A New Primary Dew-Point Generator at TUBITAK UME

An implementation of a new low-range primary humidity generator as a part of an international collaboration between TUBITAK UME and VTT MIKES was initiated as a EURAMET Project Number 1259. The dew-point generator was designed and constructed within the scope of the cooperation between TUBITAK UME and VTT MIKES in order to extend the dew-point temperature measurement capability of Humidity Laboratory of TUBITAK UME down to ??80 °C. The system was thoroughly characterized and validated at TUBITAK UME to support the evidence for dew-point temperature uncertainties. The new generator has a capability of operating in the range of ??80 °C to +10 °C, but at the moment, it was characterized down to ??60 °C. The core of the generator system is a saturator which is fully immersed in a liquid bath. Dry air is supplied to the saturator through a temperature-controlled pre-saturator. The operation of the system is based on the single-pressure generation method with a single pass, i.e., the dew-point temperature is only controlled by the saturator temperature, and the humidity-controlled air is not returned to the system after leaving of the saturator. The metrological performance of the saturator was investigated thoroughly at both National Metrology Institutes. The pre-saturator was also tested using a thermostatic bath at VTT MIKES prior to sending them to TUBITAK UME. This paper describes the principle and design of the generator in detail. The dew-point measurement system and the corresponding uncertainty analysis of the dew-point temperature scale realized with the generator in the range from ??60 °C to 10 °C is also presented.     

Performance and Validation Tests on the NIST Hybrid Humidity Generator

A new humidity generator has been constructed at the National Institute of Standards and Technology. Once fully operational, the NIST hybrid humidity generator (HHG) will generate frost/dew points from −70°C to +85°C using calibration gas-flow rates up to 150 standard liters per minute and is expected to outperform the present humidity generator at NIST 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. For dew/frost-point temperatures above −15°C, the two-pressure principle is employed. For frost points at or below −15°C, the water-vapor/air mixture is produced by mixing metered streams of moist air produced by the two-pressure method with purified, dry air. A series of performance and validation tests on the HHG in the two-pressure mode, including measurements of temperature gradients and pressure stability in the generator under various operating conditions, and comparison of the humidity generated by the HHG to that generated by the other NIST humidity-generation standards, are reported.     

The New KRISS Low Frost-Point Humidity Generator

A new low frost-point humidity generator (LFPG) has been designed, and its performance has been tested, in order to extend the calibration capabilities to the low frost-point range at KRISS. The water vapor–gas mixture is generated by saturating air with water vapor over a surface of an ice-coated saturator under the conditions of constant temperature and pressure. This LFPG covers a range of frost point from  − 99 °C to  − 40 °C. The temperature of the saturator, which is controlled by thermoelectric devices and a two-stage mechanical refrigeration system, is stable within 5 mK, and the difference between the saturator temperature and the frost point generated at the saturator outlet is less than 20 mK. This stability is achieved by using oxygen-free high-conductivity copper materials as the saturator body, and applying a precision PID temperature control system. The performance of this new LFPG system is compared with the KRISS standard two-temperature generator in the frost-point range ( − 80 to  − 40) °C, and its performance is tested with a quartz crystal microbalance (QCM), which was built at KRISS, to  − 91 °C.     

Design and Validation of the MBW Standard Humidity Generators

MBW Calibration AG (MBW) is the Designated Institute (DI) for humidity appointed by the Federal Institute of Metrology, METAS. MBW currently offers calibration and measurement capabilities (CMC) for frost/dew-point hygrometers by comparison with precision chilled-mirror transfer standards that have been calibrated using the primary standards of leading European National Metrology Institutes or DI. The design, construction and validation of two standard humidity generators to be used as the Swiss national standards for the primary realization of frost/dew-point temperature in the range from ? 90 °C to + 95 °C are presented and discussed. The generators are operated as continuous flow “single-pressure” generators in the range from ? 80 °C to ? 10 °C with saturation over ice and from 0.5 °C to + 95 °C with saturation over water. Additionally, they are used in “two-pressure” mode for saturation over ice down to frost-point temperatures of ? 90 °C and down to ? 20 °C for saturation over water. The main saturators of both generators have been designed to fit in commercially available calibration baths with either ethanol or distilled water as the heat transfer fluid for saturator temperatures below and above 0 °C, respectively. Saturator temperature is measured using standard platinum resistance thermometers and a purpose-built precision thermometer. Pressure measurements are taken with gauge pressure transducers and a separate barometric sensor, to reduce the influence of the atmospheric pressure on the measurement of the pressure ratio and make full use of the correlation of pressure measurements and enhancement factors when operating in two-pressure mode. A totally automated pre-saturation and flow control system facilitates the calibration of state-of-the-art chilled-mirror transfer for standards without manual readjustment of the generated flowrate to ensure a constant volumetric flow at the conditions of the mirror. The uncertainty budget leading to the CMC for frost/dew-point temperature realization is presented in the context of the experimental validation performed. The results in the overlapping range of both generators are presented and used as further evidence of the saturation efficiency of both standards.     

