Improved method for pressure measurement in saturation chamber of primary dew/frost point generators Danijel Sestan, Lovorka Grgec-Bermanec, Davor Zvizdic Croatian Metrology Institute/Faculty of Mechanical Engineering and Naval Architecture, Laboratory for Process Measurement (HMI/FSB-LPM), Ivana Lucica 5, 10000 Zagreb, Croatia article info Article history: Received 7 May 2018 Received in revised form 16 September 2018 Accepted 17 September 2018 Available online 27 September 2018 Keywords: Humidity generators Saturation pressure Primary dew/frost-point realization abstract This paper describes two methods for measurement of air pressure inside the saturation chambers of two primary dew/frost point generators, used as national humidity standards at Laboratory for process mea- surement (HMI/FSB-LPM) in Croatia. Together, the generators cover the dew/frost point temperature range from À70 °C to + 65 °C. Low range generator operates at air temperatures between À70 °C and 5 °C, while high range covers the range between 1 °C and 65 °C. The saturation pressures considered are in the range between atmospheric and 1060 hPa, at the air flow rates between 1.0 L min À1 and 2.5 L min À1 . The dew/frost point of the saturated air depends strongly on the saturation pressure, making improve- ment in this kind of measurements of great importance for humidity realizations. In the old method, the precise digital barometer is used for determination of saturated air pressure inside the saturation chamber. The barometer is traceable to the pressure balance, which is used as a national standard for pressure in Croatia. In the new method, the pressure was maintained directly by the primary pressure balance, achieving this way the significant improvement of related measurement uncertainties for both, pressure measurement and the dew/frost point realization. The paper will give a detailed description of both measurement setups as well as corresponding measurement uncertainty analyses. During the experiments it was also observed that in the second setup, balance stabilized the pressure oscillations as well as oscillations of the dew-point temperature measured by the chilled mirror hygrometer under calibration, making indirect improvement of related measurement uncertainties. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction The metrological system in Croatia consists of Croatian Mea- surement Institute (HMI) that has distributed architecture, coordi- nating, supporting, and representing all national standards on an international level. The national standards for temperature, pres- sure and humidity are located at the Laboratory for Process Mea- surement (LPM) which is part of the Faculty of Mechanical Engineering and Naval Architecture (FSB) at the University of Zagreb. In 2009, HMI/FSB-LPM in cooperation with national metrology institute of Finland (MIKES) designed the primary low- and high- range dew-point saturators in order to extend the existing dew- point range and to improve the uncertainties of the humidity scale realization in Croatia. The low-range saturator is used in the dew/ frost-point temperature range between 70 °C and 5 °C, while the high-range saturator covers the range from 1 °C to 60 °C. While pri- mary generators among national institutes use several basic oper- ating principles [1–7], HMI/FSB-LPM adopted design of a single- pressure, single-pass dew-point generation for both the saturators. In such a system, sample gas passes through a saturator only once, and its dew-point temperature is controlled only by controlling the temperature of the liquid bath which accommodates the saturator. Although the saturators were initially designed for air as the sam- ple gas, in this work the nitrogen supplied from the cylinder was also used. To put the saturators in operation, the HMI/FSB-LPM imple- mented the temperature, and pressure measurement equipment to the system developed the gas preparation and flow control and made the computer-based automated data acquisition software. The measurement of pressure is performed inside the satura- tion chamber and at the sensor of the instrument being calibrated. Since the saturated air flows through the tubes connecting the sat- urator and the instrument under calibration, a certain pressure drop occurs. The difference in pressures inside the saturation chamber and at the calibrated instrument reflects on the difference in corresponding dew/frost point temperatures, requiring a correc- https://doi.org/10.1016/j.measurement.2018.09.051 0263-2241/Ó 2018 Elsevier Ltd. All rights reserved. E-mail address: lovorka.grgec@fsb.hr (L. Grgec-Bermanec) Measurement 133 (2019) 162–167 Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement