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.     

宽温压力可变标准湿度发生器的研制

采用双温双压法原理研制了一套宽温度范围、测试室压力可变的标准湿度发生器。它能够测量发生温度范围为-50～90 ℃、相对压力范围为-50～0 kPa、扩展不确定度为0.3%RH～0.8%RH（k=2）的5%RH～96%RH全量程相对湿度。其测试室尺寸为150 mm×400 mm,样气流量为5～30 L/Min。通过设计高精度饱和器压力和流量自动调节装置,采用工控机与PLC相结合的操作控制方式,实现了全自动运行。冷镜式精密露点仪和重量法湿度计的实际测试结果均验证了其不确定度。     

A Simple Humidity Generator for Relative Humidity Calibrations

At present, the South African national humidity measurement standards, maintained by NMISA, consist of two chilled-mirror hygrometers operating from (−75 to 20) °Cdp and unsaturated salt solutions covering the range (5 to 95) %rh. To reduce measurement uncertainties and to obtain traceability to local measurement standards for temperature and pressure, it is desired to replace the salt solutions by a dew-point generator for relative humidity calibrations from (5 to 95) %rh (5 to 60) °C, equivalent to a dew-point range of (−30 to 59) °Cdp. Required uncertainties in relative humidity (at a coverage factor of k = 2) are (0.1 to 1.2) %rh. This article describes the design and evaluation of a two-pressure humidity generator intended to satisfy this requirement. The saturator consists of a coil of stainless steel tubing immersed in a 70 l stirred water bath. Pressure reduction is accomplished using a throttling valve adjusted by a stepper motor, and hygrometers being calibrated are sealed into a small test chamber contained in a larger temperature-controlled chamber. The results of the following performance tests are presented:

Realization of relative humidity scale from 10% to 98% at 25°C

The accurate measurements of relative humidity are increasingly needed in most of the Egyptian industries. The relative humidity has its own effect in the manufacturing of medicine, steel, metals, textile and is of great importance for controlling the environmental conditions in the factories. As a result of the above facts, it was found that the realization of Egyptian humidity scale is of prime importance for National Institute for Standards (NIS). The scale was realized using two-pressure humidity generator and humidity chamber as a medium, chilled mirror hygrometer to measure dewpoint and RTD to measure air temperature. The relative humidity scale was realized in the range from 10% to 92% using two-pressure humidity generator [1] and from 92% to 98% using humidity chamber at 25°C. All points of results were stable for more than 6 hours with stability range from 0.18% to 0.28%. The uncertainty was evaluated at a confidence level of 95% within 1.1% to 1.8% in the range from 10% to 92% and from 92% to 98%, respectively. The present measurement system is suitable for accurate calibration of different digital or chart hygrometers.     

A Humidity Generator for Temperatures up to 200?°C and Pressures up to 1.6?MPa

A new humidity generator that produces gas streams of known moisture content at temperatures from 85?°C to 200?°C, absolute pressures from 0.2?MPa to 1.6?MPa, and relative humidities from 10 % to 90 % has been developed. The generator produces a moist gas stream by injecting fixed-rate streams of dry gas and liquid water into a vaporizer, where the water evaporates into the gas. The gas stream passes into a re-entrant radio-frequency (RF) cavity, which serves as our reference hygrometer, and then a test chamber. The present standard uncertainty of the RF hygrometer is 0.6 %, limited by the uncertainty of literature values for the polarizability of water. Dry nitrogen gas purging the pressure transducer line also combines with the moist gas stream downstream of the test chamber and flows through one of a set of capillaries. Modulation of gas flow through the fixed flow impedance of the capillary gives a simple method for controlling pressure. Individual insulated, temperature-controlled aluminum ovens enclose each major component. A larger oven encloses these ovens and their connecting tubing. To minimize corrosion, critical components are constructed of high-nickel alloys. The small total volume (<1?L) and small flow rate (<0.5 L·min^?1) reduce operational hazards from steam scalding or from gas explosion.     

