An investigation of methods for cleaning stainless-steel weights

The stability of reference masses has been a long-standing concern within the SI. More recently the requirements of potential non-artefact realizations of the kilogram have added gold and its alloys to the platinum alloys that have historically been the focus of attention. Previously we proposed UV/ozone cleaning of standard-mass surfaces to improve stability with respect to carbonaceous contamination. Since then both NPL and BIPM have constructed prototypes of UV/ozone apparatus for cleaning kilogram standards. We have therefore tested a combined solvent wash and UV/ozone procedure, UVOPS or ‘UV/ozone with pre-wash and stabilization’, on both platinum and gold surfaces. X-ray photoelectron spectroscopy (XPS) shows this to be very successful in removing even gross contamination from these noble metal surfaces. Oxidation is negligible, limited to the outermost layer of noble metal atoms, and terminates at a level that is within the uncertainty of mass comparisons at the 1 kg level. This oxide has been seen in some earlier XPS studies but not others—we show that decomposition of the oxide by x-rays at XPS energies may be responsible for this disparity.

Section: The Uvops Proceduresmentioning

confidence: 99%

Stability of reference masses V: UV/ozone treatment of gold and platinum surfaces

Cumpson

Sano

2012

“…Surface effects on standard weights have previously been studied gravimetrically [6][7][8] and using different surface analysis techniques, such as x-ray photoelectron spectroscopy (XPS) [9][10][11], Auger electron spectroscopy (AES) [12], thermal desorption spectroscopy (TDS) [10,13], secondary ion mass spectrometry (SIMS) [14], time-of-flight SIMS (TOF-SIMS) [15], scanning electron microscopy (SEM) [16] and ellipsometry [5,8,17]. Each technique has its advantages and drawbacks.…”

Section: Introductionmentioning

confidence: 99%

Atomic force microscopy studies of surface contamination on stainless steel weights

Högström¹,

Korpelainen²,

Riski³

et al. 2010

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.

“…There has been a good deal of work on the effect of cleaning weights (e.g. [3][4][5][6][7][8][9][10]) showing that material adsorbed from the atmosphere can account for up to 80 µg for a kilogram weight [3] and that the quality of the air in contact with the weight is important [4]. However, this is probably not relevant to well stabilized weights in transit as they usually have very little contact with the air outside their containers.…”

Section: Introductionmentioning

confidence: 99%

An experimental study of the stability of mass standards in transit

Fen¹,

Morris²

2008

Long-distance transport of high quality mass standards for the purpose of calibration or international comparison is becoming increasingly common, and the stability of these standards in transit is of universal concern. Despite efforts to minimize disturbance, significant mass shifts have occurred during bilateral intercomparisons involving the National Measurement Institute Australia using 1 kg travelling standards of a quality equal to national secondary standards. Mass shifts have also been observed at the E 1 level.This work explores the extent to which transport and storage during transport might bring about these mass instabilities. The weights, in their boxes or transit capsules, were subjected to controlled variations in temperature, pressure and humidity and were measured before and after these environmental stresses were applied. The stresses varied from relatively mild to the very worst that could be expected during transport and storage.This study indicates that, with modest precautions, weights of class E 1 or lower can be transported by courier. If significant mass shifts do occur at this level, one can be reasonably confident that they did not happen in transit. On the other hand, a higher quality kilogram weight exhibited a mass shift in the order of the measurement uncertainty (30 µg) during a period of rather extreme environmental stresses. The common practice of 'hand-carrying' such high quality standards is therefore recommended. The effect of vibration in transit was not studied here.