Magnetic Suspension Balances

1. Rubotherm Magnetic Suspension Balances

Magnetic suspension balances allow the changes in force and mass which act on samples under controlled environments (pressure, temperature, corrosive gases or fluids), to be measured with high accuracy. By means of these measurements it is possible to determine transport quantities and state quantities very easily and accurately (sorption, diffusion, surface tension, density), chemical reactions can be investigated (corrosion, decomposition, combustion etc.) or production processes can be simulated (polymerization, coating, drying etc.)

Conventional apparatus vs. Magnetic suspension balance

Conventional measuring apparatus

The main difficulty when using conventional gravimetric instruments is the direct contact between the measuring cell (sample atmosphere) and the weighing instrument. The balance can be damaged or disturbed by the measuring atmosphere and the measuring atmosphere can be adversely affected by flushing gases and pollution. These limitations considerably reduce the field of application of conventional measuring devices.

Magnetic suspension balances

These reliable suspension balances make it possible to weigh samples contactlessly under nearly all environments. Instead of hanging directly at the balance the sample to be investigated is linked to a so-called suspension magnet which consists of a permanent magnet, a sensor core and a device for decoupling the measuring load (sample). An electromagnet, which is attached to the underfloor weighing hook of a balance, maintains a freely suspended state of the suspension magnet via an electronic control unit. Using this magnetic suspension coupling the measuring force is transmitted contactlessly from the measuring chamber to the microbalance, which is located outside the chamber under ambient atmospheric conditions. Consequently, this arrangement eliminates almost all restrictions which are inherent to conventional gravimetric measuring instruments.

1.1 Operating Principle

Operating principle of the magnetic suspension balance

A controlled suspended state is achieved by means of a direct analogous control circle (PID controller and position transducer). This modulates the voltage on the electromagnet in such a way that the suspension magnet is held constantly in a vertical position. A microcontroller driven digital set point controller superimposed to the direct PID controller allows various positions of the suspension magnet to be set up.

1.2 Automatic baseline drift correction by decoupling of the sample

Modern, high resolution balances can be tared and calibrated automatically; a precondition for this correction of zero point and sensitivity drifts is an unloaded balance, that is to say, a decoupled measuring load.

The magnetic suspension balance offers the possibility of lowering the suspension magnet in a controlled way to a second stationary position a few millimetres below the measuring position. Then, a small carrier to which the sample is connected is set down on a support. Now the sample is decoupled from the balance. The suspension magnet alone is in a freely suspended state and only its weight is now transmitted to the balance. This so-called zero point position, which corresponds to an empty balance pan in a normal weighing procedure, allows a taring and calibration of the balance at all times, even when recording measurements under process conditions in the measuring cell (pressure, temperature).

This unique option of the magnetic suspension coupling increases the measuring accuracy to levels
not previously known, particularly in the case of long term measurements.

Automatic decoupling of the measuring load in order to tare and calibrate the balance

1.3 Simultaneous weighing of two samples with one MSB

A resulting further development of being able to decouple the measuring load is the possibility to measure the mass change of two samples with only one magnetic suspension balance. In addition to the first measuring load decoupling a second is arranged in the magnetic suspension coupling. Three different vertical positions of the suspension magnet, which can be arrived at in a controlled way, correspond to three different measuring positions:

Zero point:
The permanent magnet alone is in a freely suspended state, allowing the balance to be tared and calibrated.

Measuring point 1:
The first sample is lifted up and its mass is weighed.

Measuring point 2:
The second sample is raised with the first and both masses are weighed together. By subtracting the first measuring point value from the second the mass of the second sample is given.

Main application of this unique MSB feature is the simultaneous

  • weight measurement of a reactive sample (adsorbent, catalyst, organic matter,…) placed as fist sample in the instrument and
  • measurement of the density of the fluid phase in the reactor by using an inert sinker with known volume as second sample. The density of the fluid phase is then measured highly accurate using Archimede’s principle of buoyancy.

Alternatively to that comparing measurements using two samples (e. g. sample and reference material) can be performed simultaneously under exactly the same conditions.

 

Example: Simultaneous measurement of sorption and density

1.4 Unique features and specifications

The application of a magnetic suspension balance as weighing instrument allows the design of gravimetric analyzers with unique features. The most obvious advantages and sole characteristics of magnetic suspension balance equipped instruments are:

  • Contact free weighing of all kinds of samples in hermetically closed reactors with patented magnetic suspension.
  • Hermetic separation of measurement area (sample) and weighing area (balance) to avoid any damage, destruction or interference from the process media (e.g. corrosive gases, vapours), pressure or temperature.
  • The reactor of the magnetic suspension balance is completely metal sealed, even highly aggressive or corrosive atmospheres can be used for measurements over a wide pressure and temperature range.
  • Excellent long term stability. The sample can be automatically disconnected from the balance at any time to re-tare and/or calibrate the balance. After that it is reconnected and lifted up to measuring position to continue the measurement. High precision long term measurements without baseline drift are performable without limit in measuring time.
  • The density of the measuring atmosphere in the reactor is determined highly accurate by applying Archimedes’ principle. A sinker is weighed by the balance additionally and simultaneously to the sample, so determining the buoyancy effect acting on it.
  • Modular construction: The magnetic suspension balance can flexibly be adapted to all kinds of applications by combining different modular parts of the instrument.
  • No other instrument than Rubotherm’s patented magnetic suspension balance is able to work with high pressure and high temperature at the same time and additionally handle aggressive media (e.g. H2 at high pressure and/or temperature) and vapours.
Temperature Range