Reducing the Uncertainty of Industrial Trace Humidity Generators through NIST Permeation-Tube Calibration

Permeation-tube moisture generators (PTGs) are commonly used by the semiconductor industry as transfer standards for the calibration of hygrometer systems measuring trace amounts of water vapor in gases (water vapor mole fractions typically below 1 × 10⁻⁶). They are relatively simple devices that generate a steady stream of humidified gas by diluting water vapor delivered at a constant rate from a permeable capsule with precisely metered purified gas, usually nitrogen. Here a new calibration service enabling the measurement of PTG permeation rates directly in terms of NIST primary standards of trace humidity generation is described. Rather than using commonly employed gravimetric methods for permeation-tube calibration, the method applied here links the permeation rate of the permeation tube to the thermodynamic properties of ice. Using a hygrometer based on cavity ringdown spectroscopy, we compare the water vapor concentrations produced by the NIST low frost-point generator (LFPG) and a specially constructed PTG containing the permeation tube undergoing calibration. A least squares fit of the data determines the permeation rate of the tube under test. We describe the calibration system, experimental procedure and present sample calibration data. The expanded relative uncertainty of NIST permeation-tube calibrations is 1.8% with a coverage factor k = 2, dominated by the Type A uncertainties.     

温湿度检定箱的湿度均匀度与波动度的测试方法

在温湿度检定箱的湿度均匀度与波动度测试中,针对现有湿度传感器准确度难以满足要求的情况,提出将同种型号规格的湿度传感器在同一台湿度标准装置上进行校准、并加修正值使用的测量方法,减小了测量不确定度,满足湿度均匀度和波动度的测量要求。精密露点仪测量结果验证了该方法的有效性。     

Self‐Powered Wearable Electronics Based on Moisture Enabled Electricity Generation

Most state‐of‐the‐art electronic wearable sensors are powered by batteries that require regular charging and eventual replacement, which would cause environmental issues and complex management problems. Here, a device concept is reported that can break this paradigm in ambient moisture monitoring—a new class of simple sensors themselves can generate moisture‐dependent voltage that can be used to determine the ambient humidity level directly. It is demonstrated that a moisture‐driven electrical generator, based on the diffusive flow of water in titanium dioxide (TiO₂) nanowire networks, can yield an output power density of up to 4 µW cm^?2 when exposed to a highly moist environment. This performance is two orders of magnitude better than that reported for carbon‐black generators. The output voltage is strongly dependent on humidity of ambient environment. As a big breakthrough, this new type of device is successfully used as self‐powered wearable human‐breathing monitors and touch pads, which is not achievable by any existing moisture‐induced‐electricity technology. The availability of high‐output self‐powered electrical generators will facilitate the design and application of a wide range of new innovative flexible electronic devices.     

An experimental verification of saturated salt solution-based humidity fixed points

The high instability and hysterisis of the relative humidity sensors currently available on the market render necessary simple and economic calibration methodologies that can be used as a secondary of working standards. The chemical equilibrium-type systems based on saturated aqueous salt solutions, even though simple and economical, are not always metrologically satisfactory for calibration. They can, in fact, be unreliable, when some fundamental requirements are neglected: also, unacceptable discrepancies continue to exist in the equilibrium relative humidity reference data of saturated aqueous salt solutions furnished by both literature and standards. To highlight the factors that increase the reliability of calibrations with saturated aqueous salt solutions, the authors of this paper have redetermined the equilibrium relative humidity reference data of 11 saturated aqueous salt solutions at ambient pressure and temperature. The solutions chosen were the ones generally used as fixed points to obtain a relative humidity calibration scale.     

Atmosphere generation system for the preparation of ambient air volatile organic compound standard mixtures

A dynamic generation system based on permeation for the preparation of volatile organic compounds at ambient air concentration levels is described herein. The performance of the equipment, sources of uncertainties, and overall uncertainty value are also evaluated. The system is capable of generating multicomponent mixtures at ppb levels and simulating different atmospheric conditions by changing the humidity, concentration level, temperature, and wind velocity. A minimized value for the overall uncertainty in the concentration generated by the system was determined to be ±1.9% for higher weighing time intervals of 4 weeks.     

