Two-pressure Humidity Generator Research Articles

AN APPLICATION FOR THE CALIBRATION OF THE TWO-PRESSURE HUMIDITY GENERATOR

Two-pressure humidity generator (2-PHG) has capability to produce environments of known humidity with the “two pressure” method that is proven by NIST. The 2-PHG is supplied with known humidity values, the values can be used for instrument calibration, verification and such as other processes. The set point values can be entered from the device’s front panel by an operator. 2-PHG calculates the relative humidity from the pressure and temperature measurements with the formula. The computer controls the ratios on the formula to generate a known humidity value. 2-PHG has to have traceability for using on calibration processes of other devices; 2-PHG must be calibrated. This study represents an application of the 2-PHG calibration by using the guides and articles in the literature about the device. Calibration of the 2-PHG contains examination of the each parameter of the formula that was given for humidity calculation. Four temperature and two pressure probes that are saturation temperature probe, pre-saturation temperature probe, expansion valve temperature probe, chamber temperature probe, low pressure probe, high pressure probe help 2-PHG to calculate the formula. Calibration of the 2-PHG, also, contains the calibration of the temperature probes and the pressure probes. This study expresses an application of the calibration of a 2-PHG which is located in Turkish Standard Institution Gebze Calibration Laboratories and the uncertainty calculation of this application can be seen on this paper. Each uncertainty parameter of the formula and uncertainty parameters of the probes that are affected to the total uncertainty were calculated. For different temperatures and different relative humidities, the measurement uncertainty of this application was given on this paper. This study can be used as a supplementary document in the calibrations of the 2-PGH and in the new studies.

Advanced dew-point control system in two-pressure humidity generator to stabilize dew-point generation in low dew-point range

We developed a novel dew-point control system [advanced dew-point control system (ADCS)] in a primary standard humidity generator based on the two-pressure technique. In the ADCS, the actual vapor–liquid equilibrium temperature of water was estimated for the dew-point control, and the pressure inside the main saturator was controlled by adjusting the variable opening of an expansion valve. Using the ADCS, the periodic variation (oscillation) in the dew point observed in the low dew-point range was greatly reduced. The stability of dew-point generation was improved by 23-fold.

Photoacoustic hygrometer for icing wind tunnel water content measurement: design, analysis, and intercomparison

Abstract. This work describes the latest design, calibration and application of a near-infrared laser diode-based photoacoustic (PA) hygrometer developed for total water content measurement in simulated atmospheric freezing precipitation and high ice water content conditions with relevance in fundamental icing research, aviation testing, and certification. The single-wavelength and single-pass PA absorption cell is calibrated for molar water vapor fractions with a two-pressure humidity generator integrated into the instrument. Laboratory calibration showed an estimated measurement accuracy better than 3.3 % in the water vapor mole fraction range of 510–12 360 ppm (5 % from 250–21 200 ppm) with a theoretical limit of detection (3σ) of 3.2 ppm. The hygrometer is examined in combination with a basic isokinetic evaporator probe (IKP) and sampling system designed for icing wind tunnel applications, for which a general description of total condensed water content (CWC) measurements and uncertainties are presented. Despite the current limitation of the IKP to a hydrometeor mass flux below 90 gm-2s-1, a CWC measurement accuracy better than 20 % is achieved by the instrument above a CWC of 0.14 g m−3 in cold air (−30 ∘C) with suitable background humidity measurement. Results of a comparison to the Cranfield University IKP instrument in freezing drizzle and rain show a CWC agreement of the two instruments within 20 %, which demonstrates the potential of PA hygrometers for water content measurement in atmospheric icing conditions.

Accurate and precise dew-point control system based on active pressure control in two-pressure humidity generator.

We developed an accurate and precise system for controlling dew points by using an active pressure control system in a primary standard humidity generator. By using this system, an adjustment capability to a target value of 0.0008 °C was attained for dew-point generation, compared with the previous value of approximately 0.01 °C. In a two-pressure humidity generator, the drift in the dew point with time owing to a rapid drop in the water level in the presaturator and to fluctuations in atmospheric pressure was greatly suppressed. The standard deviation of the dew point obtained with our dew-point control system was 10 times smaller than the drift observed in the absence of the system. This dew-point control system makes it possible to calibrate humidity sensors at target values with greater efficiency.

Testing the reliability of humidity generator through measurements traceable to calibration standards

This paper describes the metrological performance of NIS two-pressure humidity generator for testing the generator reliability after working for 13 years. In this work, humidity & temperature are observed and the measuring system is prepared for the calibration of humidity & temperature measurements. In the generator test chamber, relative humidity measurements are performed between 10% and 95% at fixed temperature of 25 °C and temperature measurements are performed between 10 °C and 65 °C at fixed humidity of 50% relative humidity.Results showed that the uniformity value is between ±0.13 °C and ±0.25 °C for temperature and between ±0.46% and ±0.62% for relative humidity. The value of stability is between 0.005 °C and 0.04 °C for temperature and 0.03% to 0.08% for relative humidity. Homogeneity value is in range between 0.09 °C and 0.12 °C for temperature, and between 0.43% and 0.52% for relative humidity, these satisfactory results showed the good performance of the generator. The expanded uncertainty of a coverage factor k = 2 was found to be from ±0.2 °C to ±0.32 °C for temperature and ±0.8% to ±0.96% for relative humidity. The obtained results give us the confidence that NIS two-pressure humidity generator can be used as a high accurate humid air generator and reference instrument, for calibrating all types of dew-point meters and humidity sensors with high accuracy.