Intercomparison of the Dew-Point Temperature Realizations at LPM and MIKES in the Range from ?70?°C to +?20?° C

The first European humidity key comparison EURAMET-T.K6 was completed in 2008, and it covered the dew-point temperature range from ?50?°C to +?20?°C. Both LPM and MIKES participated in the comparison, but a new low dew-point generator was introduced at LPM as a result of progress in the EUROMET P912 project. To extend the range of available comparison evidence down to ?70?°C and to study the validity of improved uncertainties of LPM, a bilateral comparison was carried out between LPM and MIKES in 2009?C2010. The applied comparison procedure was similar to that applied in EURAMET-T.K6. However, only one transfer standard was used instead of two units and the measurement point ?70?°C was added in the measurement scheme. The results show that the bilateral equivalence between LPM and MIKES is between (0.00 ± 0.06)?°C and (0.02 ± 0.08)?°C in the range from ?50?°C to +?20?°C and (0.01 ± 0.10)?°C at ?70?°C. Using MIKES results as the link to the EURAMET.T-K6, it is shown that the difference between the results obtained with the new LPM dew-point temperature standard and the EURAMET Comparison Reference Values is between (?0.02 ± 0.08)?°C at 20?°C and (+?0.02 ± 0.07)?° C at ?50?°C.     

Saturator Efficiency and Uncertainty of NMIJ Two-Pressure Two-Temperature Humidity Generator

The primary dew-point standard of National Metrology Institute of Japan (NMIJ) over the dew-point range of  −10 °C to 95 °C is a humidity generator based on the two-pressure two-temperature method. In this generator, the dew-point temperature of generated air is calculated by using the pressure and temperature, assuming that the air in the saturator is in equilibrium with liquid water. Therefore, the evaluation of the degree of saturation of water vapor in the saturator is important. In this study, the saturation efficiency of the NMIJ two-pressure two-temperature humidity generator has been re-evaluated. The NMIJ humidity generator has a presaturator that consists of a water bath and a bubbling element that can supply water vapor to the airflow into the main saturator. The amount of water vapor in the air output from the PS is altered by changing the PS temperature. The dew-point temperatures of the generated air were measured by a chilled-mirror hygrometer under various conditions of PS pressure and temperature. The saturator efficiency of the generator has been evaluated from the relationship between the measured dew-point temperature and the PS temperature. When the temperature of the PS was lower than that of the saturator, the amount of water in the air was insufficient to achieve saturation. When the temperature of the PS was slightly higher than that of the saturator, saturation was obtained.     

Improvement of the wettability of SiMn IF-HSS by liquid zinc by controlling the dew point of the annealing gas atmosphere

The wettability of low-carbon, 0.3 wt%Si–0.4 wt%Mn interstitial-free steel by liquid zinc at 450 °C was investigated using the dispensed sessile drop method. Before the wetting tests, the steel samples were annealed in a 15%H₂–Ar gas atmosphere at three different dew points, namely −60, −40, and 0 °C. It was found that as the dew point was increased from −60 to −40 °C, the wettability became poorer. However, as the dew point was increased further to 0 °C, the wettability was dramatically improved and was better than that of −60 °C. In order to understand the dramatic change in wettability, the surfaces of the steel samples after annealing were analyzed with SEM and TEM. It was found that the surface oxide changed from randomly distributed hemisphere particles of 20–30-nm high on a very thin oxide film to a film-like layer ~15-nm thick as the dew point was increased from −60 to −40 °C, and at the dew point of 0 °C, internal oxidation was so pronounced that a very thin surface oxide layer 1–2-nm thick was formed. It was believed that the improvement of the wettability at the dew point of 0 °C was caused by the short diffusion distance in the surface oxide layer.     

