Vapor pressure measurements over supercooled water in the temperature range from −10 1 °C to +10 −2 °C

An accurate measurement of saturation vapor pressure of supercooled water is a strong challenge in metrology, mainly due to difficulties concerning keeping water at a liquid state at temperatures well below the melting point; thus few experimental data covering limited temperature ranges (down to about 253 K) are reported in literature. For this reason, an investigation of the water vapor – supercooled water equilibrium along the saturation line is carried out at Istituto Nazionale di Ricerca Metrologica (INRIM). Measurements cover the temperature range from 261.26 K to 273.25 K, corresponding to a saturation vapor pressure from about 244 Pa to 611 Pa. The experimental apparatus includes a borosilicate glass sample cell, kept in a liquid bath at a constant temperature with millikelvin stability and connected to a manifold where the pressure is measured using a capacitive diaphragm pressure gauge. In this work, the water sample preparation, the measuring method and measurement corrections are reported; moreover, a comparison between experimental and literature data is conducted along with the most used vapor pressure formulations. Measurement results are discussed and uncertainty sources estimated. The resulting expanded relative uncertainty (k = 2) varies from 0.085% at 261.26 K to 0.039% at 273.25 K

Investigations on perturbations of microwave dielectric resonator thermometer

Investigations of antenna probe length, antenna-dielectric distance, cavity filling and humidity on microwave resonator thermometer with respect to Q, spurious mode depression, coupling strength, accuracy, shock resistance or sensitivity were carried out in order to improve the dielectric resonator thermometer performance. Significant improvement of Q and depression of spurious mode coupling were obtained when the antenna length was reduced. It also turns out that the Q and spurious mode coupling strength vary with the distance between dielectric and antenna pin, as well under appropriate antenna length. Filling the cavity with nitrogen increases coupling strength but decrease frequency-temperature sensitivity compared to a vacuum-pumped cavity. Besides, preliminary results on the microwave resonator sensitivity to air humidity were obtained

Assessment of the INRIM trace water generator and analysis of the uncertainty components down to -100 °C frost-point temperature

A low frost-point generator (INRIM 03) able to operate at sub-atmospheric pressure has been recently designed, constructed, and assessed at the Istituto Nazionale di Ricerca Metrologica (INRiM) with the aim of providing the metrological traceability both to instruments developed for the measurement of humidity in atmosphere and to sensors and analysers used in industry for controlling and measuring the amount of water vapour in manufacturing processes. The humidity generator operates in a single temperature single pressure mode, letting the carrier gas (nitrogen) achieve saturation in a single passage through an isothermal saturator. Its working range encompasses a frost-point temperature range from -100 °C to -20 °C, in a pressure range between 200 hPa and 1100 hPa, corresponding to an amount of water fraction range from 13·10-9 mol·mol-1 to 6.2·10-3 mol·mol-1. In a previous work its performance was assessed in the frost-point temperature range from -75 °C to -20 °C [2]. In this work, a comprehensive set of tests for its characterisation and performance evaluation between -75 °C and -100 °C is presented. A detailed uncertainty analysis in the above temperature range is reported, taking into account all the sources of uncertainty that affect the humid gas generation. An expanded uncertainty (k=2) of 0.07 °C was found for frost-point temperature measurements between -75 °C and -95 °C, while an expanded uncertainty of 0.26 °C resulted at a frost-point temperature of -100 °C. The relative expanded uncertainty (k=2) associated with water vapour amount fraction measurements was estimated equal to or better than 1.2 % between 35·10-9 mol·mol-1 and 6.1·10-3 mol·mol-1, increasing up to 6.5 % at 13∙10-9 mol·mol-1

Whispering Gallery Mode Resonators for Precision Temperature Metrology Applications

In this work, the authors exploited the whispering gallery mode (WGM) resonator properties as a thermometer. The sensor is made of a cylindrical sapphire microwave resonator in the center of a gold-plated copper cavity. Two coaxial cables act as antennas and excite the WGM standing waves in the cylindrical sapphire at selected resonance frequencies in the microwave range. The system affords a high quality factor that enables temperature measurements with a resolution better than 15 mu K and a measurement standard uncertainty of 1.2 mK, a value approximately three times better than that achieved in previous works. The developed sensor could be a promising alternative to platinum resistance thermometers, both as a transfer standard in industrial applications and as an interpolating instrument for the dissemination of the kelvin

Development of a low-frost point generator operating at sub-atmospheric pressure

A low frost point generator (INRIM 03) operating at sub-atmospheric pressure has been designed and constructed at the Istituto Nazionale di Ricerca Metrologica (INRiM) as part of a calibration facility for upper-air sounding instruments. This new humidity generator covers the frost point temperature range between -99 °C and -20 °C and works at any controlled pressure between 200 hPa and 1100 hPa, achieving a complete saturation of the carrier gas (nitrogen) in a single passage through a stainless steel isothermal saturator. The generated humid gas contains a water vapour amount fraction between 14bold dot10-9 mol/mol and 5bold dot10-3 mol/mol. In this work the design of the generator is reported together with characterisation and performance evaluation tests. A preliminary validation of the INRIM 03 against one of the INRIM humidity standards in the common region is also included. Basing on experimental test results, an initial uncertainty evaluation of the generated frost-point temperature, Tfp, and water vapour amount fraction, xw, in the limited range down to -75 °C at atmospheric pressure is reported. For the frost-point temperature, the uncertainty budget yields a total expanded uncertainty (k=2) of less than 0.028 °C, while for the mole fraction the budget yields a total expanded uncertainty of less than 10-6 mol/mol

Application of plate heat exchangers to the development of portable high-pressure humid gas generators

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.The preliminary design and testing of a portable, high-pressure humidity generator based on a corrugated-plate heat exchanger (PHE) is described. It resulted in a very simple system, consisting of a bubbling pre-saturator followed by the PHE used as a condenser. The PHE does not need to be hosted in a liquid bath, resulting in a very compact design whose operation is largely independent from a specific bath - which may be available on site – used to condition the temperature of a circulating counter-flow fluid. Experimental tests were performed on a prototype in order to demonstrate the principle and validate at an early stage the system operation. The efficiency of the PHE as condenser was assessed over a temperature range from 0 °C to 65 °C by varying input and influence parameters, such as the dew point temperature, the gas flow rate and its pressure up to 0.5 MPa. The saturation temperature of the humid gas flow compared favourably with the dew-point temperature as measured by a chilled-mirror hygrometer calibrated against a primary humidity standard. The temperature deviations were below 0.02 °C, with a maximum deviation of 0.04 °C, over the whole range of investigated temperatures and pressures.cf201

Development and preliminary investigation of a modular chamber for calibration of relative humidity instruments

In the scope of the project HUMEA – Expansion of European research capabilities in humidity measurement within the EURAMET EMPIR program, the modular chamber for calibration of relative humidity instruments was designed, manufactured and characterized. The modular chamber consists of arbitrary numbers of aluminum blocks each of which provides accommodation for the one relative humidity probe and also has the fittings for pressure and temperature probes as well as ports for gas sampling and/or supplying. The gas can be supplied from the dew/frost point generator or the larger climatic chamber. In the latter case, the airflow through the chamber can be enhanced by using an additional fan. The preliminary study was carried out to investigate the improvement in temperature uniformity using a new chamber in combination with two climatic chambers. The investigation results show significant improvement in temperature uniformity thus lowering the uncertainties of the calibration of relative humidity instruments