The technique of weighing is one of the most accurate measuring methods for experimental investigations. For numerous applications, however, it is not possible or useful to locate a balance or a weighing instrument in the measurering chamber itself (e.g. at high temperatures, high pressures, aggressive atmospheres). In many of these cases magnetic suspension balances could be a suitable solution.

By means of magnetic suspension balances it is possible to accurately measure the weight changes of a sample located in a closed measuring cell made of metal or glass. In these cases, the balance (a commercial analytical or micro balance) is located outside the measuring cell at ambient atmosphere and the sample is contactlessly weighed "through the wall" of the measuring cell. The operating principle of such magnetic suspension balances is illustrated in the following figure.

The key component of the magnetic suspension balance is the magnetic suspension coupling which consists of an electromagnet (with a soft iron core), a suspension magnet (a permanent magnet), a position sensor, and a control system. The electromagnet is attached to the underfloor weighing hook of the balance and maintains a freely suspended state of the supension magnet. To achieve the freely suspended state of this permanent magnet, its position is detected by a position sensor and controlled via a direct analog control circuit. The sample is linked to the permanent magnet via a load coupling and decoupling device (this "measuring position" is shown in the figure). By means of the magnetic suspension coupling, weight changes of the sample are contactlessly transmitted to the balance. To avoid any influence of a zero-point drift of the balance the sample can be decoupled from the suspension magnet by lowering it to a zero-point position (tare position) about 5 mm below the measuring position shown in the figure. Since only the permanent magnet is weighed in this position a simple and fast zero-point check is possible at all times. All movements of the suspension magnet and the sample vessel are electronically controlled so that any vibration is avoided. The measuring accuracy at the balance is not adversely affected by the magnetic suspension coupling.

Although the principle of contacless weighing was already developed more than 50 years ago, corresponding weighing instruments could only be produced for very restricted fields of application until the middle of the 1980s. Only by changing the control principle decisively [71, 73] it has been successful to develop a new type of magnetic suspension balances that could be used for a great variety of different applications. This new type enables the application of gravimetric measuring methods in wide ranges of pressure and temperature and with nearly any sample loads for the first time. The operating principle and the fields of application of the new magnetic suspension balance are described in detail in [71, 73]. In our laboratories, the magnectic suspension balances are especially applied for measurement of the density and the viscosity of fluids (see densimeters, combined visco-densimeter, and research projects for industry). The following figure shows two photos of a demonstration model (cutaway model) of a magnetic suspension balance (with a quartz-glass sinker for density measurements of fluids) that has especially been built up for the demonstration of the operating principle of a magnetic suspension balance.

Since the new magnetic suspension balances are suitable for many different applications and there has been a great interest of other research and metrology laboratories, magnetic suspension balances have also been commercially available in many different versions and for a lot of different applications for about ten years (Rubotherm Präzisionsmesstechnik, Germany, www.rubotherm.de). Magnetic suspension balances are currently used in numerous research institutes and industrial companies all over the world.