Low-Pressure and Low-Temperature Dew/Frost-Point Generator

Lately demands for traceability and improved accuracy in climatic humidity measurements at extreme conditions have been increasing. To address these needs a new dew/frost-point generator was developed and constructed at the Centre for Metrology MIKES of the VTT Technical Research Centre of Finland Ltd. for the range from ??90 °C to +?15 °C. The generator operates in a single-pressure principle at the absolute air pressure range from 10 hPa to 3500 hPa. With an expansion valve it can be applied also in two-pressure mode. Within this paper the new low-pressure and low-temperature dew/frost-point generator and its characterization are presented in detail. The outcomes of the characterization show that when operating in a single-pressure mode within barometric pressure range a standard uncertainty of 0.043 °C is achieved in complete temperature range. When considering the complete pressure range the standard uncertainty is larger, but it does not exceed 0.06 °C.     

New Primary Dew-Point Generators at HMI/FSB-LPM in the Range from ?70?°C to +60?°C

To extend the dew-point range and to improve the uncertainties of the humidity scale realization at HMI/FSB-LPM, new primary low- and high-range dew-point generators were developed and implemented in cooperation with MIKES, in 2009 through EUROMET Project No. 912. The low-range saturator is designed for primary realization of the dew-point temperature scale from ?70?°C to +?5?°C, while the high-range saturator covers the range from 1?°C to 60?°C. The system is designed as a single-pressure, single-pass dew-point generator. MIKES designed and constructed both the saturators to be implemented in dew-point calibration systems at LPM. The LPM took care of purchasing and adapting liquid baths, of implementing the temperature and pressure measurement equipment appropriate for use in the systems, and development of gas preparation and flow control systems as well as of the computer-based automated data acquisition. The principle and the design of the generator are described in detail and schematically depicted. The tests were performed at MIKES to investigate how close both the saturators are to an ideal saturator. Results of the tests show that both the saturators are efficient enough for a primary realization of the dew-point temperature scale from ?70?°C to +?60?°C, in the specified flow-rate ranges. The estimated standard uncertainties due to the non-ideal saturation efficiency are between 0.02?°C and 0.05?°C.     

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:

The New LMK Primary Standard for Dew-Point Sensor Calibration: Evaluation of the High-Range Saturator Efficiency

In the field of hygrometry, a primary dew-point standard can be realized according to several proven principles, such as single-pressure (1-P), two-pressure (2-P), or divided flow. Different realizations have been introduced by various national laboratories, each resulting in a stand-alone complex generation system. Recent trends in generator design favor the single-pressure principle without recirculation because it promises theoretically lower uncertainty and because it avoids problems regarding the leak tightness of the recirculation. Instead of recirculation, the efficiency of saturation, the key factor, is increased by preconditioning the inlet gas entering the saturator. For preconditioning, a presaturator or purifier is used to bring the dew point of the inlet stream close to the saturator temperature. The purpose of the paper is to identify the minimum requirements for the preconditioning system and the main saturator to assure efficient saturation for the LMK generator. Moreover, the aim is also to find out if the preconditioning system can be avoided despite the rather simple construction of the main saturator. If this proves to be the case, the generator design can be simplified while maintaining an accurate value of the generated dew point. Experiments were carried out within the scope of improving our existing primary generator in the above-ambient dew-point range up to +70°C. These results show the generated dew point is within the measurement uncertainty for any dew-point value of the inlet gas. Thus, the preconditioning subsystem can be avoided, which leads to a simplified generator design.     

Uncertainty Budget for the NIST Hybrid Humidity Generator

A detailed uncertainty budget for the new hybrid humidity generator (HHG) that has been constructed at the National Institute of Standards and Technology is presented. The HHG generates frost/dew points from ?70?°C to +85?°C using calibration gas flows up to 150 L?·? min^?1. For frost/dew points above ?15?°C, the two-pressure method is employed, and for frost points at or below ?15?°C, the divided-flow method is used (hence, the name ??hybrid??). The total expanded (k?=?2) uncertainty is estimated for HHG generation of the following quantities: frost/dew point, mole fraction, and relative humidity. The total uncertainty is estimated separately for the two-pressure and divided-flow methods.     

