | 0,08 °C |
(60 až 80) °C | 0,10 °C |
(80 až 90) °C | 0,15 °C |
The National Humidity Standard at Atmospheric Pressure is a device used to measure the dew point temperature (frost point) of air at atmospheric pressure. It is intended for the calibration of dew point hygrometers in the range (-75 to 90) °C.
The dew point temperature is the temperature at which the water vapour content of the air is in a saturated state. In humidity calculations, the relationship between the saturation vapour partial pressure pw or pi and the temperature T: is the basis for humidity calculations.
(for water)
ln pw(T) = -6096,9385·T -1 + 21,2409642 - 2,711193·10-2·T + 1,673952·10-5·T 2 + 2,433502·lnT
(for ice)
ln pi(T) = -6024,5282·T -1 + 29,32707 + 1,0613868·10-2·T - 1,3198825·10-5·T 2 - 0,49382577·lnT
where:
- T is the temperature in Kelvin [K],
- pw(T) ) is the saturated vapor pressure above water at temperature T [Pa],
- pi (T) is the saturated vapor pressure above the ice at temperature T [Pa].
In the range (-75 to 20) °C, the national standard consists of a reference saturator, cryostatic liquid bath, air pump, float flow meters, dew point hygrometer, resistance temperature sensor, alternating temperature bridge and resistance standard. The saturator, dew point hygrometer, air pump and flow meters are connected in series with stainless steel or Teflon piping in a closed circuit. The reference saturator is immersed in a liquid bath in which the desired dew point temperature is generated. A resistive temperature sensor is used in conjunction with an alternating temperature bridge and a resistive standard to accurately measure the saturator temperature. The saturator consists of a stainless steel thick-walled cylindrical vessel into which a helix-shaped inner part is inserted. A cooling coil is welded to the lid of the saturator to pre-cool the air sample. At the bottom of the saturator are a few drops of distilled water. An air pump circulates the air, which is saturated to a humidity corresponding to the desired dew point temperature. A dew point hygrometer is used to check the dew point temperature.
In the range (20 to 90) °C, the state standard consists of a presaturator, a reference saturator, two liquid baths, an air pump, a float flow meter, a dew point hygrometer, two resistance temperature sensors, an alternating temperature bridge, a resistance standard and a condensing vessel. The air pump, flowmeter, presaturator, saturator, dew point hygrometer and condensing vessel are connected in series by stainless steel or Teflon piping in an open circuit. The path from the presaturator through the saturator to the dew-point hygrometer shall consist of a heated passageway including heated connections. The temperature inside the heated duct shall be at least 5 °C above the dew point temperature generated to prevent water condensation in the duct. The presaturator shall be immersed in a liquid bath at a temperature (2 to 5) °C higher than the saturator bath. The saturator is immersed in a liquid bath in which the desired dew point temperature is generated. A resistive temperature sensor is used in conjunction with an alternating temperature bridge and a resistive standard to accurately measure the temperature of both the presaturator and saturator. The presaturator consists of a stainless steel thick-walled cylindrical vessel in the body of which metal cylindrical chambers with bores are concentrically embedded and encased in lint-free paper. This ensures perfect water suction for maximum humidification of the passing air. About half of the presaturator is filled with distilled water. The saturator consists of a stainless steel thick-walled cylindrical vessel into which the helix-shaped inner part is inserted. A coil is welded to the lid of the saturator to preheat the air sample. An air pump ensures the passage of air which is saturated to a humidity corresponding to the required dew point temperature. A dew point hygrometer is used to check the dew point temperature.