A digital hygrometer

A digital hygrometer using a polyimide capacitive humidity sensor is developed. The capacitance change of the sensor due to adsorption of water vapor in the atmosphere is detected by a switched-capacitor digital capacitance bridge controlled by a one-chip microcomputer and is displayed as relative humidity with 0.1% resolution. The accuracy of the hygrometer calibrated by a two-point method is solely determined by the temperature dependence and the long-term drift of the dielectric sensitivity of the sensor and is estimated to be 2%     

Uncertainty of the Kriss Low Frost-Point Humidity Generator

The KRISS low frost-point humidity generator (LFPG), was developed in 2006 for the frost-point range (?95 to ?40)°C in order to extend calibration capabilities. In this paper, the evaluation of the generator??s uncertainty budget is reported for which each uncertainty component was categorized and estimated by experiment and calculation. The uncertainty of the LFPG depends on the generated frost point, gas flow rate, and change of moisture concentration in transportation. The standard uncertainty of LFPG is less than 32 mK in the frost-point range from ?70?°C to ?40?°C. However, in the lower frost-point range, the uncertainty increases to 137 mK at ?90?°C, and this is mainly due to water adsorption or desorption in the transportation tubing from saturator to hygrometer.     

双压法湿度发生器校准结果的不确定度评定

给出了用于现场的双压法湿度发生器校准结果的不确定度评定方法,不确定度来源主要是测试用的精密露点仪、精密铂电阻以及湿度发生器自身.文中结合实际测试数据给出了具体的分析     

被动式液压冲击波形发生器动态特性的数学建模与仿真

基于液压节流耗能原理，提出一种被动式液压波形发生器，其设计思想是采用缝隙和小孔节流产生的阻尼来吸收消耗高速运动负载的动能，并对其紧急制动以获得设定的加速度波形；缸体上开设的可调节流孔能方便地实现对缓冲过程阻尼力的调节。通过对波形发生器内部流体动态特性的分析，建立了波形器工作机理的数学模型，其数值仿真结果表明波形发生器具有以下特点：①缓冲制动效果好；②加速度波形调节简单；③适用于动量变化大、无法用主动方式控制冲击波形的场合，尤其适合作为模拟水下爆炸环冲击境的双波冲击试验机加速度负波的产生装置。     

Inter-laboratory Comparison of Impedance-Type Hygrometer in the Range from 10 % to 95 % at 5\,^{\circ }\hbox {C} to 55\,^{\circ }\hbox {C}

Traceability in the field of relative humidity (RH) measurements is typically assured indirectly through dew point and temperature scales. Conducting an inter-laboratory comparison at the national metrology institute (NMI) level, using a direct approach with a precision RH hygrometer as a transfer standard would, therefore, be of a particular interest, especially if the measurement setups were of a different type. This paper presents an RH comparison at the NMI level between the National Metrology Institute of South Africa (NMISA) and University of Ljubljana, Faculty of Electrical Engineering, Laboratory of Metrology and Quality (MIRS/UL-FE/LMK). In scope of this inter-comparison, calibration of an impedance-type hygrometer in the range from 10 %rh to 95 %rh at air temperatures of \(5\,^\circ \hbox {C}\) , \(25\,^\circ \hbox {C}\) , and \(55\,^\circ \hbox {C}\) , respectively, was performed. It was recommended that the participants use their standard procedure for the calibration of RH sensors and, at the same time, follow the specific criteria of the review protocol for uncertainty estimation accepted by Bureau International des Poids et Mesures (BIPM), marked as BIPM CCT-WG8/CMC-10. An interesting part of the comparison was the two different calibration methods which were used by the two partners and which also have different traceability routes. MIRS/UL-FE/LMK calibrated the sensor in the humidity generator by comparison against the reference chilled mirror hygrometer, which is traceable to the MIRS/UL-FE/LMK primary dew-point generator. NMISA calibrated the transfer standard against certified salt solutions, which were kept in a temperature-controlled chamber. Results showed acceptable agreement at all 15 calibration points.     

湿度测量及校准方法

介绍了湿度测量和校准方法及其相互关系.强调指出,正确使用饱和盐法、分流法、双温法、双压法湿度发生装置对于保证湿度检测校准至关重要.     