Engaging Frost Formation in a Chilled-Mirror Hygrometer

Precise humidity measurements conducted with a chilled-mirror hygrometer (CMH) in the frost-point range require the determination of the condensate phase on the sensor’s mirror. Undetected supercooled water (SCW) on the surface of the mirror can lead to an error in frost point reading of up to 2K or more. A recent trend in CMH design is implementation of supercooled water detection or elimination abilities. Elimination of SCW can also be performed manually by cooling, though with some concerns regarding the reliability of the process and the reproducibility of the frost-point measurements. The purpose of this article is to present the possible automation of this process without compromising the instrument, especially in the case of calibration standards lacking elimination ability, when a long and valuable calibration history cannot be jeopardized. At the same time, the purpose of this investigation is also to study the reliability of automation and the effect of the process on the reproducibility of frost point. A special control device was developed to upgrade the instrument’s control loop without electrically compromising the instrument itself. In order to assess the SCW elimination process by cooling and to determine its optimal parameters, comparisons were conducted against the primary dew-point generator to obtain a stable environment. Our experiments have proven the reliability of the automation and the reproducibility of the frost point was within 0.012°C.     

Validation of Primary Water Dew-Point Generator for Methane at Pressures up to 6 MPa

In this paper, the validation of the water dew-point generator with methane as a carrier gas in the temperature range from \(-41\,^{\circ }\hbox {C}\) to \(+15\,^{\circ }\hbox {C}\) and at pressures up to 6 MPa is reported. During the validation, the generator was used with both nitrogen and methane to investigate the effect of methane on the generator and the chilled mirror dew-point meters. The effect of changing the flow rate and the dew-point temperature of the gas entering the generator, on the gas exiting the generator was investigated. As expected, methane at high pressures created hydrates in combination with water and low temperatures, thus limiting the temperature range of the generator to \(+8\,^{\circ }\hbox {C}\) to \(+15\,^{\circ }\hbox {C}\) at its maximum operating pressure of 6 MPa. A lower operating pressure extended the temperature range; for example, at 3 MPa, the temperature range was already extended down to \(-15\,^{\circ }\hbox {C}\) , and at 1 MPa, the range was extended down to \(-41\,^{\circ }\hbox {C}\) . The validation showed that, in its operating range, the generator can achieve with methane the same standard uncertainty of \(0.02\,^{\circ }\hbox {C}\) frost/dew point already demonstrated for nitrogen and air carrier gases.     

The Humidity Calibration Facility of the National Metrology Institute of South Africa (NMISA)

The NMISA Humidity Laboratory maintains the South African national measurement standards for humidity and provides traceability for commercial calibration laboratories. The laboratory performs both relative humidity and dew-point calibrations. The laboratory has been accredited since January 2004 for both parameters by the South African National Accreditation System (SANAS). Industries such as the pharmaceutical industry, power stations, and the food industry make use of the calibration services provided by the laboratory. The facilities and the measurement capabilities of the laboratory are presented in this paper. Two calibration methods are used for the calibration of relative humidity measuring instruments (RH meters). Depending on the type of instrument, they are calibrated against aqueous salt solutions or by comparison with reference RH meters. Measurements are performed over the humidity range 5–95 %rh (Best Measurement Capabilities: 0.4–1.4 %rh). Two chilled-mirror hygrometers are used as reference standards for dew-point calibrations over the range from −75°Cfp to +17°Cdp (Best Measurement Capabilities: 1.5°Cdp to 0.4°Cdp). The laboratory has participated in two international interlaboratory comparisons, one for each accredited parameter. The NMISA Humidity Laboratory is working to reduce its calibration uncertainties for relative humidity instruments: since first accredited in 2004, the best measurement capabilities have been improved from 2.1 %rh to 0.4 %rh for some humidity ranges.     

