Dew Point Hygrometer Research Articles

Long-term decay of the water pressure in the osmotic tensiometer

Matric suction is an important state variable required for the assessment of unsaturated soil properties. Tensiometers are commonly used for direct matric suction measurement but have a limited measuring range up to 90 kPa due to the cavitation problem. Osmotic tensiometer (OT) can improve the measuring range of tensiometers by increasing the osmotic pressure of water to avoid the cavitation. However, the long-term water pressure decay that appeared in OTs caused a gradual decrease in their measuring range. In this study, crosslinked poly(acrylamide-co-acrylic acid) potassium salt (PAM-co-PAAK) was used for the preparation of OTs (five in total) to explore the mechanism of water pressure decay of OTs. The maximum water pressure in the OT versus the volume fraction of polymer filled in the OT was described based on the Flory-Huggins polymer theories and validated using WP4C dewpoint hygrometer. The long-term pressure decay of OT-1, OT-2, and OT-3 was observed for 130 d and constant pressures were found for OT-1 and OT-2, indicating that the pressure decay of OT was mainly caused by the stress relaxation of the polymer hydrogels, and standard linear solid (SLS) rheological model was appropriate to fit the decay data. For OT-1, OT-2 and OT-3, the theoretical osmotic pressure that was calculated based on the mass of retrieved polymer from OTs after 130-d pressure observation was higher than the actual osmotic pressure as observed, indicating that polymer leakage cannot explain the pressure decay of the OT. The ultraviolet–visible (UV–visible) spectrophotometry examined the change in polymer concentrations in the water containers of OT-4 and OT-5 and demonstrated that there was no increase in polymer leakage during the period of pressure decay of OT-4 and OT-5. As a result, the pressure decay of OT was not caused by polymer leakage. The results of this research suggested that the viscoelastic properties of polymers should be taken into consideration in further OT development. • Water pressure versus volume fraction of the polymer filled in OT was validated. • Pressure decay in OTs was mainly due to stress relaxation of polymer hydrogels. • No increase in polymer leakage during the period of pressure decay of OTs. • Development of new OTs should consider rheological properties of polymers.

Drying and wetting soil-water retention behaviour of a highly expansive clay under varying initial density

Expansive (or swelling) clays are the most prolific problem soil across Southern Africa and many parts of the world. Large volume changes due to seasonal wetting and drying cause millions of dollars’ worth of damage to engineering infrastructure yearly. Soil-water retention behaviour is dependent ondensity, and determining the correct retention curve may be key for reliable design of infrastructure. Soilwater retention curves and shrinkage curves were measured for recompacted samples of a highly expansive bentonite clay from South Africa. Four samples were prepared at varying initial dry densities between approximately 1200 and 1500 kg/m3 and subjected to total suction testing in a dewpoint hygrometer. The water content of each sample was varied through wetting and drying. The sample volume was measured after each suction reading, allowing relationships between suction and void ratio, degree of saturation and water content to be determined. The relationship between gravimetric water content and suction was not significantly influenced by initial density. Degree of saturation at a given suction varied significantly according to initial density. Volume reductions of 25% to 36% from saturated to residual conditions were recorded, and samples tended to a residual void ratio of 0.35 to 0.4. The swelling clay showed propensity for maintaining high suctions (over 300 MPa at residual conditions) and hysteretic response between primary drying and wetting.

Determining osmotic suction through electrical conductivity for unsaturated low-plasticity soils

Determining osmotic suction from the electrical conductivity (EC) of soil pore water was widely reported in the literature. However, while dealing with unsaturated soils, they do not have enough soil pore water to be extracted for a reliable measurement of EC. In this paper, the chilled-mirror dew-point hygrometer and contact filter paper method were used to determine the total and matric suctions for low-plasticity soils with different salinities (0.05‰, 2.1‰, and 6.76‰). A new piecewise function was proposed to calculate the osmotic suction, with the piecewise point corresponding to the first occurrence of precipitated salt in mixed salt solutions (synthetic seawater). EC, ion and salt concentrations used for osmotic suction calculation were transformed from the established relationships of mixed salt solution instead of experimental measurement. The calculated osmotic suction by the proposed equation and the equations in the literature was compared with the indirectly measured one (the difference between the measured total and matric suctions). Results showed that the calculated osmotic suction, especially the one calculated using the proposed function, was in fair agreement with the indirectly measured data (especially for specimens with higher salinity of 6.76‰), suggesting that the transformation of EC and concentrations from the established relationship is a good alternative to direct measurement for low-plasticity soil. In particular, the proposed method could be applied to unsaturated low-plasticity soils which do not have enough soil pore water for a proper EC measurement.