Temperature effect in the calibration of capacitive humidity sensors

Capacitive hygrometers are widely used in industrial and environmental measurements. One of the limitations of these sensors is their temperature dependence. While in temperatures beyond 50 °C the effect has been proven to be significant, for standard conditions in calibration laboratories the magnitude of this dependence has not been rigorously studied. This paper presents the analysis and results of a study of the temperature influence in the calibration of capacitive hygrometers designed for monitoring environmental conditions. Ninety measurements were taken on 15 models of capacitive hygrometers using a two-pressure humidity generator between 30%rh and 80%rh and temperatures from 20 to 30 °C. A three-way analysis of variance (ANOVA) was used to evaluate the data. The results show that the calibration correction is independent of the temperature for the conditions considered; however, there is a correlation between the temperature and the devices, indicating that the gradients in temperature affect each technology of fabrication in a different way.

Calibration of Heat Stress Monitor and its Measurement Uncertainty

Wet-bulb globe temperature (WBGT) equation is a heat stress index that gives information for the workers in the industrial areas. WBGT equation is described in ISO Standard 7243 (ISO 7243 in Hot environments—estimation of the heat stress on working man, based on the WBGT index, ISO, Geneva, 1982). WBGT is the result of the combined quantitative effects of the natural wet-bulb temperature, dry-bulb temperature, and air temperature. WBGT is a calculated parameter. WBGT uses input estimates, and heat stress monitor measures these quantities. In this study, the calibration method of a heat stress monitor is described, and the model function for measurement uncertainty is given. Sensitivity coefficients were derived according to GUM. Two-pressure humidity generators were used to generate a controlled environment. Heat stress monitor was calibrated inside of the generator. Two-pressure humidity generator, which is located in Turkish Standard Institution, was used as the reference device. This device is traceable to national standards. Two-pressure humidity generator includes reference temperature Pt-100 sensors. The reference sensor was sheltered with a wet wick for the calibration of natural wet-bulb thermometer. The reference sensor was centred into a black globe that has got 150 mm diameter for the calibration of the black globe thermometer.

NIS two-pressure humidity generator

There are several techniques available to generate humidity references by using stream of saturated gas with well-known water vapor content. Two-pressure humidity generator (2-P) is the most commonly used for the realization of temperature and relative humidity scales at National Institute for standards (NIS) and for disseminating traceability. This generator has been shown to be highly reproducible when properly used. In order to obtain traceable measurements and best performance, the temperature sensors and pressure gauges of this generator should be calibrated. This paper explained the calibration of 2-P temperature sensors and pressure gauges and the analysis of their performance for the first time at NIS.

Characterization of KIM LIPI’s Relative Humidity Standard

At present, KIM LIPI is able to provide self-traceability in the scope of hygrometer calibrations in the range of 10–95 %RH using commercial two-pressure humidity generator as the standard which, previously, was traceable to SI unit through another NMI. In order to examine the calibration system, uncertainty of the calibration results was evaluated by performing characterization including measurement of saturator efficiency and chamber temperature distribution. A mini chamber was used to improve the chamber temperature uniformity. The validity of the RH calculation was evaluated by comparing our RH calculation with commercially available softwares. As a result, saturator efficiency gives the uncertainty 0.064 and 0.07 %RH at 20 and 27 °C. The contribution of uncertainty due to temperature non-uniformity is 0.014–0.14 %RH. The difference calculation with the commercial software is less than 0.01 %RH.

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–Raphson 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–Raphson 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.

Microwave Determination of Water Mole Fraction in Humid Gas Mixtures

A small volume (65 cm3) gold-plated quasi-spherical microwave resonator has been used to measure the water vapor mole fraction x w of H2O/N2 and H2O/air mixtures. This experimental technique exploits the high precision achievable in the determination of the cavity microwave resonance frequencies and is particularly sensitive to the presence of small concentrations of water vapor as a result of the high polarizability of this substance. The mixtures were prepared using the INRIM standard humidity generator for frost-point temperatures T fp in the range between 241 K and 270 K and a commercial two-pressure humidity generator operated at a dew-point temperature between 272 K and 291 K. The experimental measurements compare favorably with the calculated molar fractions of the mixture supplied by the humidity generators, showing a normalized error lower than 0.8.

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.

