In-Mook Choi

The 8th International Comparison of Absolute Gravimeters 2009: the first Key Comparison (CCM.G-K1) in the field of absolute gravimetry

The 8th International Comparison of Absolute Gravimeters (ICAG2009) took place at the headquarters of the International Bureau of Weights and Measures (BIPM) from September to October 2009. It was the first ICAG organized as a key comparison in the framework of the CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA) (CIPM 1999). ICAG2009 was composed of a Key Comparison (KC) as defined by the CIPM MRA, organized by the Consultative Committee for Mass and Related Quantities (CCM) and designated as CCM.G-K1. Participating gravimeters and their operators came from national metrology institutes (NMIs) or their designated institutes (DIs) as defined by the CIPM MRA. A Pilot Study (PS) was run in parallel in order to include gravimeters and their operators from other institutes which, while not signatories of the CIPM MRA, nevertheless play important roles in international gravimetry measurements. The aim of the CIPM MRA is to have international acceptance of the measurement capabilities of the participating institutes in various fields of metrology. The results of CCM.G-K1 thus constitute an accurate and consistent gravity reference traceable to the SI (International System of Units), which can be used as the global basis for geodetic, geophysical and metrological observations of gravity. The measurements performed afterwards by the KC participants can be referred to the international metrological reference, i.e. they are SI-traceable.The ICAG2009 was complemented by a number of associated measurements: the Relative Gravity Campaign (RGC2009), high-precision levelling and an accurate gravity survey in support of the BIPM watt balance project. The major measurements took place at the BIPM between July and October 2009. Altogether 24 institutes with 22 absolute gravimeters (one of the 22 AGs was ultimately withdrawn) and nine relative gravimeters participated in the ICAG/RGC campaign.This paper is focused on the absolute gravity campaign. We review the history of the ICAGs and present the organization, data processing and the final results of the ICAG2009.After almost thirty years of hosting eight successive ICAGs, the CIPM decided to transfer the responsibility for piloting the future ICAGs to NMIs, although maintaining a supervisory role through its Consultative Committee for Mass and Related Quantities.

Application of carbon nanotube field emission effect to an ionization gauge

Using the field emission effect of a carbon nanotube (CNT), we characterized a new type of technology for detecting low pressure. The fabricated low pressure sensor is of a triode type, consisting of a cathode (carbon nanotubes field emitter arrays), a grid, and a collector. The gauge described here has a triode configuration similar to that of a conventional hot cathode ionization gauge but also has a cold emission source. Due to the excellent field emission characteristics of CNT, it is possible to make a cost effective cold cathode type ionization gauge. For an effective CNT cathode, we used the screen-printing method and also, we controlled the collector and the grid potentials in order to obtain a high ionization current. We found that the ratio of the ionization current to the CNT cathode current changes according to the pressure in the chamber. In short, we elaborate the various metrological characteristics of a home-made pressure sensor that uses CNTs.

Improved metrological characteristics of a carbon-nanotube-based ionization gauge

A low-pressure detection technology utilizing the field emission of carbon nanotubes (CNTs) is introduced and its performance is characterized. To overcome the limitations of conventional ionization gauges, the CNTs had been proposed as an electron source. However, the simple triode structure of previous CNT pressure sensor not only has suffered from a limited measurement range but is also subject to a serious CNT array degradation problem. In this letter, the authors report the characterization results of an improved CNT-based pressure gauge, which is implemented by a screen-printing method, a modified mesh structure, and a pulse width modulation signal.

Parallelism error analysis and compensation for micro-force measurement

The null balance method for micro- and nano-force measurement is widely used in various precision industries. A parallel spring and a lever mechanism are designed and analysed for an electrostatic force balance by the null balance method. Various design parameters are required to obtain high resolution, a large measurement range, and fast response. The corner loading error is one of the most dominant error sources that should be removed. A parallel spring known as the Roberval mechanism, used in the micro-weighing device, can be used for one-axis force measurement since it is very sensitive to the vertical direction force, but insensitive to other force components. However, precise control of parallelism to below a micrometre is required not only to compensate for but also to remove the corner loading error. In this paper, the effects of the parallelism error have been analysed using the Lagrange method and verified by the FEA method. It was found that parallelism control by the proposed precise compensation mechanism enables the simultaneous elimination of and compensation for the corner loading error.