Investigating the salinity effect on water retention property and microstructure changes along water retention curves for lime-treated soil

Lime treatment is a widely applied technique in improving the workability and geotechnical properties of soil. The water retention property and microstructure are highly related to the hydro-mechanical behaviour of unsaturated lime-treated soil. In this study, the water retention property and the evolution of pore size distribution (PSD) along the water retention curves (during drying) were studied for a lime-treated soil, with emphasis put on the curing time and salinity effects. Two soil powders with different soil salinities were prepared and stabilised by 2% lime. The chilled-mirror dew-point hygrometer and the contact filter paper method were used to measure the total and matric suctions, respectively. The PSD of lime-treated soil at various water contents was obtained using mercury intrusion porosimetry (MIP). Results showed that the matric suction increased significantly, while the total suction varied slightly during curing. At a given curing time, the specimens with higher salinity exhibited higher matric and total suctions. The difference between the soil water retention curves (SWRCs) determined by the filter paper method and from the PSD became more significant at longer curing time, as the production of cementitious compounds did not contribute to the SWRC from PSD, but contributed to the increase of matric suction measured by the filter paper method. The PSD of lime-treated soil changed from bi-modal characteristics (w > 14 %) to tri-modal pattern (w ≈ 8 %), and finally recovered to bi-modal characteristics (w ≈ 3 %), due to the shrinkage-related cracking of the clay fraction. The lime treatment inhibited the clay shrinkage, whereas the curing time and salinity effects on the drying-induced microstructure were insignificant.

Response Characteristics of Dew Point Hygrometer by Connection Order in Series in Calibration

Response Characteristics of Dew Point Hygrometer by Connection Order in Series in Calibration

Alternative methods of estimating the water potential at turgor loss point in Acer genotypes

BackgroundSelecting for drought tolerance in urban tree species can have a significant influence on survival rates, aftercare requirements and performance. The water potential at turgor loss point (πtlp) is gaining popularity as a trait to help determine drought tolerance to aid tree selection. Therefore, it is important to understand if differing methods used to measure or calculate πtlp deliver consistent results.ResultsThe sensitivity of three methods used to determine this valuable selection parameter were evaluated. A classical pressure chamber, pressure–volume (P–V) curve method was compared with vapour-pressure osmometer (Vapro®) and dewpoint hygrometer (WP4C) methods. These methods were evaluated using closely related cultivars of Acer platanoides and A. pseudoplatanus ‘Negenia’.ConclusionBoth the osmometer and hygrometer methods ranked genotypes with a very high similarity (Rs = 1, R2 = 0.96) and were able to identify significant differences between cultivars. This is the first study to demonstrate suitability of the dewpoint hygrometer in comparison to the vapour-pressure osmometer to measure πtlp. The P–V method was unable to identify differences between the cultivars tested. The Vapro and WP4C provide greater applicability than the conventional P–V method to studies requiring both high throughput and high sensitivity. Consistency of measurement type is however highly recommended in future studies as some differences were observed between Vapro and WP4C.

Analysis and Test on Influence Factors of Dew Drop Condensation in Dew Point Hygrometer

Analysis and Test on Influence Factors of Dew Drop Condensation in Dew Point Hygrometer

An experimental investigation on the water retention behaviour of a silty soil for the computation of the lateral earth thrust on a retaining wall

Retaining structures often interact with soils in unsaturated conditions and their performance is influenced by environmental actions. Currently, geotechnical analysis and design approaches mainly consider the soils either totally dry or totally saturated. Environmental actions, like infiltration due to rainfall, can significantly affect the lateral earth pressure of soils, influencing the performance of both temporary or permanent retaining structures. This paper deals with the investigation of the water retention behaviour of a silty soil for the computation of the soil thrust on a retaining wall during rainfall events. In this regard, the retention property of the involved geomaterial is investigated in laboratory through the combined use of high capacity tensiometers (HCT) and a dew-point hygrometer (WP4C). Considering drying and wetting paths, the experimental results are employed to calibrate the water retention behaviour for the computation of the failure shear strength of the geomaterial. The importance to monitor volume change during the characterization of the water retention behaviour of fine soils is highlighted. A series of analytical uncoupled hydro-mechanical analyses is performed to estimate the changing in the thrust of an unsaturated soil on a retaining wall under several infiltration rates. An appropriate modelling of the soil water retention behaviour is resulted to be crucial for the computation of lateral earth thrust.

