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We’ve all experienced those hot and humid summer days. A feeling of inescapable airlessness, no matter how many windows you open or how few clothes you wear. This is down to the amount of water vapour present in the air. In a normal atmosphere water vapour is around one per cent, but this can fluctuate wildly.
While this may feel physically uncomfortable for us, in an industrial environment this translates into the increased potential for condensation, mould, corrosion and warping.
Humidity is most commonly referred to as a percentage in terms of relative humidity (rh), which signifies the level of water vapour saturation in a space.
Humidity is measured using hygrometers and it is important that hygrometers are calibrated. The calibration ensures that your hygrometer gives equivalent readings to any other calibrated hygrometer. Measurements are only ever estimates. As expensive as your kit may be, as meticulously you work, your findings will always be subject to some uncertainty. That’s not to say that we should stop trying, but we should find an uncertainty level appropriate to our needs.
Good measurement practices reduce uncertainty; for example, regular calibration providing traceability to the International System of Units (SI), through an unbroken chain of calibrations tracing back to a national standards institute.
Calibration of hygrometers today
Reliability from calibration
Reliability of humidity measurements is important, e.g. in storage of wood, paper and food, in aviation and environmental monitoring, and in diverse fields of industry and research. Calibration of hygrometers at regular intervals and monitoring their stability is an essential part of verification of measurements. It is important to obtain high-quality calibration services for instruments measuring humidity of gases and expert services on research and development related to humidity measurements and their reliability.
Traceability to humidity measurements
MIKES creates conditions for traceable humidity measurements in Finland by developing and maintaining measurement standards for humidity and by offering calibration and expert services. The high quality of the humidity laboratory is maintained by taking part in international research and comparison projects and by carrying out our own research projects.
Traceability
Traceability of humidity measurement is based on a dew-point temperature scale. The scale is realised by using a humidity generator, which is the national measurement standard in Finland. The core of a dew-point generator is a saturator in which total saturation of air with respect to water or ice is reached. The dew-point temperature of the air coming out of the generator is calculated from the saturator temperature and from the pressure difference between the saturator and the device under calibration. When saturated air is led into the measurement chamber of the generator, the equipment is also suitable for calibration of relative humidity sensors.
The dew-point meter under calibration is directly connected to the dew-point generator. In calibration of a relative humidity sensor, the sensor is placed in the measurement chamber system. The reading of the sensor is compared to the value of relative humidity that is calculated from the dew-point temperature and the air temperature inside the chamber.
Calibration services
Most dew-point meters are calibrated using a dew-point generator. The measurement standards of the humidity laboratory at MIKES cover the dew-point temperature range -80°C to +84°C. At best the calibration uncertainty is 0.05°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 dew-point temperature and the air temperature in the chamber are measured by using a chilled mirror hygrometer and a digital thermometer, respectively. 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 (rh) sensors are calibrated in the range 10%rh to 95%rh at temperatures between -20°C and +85°C. At best the calibration uncertainty varies from 0.2%rh at 10%rh to 0.7%rh at 95%rh.
In cases of other humidity quantities, calibrations are performed with the same equipment, the relative humidity calibration systems. The values of these quantities are calculated from measured dew point temperature, temperature and pressure.
Future challenges
Water vapour plays a key role in climate and global warming. New and more accurate radiosondes are being developed for scanning atmospheric humidity up to 30km height and beyond. During the flight of the device the temperature drops down to -80°C and the pressure reduces from about 1,000hPa down to 10hPa. MIKES is one of the first metrology institutes in the world developing a reference calibration system applicable to these conditions. The research is carried out within a European Metrology Research Programme (EMRP) project: MeteoMet2.
It’s not only air where we need to know the amount of water vapour. For safety reasons and pricing, it’s essential to measure humidity accurately in gas grids. This is becoming more challenging with increasing usage of biogas from various sources. MIKES is carrying out research to improve these humidity measurements in cooperation with other European partners in the EMRP Biogas project.
About 15% of all energy is consumed in industrial drying. In Europe, this costs about €30 billion per year. Humidity and moisture measurements have a key role in optimising drying processes, but the measurement accuracy is highly limited by available calibration methods. MIKES is leading pioneering research to develop new and more fundamental calibration methods for humidity at transient conditions and temperatures above 100°C and for moisture in various solid materials. The work is done within a European Metrology Programme for Innovation and Research (EMPIR) project HIT and an EMRP METefnet project.
Calibration of thermometers
In most cases when you are measuring humidity you need to measure temperature as well. The same relative humidity value means very different amounts of water in air at different temperatures. For example, if the temperature of air at 90%rh changes from 25°C to 28°C, the relative humidity drops to 75%rh. Temperature is the most important parameter in environmental monitoring and most frequently measured parameter in industry. Measurements are done in a wide range from below -273°C to above 3,000°C.
When using a contact thermometer, e.g. thermocouple and platinum resistance thermometer, it’s essential that your device is at the same temperature as your target through a good contact. An infrared thermometer is a non-contact device that determines the temperature of the target from the thermal radiation.
The most accurate platinum resistance thermometers are calibrated to the fixed points of the ITS-90 temperature scale. A fixed point cell usually contains pure metal, e.g. tin, zinc, aluminium or silver (as shown in Table 1) sealed in a crucible of purified graphite. The purity of the metal is typically ca. 99.99995%.
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 super cooled 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 solidification state can be maintained for up to 10 hours and the temperature of the fixed point cell stays within ±0.5 mK. A 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. These fixed point calibrated thermometers are used for calibrating other contact thermometers in liquid baths or furnaces.
Blackbody radiators are used in calibration of infrared and other radiation thermometers. The operation range of MIKES radiators is -40°C to 1,500°C. The temperature of a blackbody radiator can be measured using either a temperature sensor embedded in the radiator wall or a reference radiation thermometer. When calculating the radiation temperature from the measured temperature, the emissivity of the wall and bottom materials of the radiating cavity and the geometry of the blackbody radiator as well as the temperature gradients are taken into account.
The radiation temperature measured by a radiation thermometer is often lower than the surface temperature of the measured object, since the surface emissivity is usually lower than the emissivity of an ideal blackbody (the emissivity of a blackbody is one, but the emissivity of a glossy copper surface is 0.1). Above 960°C, the MIKES reference radiation thermometer is calibrated using silver and copper fixed point blackbody radiators. Similar to the fixed point cells for contact thermometers, the temperature of these radiators is fixed by pure metal between the inner cavity wall and the outer cell wall.
ITS-90 as the source of traceability
Today the international temperature scale ITS-90 is the most ultimate source of traceability in thermometry. At MIKES, this scale is realised from -189°C to +962°C with appropriate platinum resistance thermometers and fixed point cells. At higher temperatures up to 1,600°C, the scale is realised using a reference radiation thermometer and fixed-point radiators (962°C and 1,085°C). The best uncertainty is achieved in triple point of water temperature (0.01°C): 0.0002°C.
In addition to the direct ITS-90 realisation, MIKES provides calibrations by comparison in liquid baths, furnaces and climate chambers in the range from -196°C to +1,600°C.
Major scientific discoveries in physics are nowadays obtained at very low temperatures, i.e. below -270°C. Also a significant part of material research is focused in this range. To underpin these future orientated activities, MIKES is developing more accurate temperature measurement methods for this range within the EMRP InK project. In the same project, MIKES is also developing optical temperature measurement methods for high temperatures.
Published: 16th Feb 2016 in AWE International
