Martti Heinonen

A humidity generator with a test chamber system

Abstract For the calibration of relative humidity hygrometers and dew-point hygrometers with an open sensor, a test chamber system was constructed and connected to a dew-point generator. The system includes several small chambers connected in series. By reversing the flow direction, humidity and temperature gradients in the test chamber system are monitored during calibration measurements. This paper reports the construction of the test chamber system and the improvements in the construction and the analysis of the dew-point generator. A full uncertainty analysis is presented. The expanded uncertainty of the dew-point temperature and the relative humidity with the coverage factor k =2 are from ±0.05°C to ±0.08°C and from ±0.1%rh to ±1.0%rh, respectively. Although the dew-point temperature range is from −40°C to +77°C, the limits for the temperature range of the test chamber system are −20°C and +60°C. The system was not tested with relative humidity values lower than 10% or higher than 95%.

The CMA humidity standard

Abstract A two-temperature humidity generator has been developed at the Centre for Metrology and Accreditation (CMA) in order to have a reliable humidity standard. The system has been tested and the analysis of uncertainty has been carried out. The disturbance of the saturation equilibrium, the accuracy of temperature measurements, pressure differences, leaks, desorption and contamination have been investigated. The measurement range of the system is from −40 to +60°C with an uncertainty of less than ±0.25°C (k=2). The system was found suitable to be used as the CMA humidity standard.

Study of the PM Gas-Phase Filter Artifact Using a Setup for Mixing Diesel-Like Soot and Hydrocarbons

The filter artifact is a significant source of error in gravimetric measurements of particulate matter (PM) exhaust. However, only a few studies on the subject exist. Results from these studies show a large discrepancy mainly because the experiments were performed using real diesel vehicle exhaust with varying exhaust composition. In this study, a setup for mixing diesel-like soot and hydrocarbon vapor was constructed for generating a stable exhaust aerosol with adjustable composition. The particle size distribution of the diesel-fueled soot generator (GMD [geometric mean diameter] adjustable between 27 and 164 nm) was found to represent “real” exhaust particulate emission. This setup was applied for studying the filter artifact on Teflon-coated glass fiber filters using pentadecane as the hydrocarbon vapor. Experiments were performed using particle and hydrocarbon concentrations of 130–700 μg/m3 and 10–12 ppm, respectively. It was found that the particle concentration of the aerosol affects the filter artifact. At lower particle concentrations, more hydrocarbon adsorption was detected. In the absence of particles, the adsorption was highest. Furthermore, filter soot load, corresponding to 0.13%–0.66% of the clean filter mass, was found to affect adsorption. Sooty filters adsorbed less vapor than clean filters. However, increasing the soot load resulted in more adsorption. Moreover, it was found that the backup filter serves as a reasonable estimate of the filter artifact only for low particle concentrations and filter soot loads. These results indicate that the filter soot load is an important parameter influencing the filter artifact, and therefore, it should be considered when performing gravimetric sampling. The setup was proven to be a unique tool for quantitative studies of the filter artifact. Copyright 2012 American Association for Aerosol Research

Validating the single charged aerosol reference (SCAR) as a traceable particle number concentration standard for 10 nm to 500 nm aerosol particles

Measurement of nanometre-sized aerosol particles is based on particle number concentration measurements. The commonly used method for providing traceability for these measurements involves charging and electrical counting of aerosol particles. This method requires that the particles are singly charged or that the average charge is exactly known, neither of which is easy to ensure. In the device called a single charged aerosol reference (SCAR), the fraction of multiply charged particles is minimal due to the novel operating principle of electrical charging and subsequent growth. In this study the SCAR was validated as a primary particle number concentration standard. The average charge of the output aerosol was evaluated for the whole operational particle size range. For this, the effect of the size distribution of the primary nanoaerosol and the output number concentration on the fraction of doubly charged and neutral particles was measured. It was found that the uncertainty caused by assuming singly charged particles is only 0.16%. A full uncertainty analysis was carried out for a condensation particle counter (CPC) calibration. According to the results, the relative expanded uncertainty of calibration was 3.0%. This represents a typical uncertainty level achieved in CPC calibrations performed with SCAR. As a result of this study, SCAR was validated as a particle number concentration standard suitable for traceable calibration of particle counting instruments in the particle size range from 10 nm to 500 nm.