Evaluation of the IAGOS-Core GHG package H<sub>2</sub>O measurements during the DENCHAR airborne inter-comparison campaign in 2011

Abstract. As part of the DENCHAR (Development and Evaluation of Novel Compact Hygrometer for Airborne Research) inter-comparison campaign in northern Germany in 2011, a commercial cavity ring-down spectroscopy (CRDS) based gas analyzer (G2401-m, Picarro Inc., US) was installed on a Learjet to measure atmospheric water vapor, CO2, CH4, and CO. The CRDS components were identical to those chosen for integration aboard commercial airliners within the IAGOS (In-service Aircraft for a Global Observing System) project. Since the quantitative capabilities of the CRDS water vapor measurements were never evaluated and reviewed in detail in a publication before, the campaign allowed for an initial assessment of the long-term IAGOS water vapor measurements by CRDS against reference instruments with a long performance record (Fast In-situ Stratospheric Hygrometer (FISH) and CR-2 frost point hygrometer (Buck Research Instruments L.L.C., US), both operated by Research Centre Jülich). For the initial water calibration of the instrument it was compared against a dew point mirror (Dewmet TDH, Michell Instruments Ltd., UK) in the range from 70 000 to 25 000 ppm water vapor mole fraction. During the inter-comparison campaign the analyzer was compared on the ground over the range from 2 to 600 ppm against the dew point hygrometer used for calibration of the FISH reference instrument. A new, independent calibration method based on the dilution effect of water vapor on CO2 was evaluated. Comparison of the in-flight data against the reference instruments showed that the analyzer is reliable and has a good long-term stability. The flight data suggest a conservative precision estimate for measurements made at 0.4 Hz (2.5 s measurement interval) of 4 ppm for H2O < 10 ppm, 20 % or 10 ppm (whichever is smaller) for 10 ppm < H2O < 100 ppm, and 5 % or 30 ppm (whichever is smaller) for H2O > 100 ppm. Accuracy of the CRDS instrument was estimated, based on laboratory calibrations, as 1 % for the water vapor range from 25 000 ppm down to 7000 ppm, increasing to 5 % at 50 ppm water vapor. Accuracy at water vapor mole fractions below 50 ppm was difficult to assess, as the reference systems suffered from lack of data availability.

Measurement uncertainty of coulometric trace humidity sensors

Abstract Especially trace amounts of water vapour in gases can be reliably determined by coulometric trace humidity sensors. The principle of these sensors is based on water vapour absorption in a hygroscopic layer and its subsequent electrolytic decomposition. The calibration of sensors was performed in the humidity range, expressed as frost point temperature, from − 30 ° C -30\hspace{0.1667em}^\circ \mathrm{C} to − 80 ° C -80\hspace{0.1667em}^\circ \mathrm{C} . This range is equivalent to volume fractions smaller than 376 µL·L−1. Generated humidity was measured with coulometric sensors and a chilled dew point hygrometer that was used as reference. An empirical non-linear function was found between sensor signal and measured reference humidity. This function consists of two parameters with a measurement uncertainty. Both calibration parameters were checked by means of one-way analysis of variance. It showed that gas specific function can be used for humidity measurement in nitrogen, hydrogen, dinitrogen monoxide, compressed and synthetic air. It is possible to determine trace humidity in all tested gases with an expanded uncertainty less than 2.1 K (coverage factor k = 2 k=2 ) regarding frost point temperature.

Coulometric trace humidity measurement in technical gases.

Trace humidity was measured by using miniaturized planar coulometric sensors in technical gases such as hydrogen, nitrogen, helium, nitrous oxide, and synthetic air. Frost point temperatures in the gases ranged from -60 °C to -30 °C, which is equivalent to a vapour mole fraction from 10 μmol mol-1 to 376 μmol mol-1. In addition, the generated humidity was determined by using a precision dew point hygrometer as reference. Nonlinear calibration functions were calculated that correlated electric current (sensor signal) and reference humidity. Parameters of functions were tested with one-way analysis of variances (ANOVA) to prove if all used sensors had a similar behavior in the same gas during experiments. Results of ANOVA confirmed that averaged functions can be applied for trace humidity measurement in nitrogen, helium, nitrous oxide, and synthetic air. The calculated functions were negligibly different for nitrogen, helium, and synthetic air. In humidified nitrous oxide, a minor change of parameters was observed due to lower electrical currents. In total contrast to that, the measured sensor signals were significantly higher in humidified hydrogen and each sensor required its own calibration function. The reason was a recombination effect that favoured multiple measurements of water molecules. Nevertheless, it was possible to measure continuously trace humidity in all tested gases by using coulometric sensors with an expanded uncertainty below 2 K (k = 2).