Thermal expansion and theoretical density of 2,2′,4,4′,6,6′-hexanitrostilbene

The linear coefficient of thermal expansion (CTE) and the theoretical density are important for energetic materials. To obtain the CTE and theoretical density of 2,2′,4,4′,6,6′-hexanitrostilbene (HNS), X-ray powder diffraction (XRD) together with Rietveld refinement was employed to estimate the dimension and density change at a crystal lattice level, in the range of temperature 30–240 °C. The CTE of a-, b-, c-axis and volume were obtained as 7.6719 × 10⁻⁵/°C, 6.8044 × 10⁻⁵/°C, 1.1192 × 10⁻⁵/°C and 16.725 × 10⁻⁵/°C, respectively. Also, the possible reasons for the expansion property of HNS have been discussed by comparing its structure with 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Based on the refined lattice parameters, the theoretical densities of HNS at various temperatures were obtained. By extrapolation of linear fitting the theoretical density of HNS at 20 °C was gotten as 1.7453 g/cm³. Furthermore, a good thermal resilience of HNS has also been observed when the temperature returned from 240 to 30 °C.     

Results of comparisons of a laser interferometric oil manometer at VNIIM with the national standards of Finland

Results of comparisons of a laser interferometric oil manometer developed at the Mendeleev Institute (VNIIM), which reproduces the unit of pressure over the range 1–1000 Pa, with the national standards at the Metrology and Accreditation Center of Finland (MIKES) are described. The comparisons confirm the metrological characteristics of the VNIIM manometer and show that the size of the pressure unit reproduced by it agrees with the Finnish national standard.     

ITS-90 Scale Realization on the New Radiation Thermometer Calibration Facility at NMi VSL

In the first half of 2005, Nederlands Meetinstituut Van Swinden Laboratorium B.V. (NMi VSL) redesigned their facilities for radiation thermometry in a new laboratory building and an opportunity arose to implement new measurement methods. The new facility is used for ITS-90 realization and dissemination in the temperature range from  − 50 °C to 3,000 °C. A study was performed to compare a silver-point realization with a fixed-point blackbody radiator (FP-BBR) to a sodium heat-pipe blackbody radiator (HP-BBR) traceable via a HTSPRT to a contact thermometry silver point. It was found that the fixed-point realization transfer to the sodium heat pipe results in an uncertainty from 0.2 K to 2.4 K for the ITS-90 over the temperature range from 961.78 °C to 3,000 °C.     

Compressive deformation behavior of ternary compound Cr<Subscript>2</Subscript>AlC

The compressive properties of ternary compound Cr₂AlC at different temperatures and strain rates were studied. When tested at a strain rate of 5.6 × 10⁻⁴ s⁻¹, the compressive strength decreases continuously from 997 ± 29 MPa at room temperature to 523 ± 7 MPa at 900 °C. The ductile-to-brittle transition temperature is measured to be in the range of 700 to 800 °C. When tested in the strain rate range of 5.6 × 10⁻⁵ to 5.6 × 10⁻³ s⁻¹, Cr₂AlC fails in a brittle mode at room temperature, whereas the deformation mode changes from a brittle to a ductile as the strain rate is lower than 5.6 × 10⁻⁴ s⁻¹ when compressed at 800 °C. The compressive strength increases slightly with increasing strain rate at room temperature and it is less dependent on strain rate when tested at 800 °C. The plastic deformation mechanism of Cr₂AlC was discussed in terms of dislocation-related activities, such as kink band formation, delamination, decohesion of grain boundary, and microcrack formation.     

Fabrication of dense β-calcium orthophosphate with submicrometer-sized grains and its high-temperature superplastic deformation

High-density β-calcium orthophosphate (β-Ca₃(PO₄)₂, also called β-tricalcium phosphate: β-TCP) ceramics with submicrometer-sized grains were fabricated using a pulse-current pressure firing route. The maximum relative density of the β-TCP compacts was 98.7% at 1050 °C and this was accompanied by a translucent appearance. The mean grain size of the β-TCP compacts increased slightly with temperature to reach 0.78 μm at 1000 °C. However, upon further increasing the firing temperature to 1050 °C the mean grain size increased significantly to 1.6 μm. The extent of plastic deformation during tensile testing was examined at temperatures between 900 and 1100 °C using a strain rate in the range 9.26 × 10⁻⁵ to 4.44 × 10⁻⁴ s⁻¹. The maximum tensile strain achieved was 145% for a test temperature of 1000 °C and strain rate of 1.48 × 10⁻⁴ s⁻¹ and this was attributed to the relatively high density and small grain size.