Report to the CCT on key comparison EUROMET.T-K6 (EUROMET Project no. 621): Comparison of the realizations of local dew/frost-point temperature scales in the range −50 °C to +20 °C

The first humidity CIPM key comparison, CCT-K6, will be completed in 2010. The corresponding European regional key comparison, EUROMET.T-K6, was carried out in 2004 to 2008. National metrology institutes from 24 countries participated in the comparison. The comparison covered the dew-point temperature range from −50 °C to +20 °C. It was organized as three parallel loops with two specially manufactured precision chilled mirror hygrometers as transfer standards in each loop. The comparison scheme was designed to ensure high quality results with evenly spread workload for the participants. MIKES was coordinating the project. This report presents the results of the comparison and provides detailed information on the measurements performed by all participating laboratories and the analysis of the results. Conclusions on the equivalence of the dew-point temperature standards are drawn on the basis of calculated bilateral degrees of equivalence and deviations from EURAMET comparison reference values (ERV). Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Investigation of the dew-point temperature scale maintained at the MIKES

For the investigation of the dew-point temperature scale realized by the MIKES primary dew-point generator, a two-pressure generator and a dew-point indicator based on condensation in a cooled coil were constructed and tested. In addition, a chilled mirror hygrometer was validated by means of an uncertainty analysis. The comparison of these systems was focused on the dew-point temperature range from to but measurements were made up to . The generator systems were compared using a dew-point comparator based on two relative humidity sensors. According to the results of the comparisons, the differences between the measurement systems were less than , while the expanded uncertainty of the MIKES generator was between and . The uncertainty of the other systems was from to . It was concluded that the dew-point temperature scale was not dependent on the realization method.

The effect of density gradients on hydrometers

Hydrometers are simple but effective instruments for measuring the density of liquids. In this work, we studied the effect of non-uniform density of liquid on a hydrometer reading. The effect induced by vertical temperature gradients was investigated theoretically and experimentally. A method for compensating for the effect mathematically was developed and tested with experimental data obtained with the MIKES hydrometer calibration system. In the tests, the method was found reliable. However, the reliability depends on the available information on the hydrometer dimensions and density gradients.

Atomic force microscopy studies of surface contamination on stainless steel weights

The SI unit of mass will probably be redefined within the next few years using an invariable natural constant. Nevertheless, dissemination of the kilogram will still be realized by weighing using physical weights prone to contamination. Published data on cleaning, humidity effects and long term stability of weights show large discrepancies, indicating that not all factors affecting adsorption characteristics of weights are known. In the work reported here, an atomic force microscope (AFM) was used to study surface effects of stainless steel weights at the nanometre scale. Effects of transfer between air and vacuum as well as effects of cleaning were studied by recording topography images of the surface before and after each procedure. An image processing method was developed for improving the sensitivity of detecting changes in images. Ultrasonic cleaning in ethanol removed contamination mainly from the grooves in the surface, while vacuum exposure caused contamination to build up in the grooves. The results show that the surface microstructure of stainless steel weights affects adsorption of contaminants in such a way that grooves seem to be preferential sites for adsorption. AFM has proven to be a valuable tool for studying surface effects of standard weights at ambient pressure with near nanometre resolution.

Optical temperature measurements of silicon microbridge emitters

Microbridges are miniature suspended structures fabricated in silicon. Passing a current through the microbridge can heat it up to the point of incandescence. A glowing microbridge can be used as a wideband light source. This study presents a method for optical measurement of the temperature of a microbridge. Spectroscopic measurements of microbridges are optically challenging, because the multilayer structures cause interference effects. To determine the temperature from the emitted spectrum, the emissivity was modeled with thin-film Fresnel equations. Temperatures of 500-1100 degrees C were obtained from the measured spectra at different levels of applied power. The range is limited by the sensitivity of the detectors at lower power levels and by the stability of the bridge at higher levels. Results of the optical measurements were compared with contact temperature measurements made with a microthermocouple in the same temperature range. The results of the two methods agree within 100 K.

The contribution of varying shear stress to the uncertainty in gravimetric gas mass flow measurements

The effect of natural convection on the uncertainty calculation of the MIKES dynamic gravimetric gas mass flow measurement system (DWS) is studied in this paper. The magnitude of the uncertainty component due to varying shear stress rate at the wall of the cylinder is studied theoretically at two different varying temperature models. Based on the obtained results, the contribution of the component may be significant, if the temperature difference between the wall of a gas cylinder and ambient air increases 1 K during one measurement cycle. Then the contribution of the shear stress can be above 92% of the combined standard uncertainty at the gas mass flow rate 0.1 mg s−1. Correspondingly, the contribution at the same temperature difference is below 1% at the mass flow rate 625 mg s−1. Applying the Monte Carlo uncertainty estimation method, the assumption of independent variables was shown to be reasonable.