Multilayer inkjet printed dew point hygrometer

This paper describes the dew point temperature hygrometer as a complex microsystem manufactured using inkjet printing directly on the surface of a Peltier couple heat pump. The principle of the structure’s fabrication is similar to that of other sensors printed directly on textiles, paper or thin foils, but is much more difficult to perform. Both the principle of specific detector construction as well as some technological aspects are discussed in the paper. All three elements of the detector i.e. thermoresistor, heater and impedance dew detector are widely characterized to ascertain the performance of each. Final tests of the whole dew point hygrometer operation parameters are carefully reported and discussed in the conclusions.

Coagulation Detection Method Based on the Image Information Entropy from a New Microscopic Dew-point Hygrometer

It is difficult to accurately identify coagulation timing and phase of coagulation for dew-point hygrometer. In this paper, an imaging dew-point device is developed and a method is presented to extract textural feature of the information entropy of the mirror image of dew-point instrument according to image texture characteristics of different types of mirror. The method of detection and identification of condensation of dew-point instrument based on texture was designed. Mirror images are first extracted the textural feature using the gray scale symbiotic matrix. The information entropy of the image is extracted, and then the characteristics of entropy of different types of images were analyzed. The entropy value threshold of different types of images was established and tested. It is shown that we are able to achieve an average detection accuracy rate of 97.8% for different types of mirrors. It is a great solution combined with the continuous observation method of the detection and identification of dew-point condensation.

New Submersed Chamber for Calibration of Relative Humidity Instruments at HMI/FSB-LPM

This paper gives a detailed description of a new chamber designed for calibration of relative humidity (RH) instruments at Laboratory for Process Measurement (HMI/FSB-LPM). To the present time, the calibrations of RH instruments at the HMI/FSB-LPM were done by comparison method using a climatic chamber of large volume and calibrated dew point hygrometer with an additional thermometer. Since 2010, HMI/FSB-LPM in cooperation with Centre for Metrology and Accreditation in Finland (MIKES) developed the two primary dew point generators which cover the dew point temperature range between \(-\,70~{^{\circ }}\hbox {C}\) and \(60~{^{\circ }}\hbox {C}\). In order to utilize these facilities for calibrations of the RH instruments, the new chamber was designed, manufactured and installed in the existing system, aiming to extend its range and reduce the related calibration uncertainties. The chamber construction allows its use in a thermostatic bath of larger volume as well as in the climatic chambers. In the scope of this paper, performances of the new chamber were tested while it was submersed in a thermostated bath. The chamber can simultaneously accommodate up to three RH sensors. In order to keep the design of the chamber simple, only cylindrical RH sensors detachable from display units can be calibrated. Possible optimizations are also discussed, and improvements in the design proposed. By using the new chamber, HMI/FSB-LPM reduced the expanded calibration uncertainties (level of confidence 95 %, coverage factor \(k=2\)) from 0.6 %rh to 0.25 %rh at 30 %rh \((23~{^{\circ }}\hbox {C})\), and from 0.8 %rh to 0.53 %rh at 70 %rh \((23~{^{\circ }}\hbox {C})\).

Improving the Traceability of Meteorological Measurements at Automatic Weather Stations in Thailand

A joint project between the National Institute of Metrology Thailand (NIMT) and the Thai Meteorology Department (TMD) was established for improving the traceability of meteorology measurements at automatic weather stations (AWSs) in Thailand. The project aimed to improve traceability of air temperature, relative humidity and atmospheric pressure by implementing on-site calibration facilities and developing of new calibration procedures. First, new portable calibration facilities for air temperature, humidity and pressure were set up as working standard of the TMD. A portable humidity calibrator was applied as a uniform and stable source for calibration of thermo-hygrometers. A dew-point hygrometer was employed as reference hygrometer and a platinum resistance thermometer (PRT) traceable to NIMT was used as reference thermometer. The uniformity and stability in both temperature and relative humidity were characterized at NIMT. A transportable pressure calibrator was used for calibration of air pressure sensor. The estimate overall uncertainty of the calibration setup is 0.2 K for air temperature, 1.0 % for relative humidity and 0.2 hPa for atmospheric pressure, respectively. Second, on-site calibration procedures were developed and four AWSs in the central part and the northern of Thailand were chosen as pilot stations for on-site calibration using the new calibration setups and developed calibration procedures. At each station, the calibration was done at the minimum temperature, average temperature and maximum temperature of the year, for air temperature, 20 %, 55 % and 90 % for relative humidity at the average air temperature of that station and at a one-year statistics pressure range for atmospheric pressure at ambient temperature. Additional in-field uncertainty contributions such as the temperature dependence on relative humidity measurement were evaluated and included in the overall uncertainty budget. Preliminary calibration results showed that using a separate PRT probe at these AWSs would be recommended for improving the accuracy of air temperature measurement. In case of relative humidity measurement, the data logger software is needed to be upgraded for achieving higher accuracy of less than 3 %. For atmospheric pressure measurement, a higher accuracy barometer traceable to NIMT could be used to reduce the calibration uncertainty to below 0.2 hPa.