In the table below the key specifications of the standard versions of Rubotherm’s MSB are summarized. Please note, that the given temperature specifications, belong to the upper part of the pressure vessel, the magnetic suspension balance. The sample temperature in the measuring cell can be much higher or lower. The sample can be heated / cooled separately to higher or lower temperatures, depending on the type of measuring cell and thermostat used.

This principle of thermal separation between the MSB and the sample is schematically shown in the figure. For measurements at sample temperatures which are in the range of the magnetic suspension balance the whole pressure vessel is at the same temperature. For all other sample temperatures the sample is thermostated separately from the balance.

MSB TypePressure range PTemperature Range TWeighing range m
Standard
Versions
LP Vac…1 bar
Vac…14.5 psi
-40…200°C
233…473 K
0…60 g
HP Vac…150 bar
Vac…2175 psi
-40…200°C
233…473 K 
0…60 g
HPII Vac…350 bar
Vac…5075 psi
-40…200°C
233…473 K
0…60 g
HPIII Vac…700 bar
Vac…10150 psi
RT...150
293...423
0…60 g
Special Versions Vac…1000 bar
Vac…14500 psi
-196...400
77...673
0…200 g

The type of instrument is determined by the selection of a measuring cell / reactor and thermostat suitable to cover the application range. Different standard product lines were designed offering standardized yet flexible configurations. For details about the Rubotherm product lines please refer to the product sites

2. Historical development

2.1 Early versions of the magnetic suspension balance.

Since the 1940's researchers in mainly the USA and Germany worked on methods for magnetic force transmission through the wall of a reaction vessels. One aim was to realize weighing of a sample inside a reactor under a potentially harmful atmosphere, high pressures and/or temperatures with sufficient accuracy.

The first magnetic suspension balance, a demonstration apparatus without practical use and application, was built in the US 1947 [1].

Research at NBS (later NIST) in the USA was focused on the development of a magnetic suspension balance for liquid density measuring. In the first concept a small ferromagnetic buoy was kept in a free suspension state by means of an electrically generated magnetic field and an analytical balance was used for a tricky mass determination by weighing the measuring cell, first with the sinker being in a free suspension state and second without being freely suspended and therefore laying on the bottom of the cell [2].

The first commercialized magnetic suspension balance was developed by Gast 1969 at the Technical University of Berlin [3] and produced by the German balance manufacturer Sartorius. This instrument was of real use for certain applications from vacuum to ambient pressure.

The Sartorius Magnetic Suspension Balance

The Sartorius Magnetic Suspension Balance

  1. Balance
  2. Electric Magnet
  3. Permanent Magnet
  4. Glass Housing
  5. Sample
  6. Windows

A permanent magnet, the sample was attached to, was kept in a free suspension state by means of an electro magnet fixed to the under floor weighing hook of an analytical balance. The analytical balance was used for mass determination by weighing the electromagnet, permanent magnet and sample together – thus measuring the sample’s mass. The magnetic suspension bearing was used for contactless force transmission of the sample to the balance located outside the measuring cell under ambient conditions.

A first suspension balance for high pressure use was built in the US 1976 to measure liquid density. In this apparatus again a small ferromagnetic buoy was kept in a free suspension state by means of an electrically generated magnetic field. But now the current in the electromagnet coils is used for weight measuring [4].

Short time later at the University of Bochum in Germany the Sartorius suspension balance was adapted by a user for thermogravimetry under elevated pressure [5]. Two new suspension balances for fluid density measuring at elevated pressure were developed in 1984, one at the NIST in Bolder [6], US and one at the University of Bochum [7] in Germany. Both apparatus use an analytical balance for mass recording and a magnetic suspension coupling for force transmission only.

Up to this point all versions of Magnetic suspension balance were very limited in regard of their measuring and application range, accuracy and reliability. All of them use glass housings or at least glass or sapphire separating walls at the force transmission area. Zero point recording and correction was not possible. The same applies to balance calibration.

2.2 Rubotherm Magnetic Suspension Balance

In the 1980's researchers from the Ruhr-University Bochum developed a new kind of magnetic suspension balance [7] using also a commercial balance for weighing, while the suspension coupling only transmits the sample weight contactless from the pressurized reactor to a microbalance at ambient atmosphere.

The new magnetic suspension balance was patented in Europe and the USA. It overcame the limitations of the existing instruments mainly by applying a new electronic controlling and construction principle, allowing entire metal high pressure housings also in the separation area. Additionally by means of these new instruments it was now possible to perform zero point correction and balance calibration during recording measurements at any time.