Practical Study of Psychrometer Calibrations

Psychrometers remain the most widely used instruments for controlling the humidity in climatic test chambers, yet the calibration of these instruments is particularly challenging. Psychrometer calibrations require careful consideration of influence variables such as the fitting and cleanliness of the wick, the effect of the calibration chamber on the air flow past the sensors, on radiation incident on the sensors, and on the dissipation heat from the built-in fan (if included). In addition, uncertainty requirements for calibration of such psychrometers are typically around 1?%rh to 2?%rh, i.e., close to the best calibration and measurement uncertainties (CMCs) claimed by national metrology institutes (NMIs). As well as their role in supporting CMCs, inter-comparisons provide a good test-ground to ensure all influence variables are controlled or otherwise accounted for in the uncertainty budget. This paper presents the results of a comparison of psychrometer calibrations performed by the NMIs in Denmark, Slovenia, and Finland. The comparison was carried out under EURAMET Project No. 1033 with the aim to investigate the equivalence of psychrometer calibrations performed at the highest level and to gather practical experience to be used in similar comparisons in the future. An aspirated electro-psychrometer was used for the comparison, and calibrations were carried out in the range from 15 %rh to 93?%rh in a temperature range from 15?°C to 70?°C. While the results show good agreement at high relative humidity, significant differences at low relative humidity are reported. It is suggested that the differences are caused by a combination of psychrometer wick contamination and a difference in the wick-wetting methods used by the participant laboratories.     

Measurement Uncertainty of Dew-Point Temperature in a Two-Pressure Humidity Generator

This article describes the measurement uncertainty evaluation of the dew-point temperature when using a two-pressure humidity generator as a reference standard. The estimation of the dew-point temperature involves the solution of a non-linear equation for which iterative solution techniques, such as the Newton?CRaphson method, are required. Previous studies have already been carried out using the GUM method and the Monte Carlo method but have not discussed the impact of the approximate numerical method used to provide the temperature estimation. One of the aims of this article is to take this approximation into account. Following the guidelines presented in the GUM Supplement 1, two alternative approaches can be developed: the forward measurement uncertainty propagation by the Monte Carlo method when using the Newton?CRaphson numerical procedure; and the inverse measurement uncertainty propagation by Bayesian inference, based on prior available information regarding the usual dispersion of values obtained by the calibration process. The measurement uncertainties obtained using these two methods can be compared with previous results. Other relevant issues concerning this research are the broad application to measurements that require hygrometric conditions obtained from two-pressure humidity generators and, also, the ability to provide a solution that can be applied to similar iterative models. The research also studied the factors influencing both the use of the Monte Carlo method (such as the seed value and the convergence parameter) and the inverse uncertainty propagation using Bayesian inference (such as the pre-assigned tolerance, prior estimate, and standard deviation) in terms of their accuracy and adequacy.     

Uncertainty quantification models for micro‐scale squeeze‐film damping

Two squeeze‐film gas damping models are proposed to quantify uncertainties associated with the gap size and the ambient pressure. Modeling of gas damping has become a subject of increased interest in recent years due to its importance in micro‐electro‐mechanical systems (MEMS). In addition to the need for gas damping models for design of MEMS with movable micro‐structures, knowledge of parameter dependence in gas damping contributes to the understanding of device‐level reliability. In this work, two damping models quantifying the uncertainty in parameters are generated based on rarefied flow simulations. One is a generalized polynomial chaos (gPC) model, which is a general strategy for uncertainty quantification, and the other is a compact model developed specifically for this problem in an early work. Convergence and statistical analysis have been conducted to verify both models. By taking the gap size and ambient pressure as random fields with known probability distribution functions (PDF), the output PDF for the damping coefficient can be obtained. The first four central moments are used in comparisons of the resulting non‐parametric distributions. A good agreement has been found, within 1%, for the relative difference for damping coefficient mean values. In study of geometric uncertainty, it is found that the average damping coefficient can deviate up to 13% from the damping coefficient corresponding to the average gap size. The difference is significant at the nonlinear region where the flow is in slip or transitional rarefied regimes. Copyright © 2010 John Wiley & Sons, Ltd.     

A hygrometer comprising a porous silicon humidity sensor with phase-detection electronics

A novel hygrometer is presented, comprising a capacitive humidity sensor with a porous silicon (PS) dielectric and electronics. The adsorption of water vapor by the PS layer leading to change of its effective dielectric constant is modeled with an effective medium approximation (EMA). A simple, but precise, phase-sensitive electronic circuit has been developed. This detects any change of phase of a sinusoidal signal transmitted through the PS dielectric and correlates to ambient humidity. It is outlined how the nonlinear response of the sensor is compensated through piecewise linearization. The sensor is tested in combination with the phase detection circuitry. Excellent linearity over the entire range of relative humidity is achieved. Experimental results show a resolution better than 0.1% and an accuracy of 2% (near the transition region) and better than 0.1% (otherwise). The response time is less than 10 s with good stability.