산화 알루미늄 및 냉각거울 노점계의 온도 의존성에 관한 연구

The measurement of absolute humidity of gases is essential in many industries. The effect of temperature on aluminum oxide and chilled mirror dew-point hygrometers is investigated. The temperature of laboratory, pipe line, and sensor is varied and the dew point is measured by two different aluminum oxide hygrometers. In all cases, the dew point of hygrometers is increased as the temperature is elevated. The reason behind this observation is due to desorption of water from the inside of pipe line and/or sensor surroundings at elevated temperature that result in the increase of the absolute humidity. Moreover, the sensor itself shows a certain degree of temperature dependency in sensing the humidity especially at low temperature. It is also studied that chilled mirror dew-point hygrometer may indicate a higher dew point than the reference at high temperature because the cooling capability of mirror is decreased at high temperature. Our study will provide evidences in the incorporation of the temperature effect as uncertainty factors in the standard calibration procedure for dew point hygrometers.

Verification of Alternative Dew Point Hygrometer for CV-LRT in Monju

Verification of Alternative Dew Point Hygrometer for CV-LRT in Monju

An ultrahigh-accuracy Miniature Dew Point Sensor based on an Integrated Photonics Platform

The dew point is the temperature at which vapour begins to condense out of the gaseous phase. The deterministic relationship between the dew point and humidity is the basis for the industry-standard “chilled-mirror” dew point hygrometers used for highly accurate humidity measurements, which are essential for a broad range of industrial and metrological applications. However, these instruments have several limitations, such as high cost, large size and slow response. In this report, we demonstrate a compact, integrated photonic dew point sensor (DPS) that features high accuracy, a small footprint, and fast response. The fundamental component of this DPS is a partially exposed photonic micro-ring resonator, which serves two functions simultaneously: 1) sensing the condensed water droplets via evanescent fields and 2) functioning as a highly accurate, in situ temperature sensor based on the thermo-optic effect (TOE). This device virtually eliminates most of the temperature-related errors that affect conventional “chilled-mirror” hygrometers. Moreover, this DPS outperforms conventional “chilled-mirror” hygrometers with respect to size, cost and response time, paving the way for on-chip dew point detection and extension to applications for which the conventional technology is unsuitable because of size, cost, and other constraints.

Novel dew point hygrometer fabricated with inkjet printing technology

This paper reports a novel, first time made Dew Point Hygrometer (DPH) detector, which was fully fabricated with the inkjet printing technology. This type of the sensor operation was already known in DPH which is based on a silicon complex dew detector (Jachowicz and Senturia, 1981; Weremczuk et al., 2006) [1,2]. However, in our device the silicon substrate has been replaced by a thin, flexible Kapton foil. All components integrated in the detector: the interdigitated capacitive sensor electrodes, the thermoresistor and the heater were made by ink-jet printing technology. All individual elements of the printed dew point hygrometer detector have been characterized, and finally, their measurement features were described.

Aggregate size effect on the water retention properties of a lime-treated compacted silt during curing

The main drying paths of the water retention curves of lime-treated soils were measured. Aggregate size effect on the water retention property was emphasised. Four soil powders with different maximum aggregate sizes (Dmax = 5, 2, 1 and 0.4 mm) were prepared and mixed with 2% quicklime (by weight of dry soil). Samples were prepared by static compaction at dry side of optimum water content (w = 17%) with a dry density of 1.65 Mg/m3. Suction measurement was performed by a dew-point hygrometer at different curing periods (t = 7, 28 and 90 days). The results obtained show that: i) aggregate size effect is insignificant on the water retention curve of untreated soil in the studied suction range (ca 500 kPa ~ 55 MPa); ii) lime treatment gradually improves the soil water retention capacity with the increasing of curing time; iii) aggregate size effect on water retention property of lime-treated soil becomes significant in the suction range from ca 860 kPa to 9 MPa on the long curing term: treated soils prepared with smaller aggregate size (S0.4 and S1) have a higher water retention capacity rather than soils prepared with larger aggregate size (S5).