Working with precision- and microbalances is part of our daily routine. Any scientist or experimenter who has ever dealt with possible influences on highly sensitive measurement technology, especially on microbalances and high-precision mass comparators , knows how many factors need to be taken into account during measurement and, in particular, during subsequent evaluation.
One problem in microbalance technology is buoyancy, which, depending on external climatic conditions such as air pressure, temperature and humidity, ensures that weighing under atmospheric conditions delivers values that are either above or below the true value.
In order to obtain the correct value, the so-called corrected mass , , it is sometimes necessary to carry out quite complex correction calculations. And even then not all influencing factors have been recorded because even fine adsorption layers (mostly water from the atmosphere) falsify the weighing result , .
There are various ways of dealing with these problems but weighing in a vacuum eliminates a large proportion of these factors at a stroke.
Why not always weigh in a vacuum?
Well, here’s the problem: Commercially available precision and microbalances, but also commonly used comparators work according to the principle of electromagnetic compensation . They are therefore dependent on an external or internal voltage supply, have highly sensitive electronics inside and heat up during operation like other electrical devices. Components that could outgass or even be damaged in a vacuum (e.g. electrolytic capacitors that would burst in a vacuum) and in particular the unavoidable waste heat, which would lead to overheating of the device due to the lack of convection and heat conduction in the absence of carrier gases, pose a major problem.
The currently available options for carrying out weighing operations in a vacuum are very rare and are usually associated with high costs. The Ilmenau Technical University (Institute for Process Measurement and Sensor Technology, Department of Process Measurement Technology headed by Prof. Thomas Fröhlich) and the PTB (FB 1.1, AG 1.11) each have a 1 kg vacuum mass comparator from the company Sartorius . However, these are generally not accessible for normal basic research, especially not for long-term measurements, which are often necessary, for example, in our experiments. Other alternatives to load cells with electromagnetic compensation that are currently available on the market have proven to be too expensive for our purposes.
With the help of existing vacuum equipment and a microbalance (Mettler-Toledo, WXS206SDU/15, reading accuracy: 1 µg), a small structure was designed in which, among other things, heat dissipation and active cooling of the balance in a high vacuum (10 -5 to 10 -6 mbar) can be tested.
After completion of initial tests, which are intended to provide information on the linearity behaviour of the microbalance, for example, we are planning – in cooperation with a Swiss development laboratory at Mettler-Toledo – to adapt the microbalance so that it can measure reliably in a high vacuum for many days.
Literature: M. Kochsiek, M. Gläser: “Mass determination”, VCH, 1997.  M. Gläser: “Response of apparent mass to thermal gradients”, Metrologia 27, 95-100, 1990.  S. Davidson et al.: “The measurement of mass and weight”, Measurement Good Practice Guide No. 71, National Physical Laboratory, 2004.  M. Kochsiek: “Measurement of water adsorption layers on metal surfaces”, Metrologia 18, 153-159, 1982.  R. Schwartz: “Precision determination of adsorption layers on stainless steel mass standards by mass comparison and ellipsometry. Part I: Adsorption isotherms in air”, Metrologia 31, 1994.  R. Schwartz et al.: “Guideline for high accuracy mass determinations”, PTB-MA-80, 2006.  M. Borys et al.: “Design and performance of the new Sartorius 1 kg vacuum mass comparator at PTB”, XVII Imeko World Congress – Metrology for a sustainable development, September, 17-22, 2006, Rio de Janeiro, Brazil.