Design and Performance of the New NIST Hybrid Humidity Generator

Abstract:A new humidity generator has been constructed at the National Institute of Standards and Technology and is now fully operational. The NIST Hybrid Humidity Generator (HHG) has replaced the Two-Pressure (2-P) Humidity Generator Mark II as the NIST primary humidity generation standard for frost/dew points from −70 °C to +25 °C using calibration gas flows up to 150 standard liters per minute. The HHG extends the NIST humidity generation range up to 85 °C, and outperforms the 2-P Generator in terms of accuracy. The HHG combines the two-pressure and divided-flow humidity generation techniques (hence the name “hybrid”). The centerpiece of the HHG is a heat-exchanger/saturator that is immersed in a temperature-controlled bath stable to within 1 mK. A precisely regulated pre-saturation process minimizes sensible and latent heat loading on the final saturator. For dew/frost point temperatures above −15 °C, the two-pressure principle is employed. For frost points at or below −15 °C, the divided-flow method ...

Bilateral Comparison of Humidity Standards Between UME and MIKES

In order to assure the quality of dew-point measurements, a bilateral comparison between the National Metrology Institute of Turkey (UME) and the Centre for Metrology and Accreditation (MIKES), Finland, was carried out in 2006. This comparison is registered as EUROMET Project No. 906. Two chilled-mirror hygrometers (MBW 373 and MBW 373H) were used to compare the UME two-pressure humidity generator with the MIKES dew/frost-point generator. An MBW 373 was used in the range from a frost point of −20 °C to a dew point of +20 °C and an MBW 373H in the range from a frost point of −20 °C to a dew point of +60 °C. Dew-point temperature measurements were performed at six dew-point temperatures in the range from a frost point of −20 °C to a dew point of +60 °C at UME and MIKES. According to the results, the mean difference between the laboratories varies from (−0.11 to +0.07) °C over the range. The estimated expanded uncertainty (k = 2) varies from (0.13 to 0.28) °C. A detailed description of the equipment, results and uncertainty analysis is presented in this paper.

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: (i) comparison of the output when the air stream was oversaturated before entry to the saturator to that when it was dry, (ii) comparison with a chilled-mirror hygrometer from (−25 to 20) °Cdp, and (iii) comparison with relative humidity hygrometers that had previously been calibrated against salt solutions.

Miniaturized Two-Pressure Generator for Relative Humidity

The concept and design of a miniaturized two-pressure humidity generator are presented. The generator is suitable for achieving relative humidity ranging from 10% to 95% with uncertainties of under 1% at ambient temperature and can be used for the calibration of relative humidity instruments in the laboratory and on site. By virtue of the concept, the relative humidity achieved is traced to only two pressure measurements. Further references are not necessary. A special adjustment algorithm enables standard industrial pressure sensors to be used. The two pressure sensors are synchronized by aligning their readings at ambient pressure. The resulting correlation of the uncertainties leads to very small systematic errors in the humidity display. Each humidity generator can still be individually adjusted through comparison with a chilled-mirror hygrometer, the total uncertainty of the generator being largely determined by the uncertainty of this hygrometer. Any drift of the pressure sensors that occurs can be compensated at any time by performing an alignment at ambient pressure without changing the individual adjustment of the generator. It can be demonstrated that the uncertainty of the displayed humidity remains practically unchanged over the course of a year by virtue of this alignment process.

Sensor calibration of a SAW resonator for absolute humidity measurements in microelectronic packages

This paper describes sensor calibration of a SAW resonator using the NIST two-pressure humidity generator as the reference standard. It involves saturating a continuous steam of air with vapor from pure water in the saturator at a constant pressure and temperature. The saturated air then flows through an expansion valve where it is isothermally expanded into a test chamber at a lower pressure, usually atmospheric pressure. The SAW sensor is connected to an outlet of the test chamber. Accurate measurements of the pressure and temperature within the saturator and the SAW sensor are made to determine the water-vapor content of the air stream in the sensor. Changes in resonance frequency of the sensor are measured as a function of water-vapor concentration in moist air in the range 2000–7000 ppm v, corresponding to absolute humidity of water-vapor density of 1.4–4.8 g m −3.

The NBS Two-Pressure Humidity Generator, Mark 2

A new humidity calibration facility which uses the two-pressure principle for generating gas of known humidity has been developed at NBS for calibrating and testing hygrometers. The relative humidity range of the two-pressure humidity generator is 3 to 98 percent for ambient temperatures −60 ° to 80 °C and test chamber pressures 5 to 200 kPa (absolute). This is equivalent to a nominal dew/frost point range of −80 ° to 80 °C. Intercomparison tests were made with the NBS standard gravimetric hygrometer over a portion of the generator’s operating range. The estimated maximum uncertainty (three standard deviations) is 0.2 percent RH for temperatures 0 ° to 80 °C which in units of dew point corresponds to an estimated maximum uncertainty of 0.04 °C for dew points −35 ° to 80 °C.

A two-pressure humidity generator for calibrating electrical hygrometers used in meteorology

A two-pressure humidity generator has been developed in the Meteorological Office for calibrating electrical hygrometers. The generator incorporates two parallel saturating systems which can be held at different pressures, allowing the relative humidity and dewpoint in the test chamber to be changed in less than 10 s when experiments on the lag of instruments are performed. The system also incorporates an automatic dewpoint hygrometer and thermometer which is used to check the calculated dewpoint and relative humidity in the test chamber.