As a result of that for the first time mass changes of a sample could be recorded even under extreme conditions with the utmost accuracy. Since these days magnetic suspension balances have been applied to fundamental research work with enormous success. Because of the big interest from researchers in industry and academia in the new MSB instruments [8], the inventors from the University Bochum, Hans Wilhelm Lösch, Reiner Kleinrahm and Wolfgang Wagner founded the company Rubotherm in 1990.

Our weighing technology has since then been enjoying an even wider circle of application throughout the world in both research and industry. Rubotherm continuously developed the balance itself and accessories – special types of reactors, gas dosing and mixing systems, vapour generators, pressure controllers, gas sampling devices, etc. – for different scientific applications (please refer to the literature reference list with publications of our customers). Big steps have been binary sorption measurements in gas mixtures using the new MSB, [9,10], the development of the simultaneous sorption and density measuring [11,12] and the development of a new double sinker densimeter [12,13]. Up to now this all led to three balance product lines with more than 20 instruments with different specifications. Still today Rubotherm often follows customer requests for new and customized models which are designed by our committed team of engineers for interesting applications.

Magnetic Suspension Balance

2.3 Rubotherm current MSB development

A completely new type of magnetic suspension balance is developed from 2009 on in co-operation with the Swiss Federal Institute of Technology (ETH Zürich). Researchers there had invented a new suspension balance principle which allows weighing of relatively small samples with extraordinary high resolution.

This new, patented technology is currently further developed into commercial instruments by Rubotherm. Extremely fascinating is the possibility to reach resolutions in the nanogram range when weighing samples of several 10s or 100s milligram weight.

For application of the new MSB in sorption measurements at rather low temperatures but up to highest pressures a version with hanging sample is designed. The small inner volume and diameter of the balance allows highest pressures in the range of 1000 bar or more to be realized.

MSB development

Another version of the new balance with toploading sample is designated for thermogravimetric measurements at high temperatures up to moderately high pressures. The hot sample area is located above the cooler balance. This setup leads to drastically reduced convection effect in the reactor of the instrument, even if measurements are carried out at high temperatures and high pressures.

References

[1]
Clark, J. W.
An Electronic Analytical Balance
Rev. Sci. Instr. 18 (1947) 915-918.

[2]
Beams, J. W. and Clark, A. M.
Magnetic Suspension Balance Method for Determining Densities and Partial Specific Volumes
Rev. Sc. Instr. 33 (1962) 750-753.

[3]
Gast, Th.
Waagen mit freischwebender magnetischer Aufhängung
Naturwissenschaften 56 (1969) 434-438.

[4]
Haynes, N. M.; Hiza, N. J.; Frederick, N. V.
Magnetic Suspension Densimeter for Measurements on Fluids of Cryogenic Interest
Rev. Sci. Instrum. 47 (1976) 1237-1250.

[5]
Sabrowsky, H.; Deckert, H. G.
Thermogravimetrie unter erhöhten Drücken, Teil 1
Chem. Ing. Technik 50 (1978) 217-219.

[6]
Masui, R.; Davis, H. A.; Levelt Sengers, J. M. H.
A New Magnetic Suspension Densimeter for Determining Fluid Densities by Weighing
Proc. of the Eight Symp. on Thermophysical Properties
Vol. 1, ASME, New York (1982) 128-133.

[7]
Lösch, H.W.
Entwicklung und Aufbau von neuen Magnetschwebewaagen zur berührungsfreien Messung vertikaler Kräfte
Fortschr. –Ber. VDI-Z, Reihe 3, Nr. 138, VDI- Verlag, Düsseldorf (1987).

[8]
Lösch, H.W.; Kleinrahm, R.; Wagner, W.
Neue Magnetschwebewaagen für gravimetrische Messungen in der Verfahrenstechnik
Jahrbuch 1994 „Verfahrenstechnik und Chemieingenieurwesen“, VDI- Verlag, Düsseldorf (1987).

[9]
Dreisbach, F., R. Staudt, M. Tomalla and J.U. Keller
Measurement of Adsorption Equilibria of Pure and Mixed Corrosive Gases: The Magnetic Suspension Balance Fundamentals of Adsorption,
M.D. LeVan (Ed.), Kluwer Academic Publishers, p. 259-268, 1996.

[10]
Dreisbach, F., R. Seif A. H. and H. W. Lösch
Messmethoden für Gasphasen-Adsorptionsgleichgewichte
Chemie Ingenieur Technik, 74 (2002) Nr. 10, S. 1353-1366.

[11]
Seif Amir Hosseini, Reza
Entwicklung und Aufbau neuer Sorptionsmessapparaturen auf der Basis von Magnetschwebewaagen,
2003.

[12]
Lösch-Will, Cornelia
Entwicklung und Aufbau neuer Dichtmessapparaturen auf der Basis von Magnetschwebewaagen,
2005.

[13]
Marc O. McLinden , Cornelia Lösch-Will
Apparatus for Wide-Ranging, High-Accuracy Fluid p- ρ-T Measurements Based on a Compact Two-Sinker Densimeter,
Journal of Chemical Thermodynamics No.39 (2006) pp.507-530.