CCM.G-K2 key comparison

In November 2013 an International Key Comparison, CCM.G-K2, was organized in the Underground Laboratory for Geodynamics in Walferdange. The comparison has assembled 25 participants coming from 19 countries and four different continents. The comparison was divided into two parts: the key comparison that included 10 NMIs or DIs, and the pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent to each other. The results obtained for the key comparison confirm a good agreement between the NMI instruments. 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 CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Development of low pressure sensor based on carbon nanotube field emission

A newly developed low pressure detection technology using carbon nanotube (CNT) field emission effect has been designed and manufactured. The fabricated pressure sensor is of a triode type, consisting of a cathode (CNTs field emitter arrays), a grid and a collector. The principle described here is that, for a constant number of electrons available for ionization emitted from CNT arrays by a grid potential, a constant fraction of gas molecules will be ionized and the number of ions collected in a collector will be proportional to the number of gas molecules in the chamber traversed by the electrons. Due to the excellent field emission characteristics of CNT, it is possible to make a cost-effective cold cathode type ion gauge. A screen-printing method has been used to make the CNT cathode. A glass grid with Cr deposited by an e-beam has been put on the cathode with a gap of 200 µm between two electrodes. Due to the voltage applied to the grid, the electrons emitted from the CNTs ionize gas molecules in the chamber and the ionized molecules are gathered to the collector. Then, the collector voltage is made lower than the grid voltage to obtain a large ionization ratio. The current detected in the collector is proportional to the pressure in the chamber. The ionization characteristics are dependent on the gas and the voltage applied to the grid and the collector. In this paper we will show the various metrological characteristics of the home-made pressure sensor utilizing CNTs.

Final report of key comparison CCM.G-K1: International comparison of absolute gravimeters ICAG2009

The International Comparison of Absolute Gravimeters 2009 (ICAG2009) was carried out in September 2009 at the International Bureau of Weights and Measures (BIPM). The ICAG2009 and its data processing were performed in accordance with the agreed Technical Protocol which was approved by the Steering Committee. The pilot laboratory (BIPM) evaluated the final results of the key comparison from the measurements made at the BIPM in the framework of ICAG2009. A pilot study run in parallel allowed evaluation of the offsets for absolute gravimeters from institutes not fulfilling the requirements for participating in the key comparison. The gravity values at the five stations, and the offsets for each absolute gravimeter, were obtained by a weighted least squares fit. The results indicate that all participating gravimeters are consistent within their expanded uncertainties. 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 CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Frequency analysis of a step dynamic pressure calibrator

A dynamic high pressure standard is becoming more essential in the fields of mobile engines, space science, and especially the area of defense such as long-range missile development. However, a complication arises when a dynamic high pressure sensor is compared with a reference dynamic pressure gauge calibrated in static mode. Also, it is difficult to determine a reference dynamic pressure signal from the calibrator because a dynamic high pressure calibrator generates unnecessary oscillations in a positive-going pressure step method. A dynamic high pressure calibrator, using a quick-opening ball valve, generates a fast step pressure change within 1 ms; however, the calibrator also generates a big impulse force that can lead to a short life-time of the system and to oscillating characteristics in response to the dynamic sensor to be calibrated. In this paper, unnecessary additional resonant frequencies besides those of the step function are characterized using frequency analysis. Accordingly, the main sources of resonance are described. In order to remove unnecessary frequencies, the post processing results, obtained by a filter, are given; also, a method for the modification of the dynamic calibration system is proposed.

An analysis and design of the mechanical characteristics of the knife edges used in the NPL watt balance

Of the seven base units of the international system of units, the SI, only the kilogram is still defined in terms of a material artefact. The watt balance is an experimental approach which supports a new definition of the kilogram in terms of constants of nature. Some watt balances, such as the NPL Mk II watt balance, use knives and flats for the balance pivots as they have the advantages of greater robustness over flexure pivots in this particular application. However, hysteresis in the knives can produce errors in the weighing through systematic shifts in the equilibrium point of the balance and it is desirable to investigate this effect as an aid to its elimination. This paper analyses the hysteresis mechanism using both finite element techniques and direct measurement. It was found that the cause of hysteresis is not normal stress but shear stress and that the deformation of the flat, rather than that of the knife, is an important factor. Problems that could not be analysed in a two-dimensional finite element model were investigated using a simplified stand-alone balance. The combined results from the finite element analysis and the stand-alone balance suggest that material properties and tip radius are more important than friction coefficient and tip angle. At loads where knife and flat distortion are low the straightness of the knife and the roughness of the surfaces can have a significant effect.

Vacuum measurement by carbon nanotube field emissiona)

A vacuum measurement technology utilizing the carbon nanotube (CNT) field-emission effect has been developed and characterized. The fabricated pressure sensor is a triode type similar to a conventional ionization gauge, but has a planar structure similar to a field-emission display. Owing to the excellent field-emission characteristics of CNTs, it is possible to make a cost effective cold cathode-type ion gauge. The triode-type CNT sensor has been manufactured by a screen-printing method and by a thermal chemical-vapor deposition growth method. A modified structure with an electron filter has also been introduced and characterized so as to improve the performances, such as sensitivity and stability. A glass grid with Cr deposited by e-beam is placed on the cathode. By the voltage applied to the grid, electrons are emitted from the CNTs and they ionize gas molecules in the chamber. Two modes are available to detect the gas density in the chamber, an electron emission mode and an ionization mode. In the ele...