It's all in the balance

Modern microbalances, or more properly recording microbalances, have little in common with the traditional concept of weight measurement. Here, Tony Roberts looks at the applications for microbalances in R&D and quality control

Weighing in its many forms has a history going back over millennia and has clearly been an essential element in the development of civilisation. The construction of highly sensitive mechanical beam balances having weighing ranges from tons down to microgram levels of accuracy, dates back to the 19th Century. Typically these were massive structures that needed to be installed and operated in a highly stable, vibration-free and temperature-controlled environment. Objects to be weighed had to be brought to the balance room and the highest precision was achieved by following time-consuming and rigidly defined weighing procedures. This approach successfully provided a means to measure absolute weight, to maintain standards and to calibrate instrumentation. These activities centred on the accurate weight of a sample as the primary objective.

During the first half of the 20th Century a range of mechanical balances and scales were developed using either the beam or spring balance principle for their operation. Over this period a need developed for experimental procedures in which the change of weight of a material could be determined while it was subjected to an imposed “hostile” environment. This could be achieved by adapting available balances though often the amount of data recorded was limited, for example where the sample had to be physically transferred to and from the weighing location. These applications were mostly concerned with elapsed time as a defining parameter (ie they needed to provide a record of the weight history of a sample by consecutive measurements at time intervals which were recorded). Thus a weight or mass profile could be developed for the sample.

Alternatively the sample’s environment may be constant or it may be varied in a controlled and recorded manner. The desired environment may take any of a wide variety of forms. Commonly the sample is located in an oven where it is exposed to a controlled range of temperatures such that its weight is related to time and temperature. Other situations may involve weighing in a vacuum, or while subject to elevated gaseous pressure, at controlled levels of humidity, exposure to corrosive vapour or exposure to a magnetic field. The sample may be a mixture of materials (eg powders, liquids, solids) which react together thereby gaining or losing weight due to interaction between their several components. The variation in weight may be a slow process or it may occur rapidly, for example due to a chemical reaction which takes place at a critical temperature or humidity or any of the other controlled parameters.

Experimental procedures designed to acquire fundamental data in any of these situations have evolved over many years making use of the balance and the environmental control technology available at the time. Ideally the balance mechanism should be in close proximity to the sample being measured, without itself being affected by the environment. Often this has not been possible, and the rate of acquisition of data and its accuracy has been compromised where the sample has had to be transferred to the balance for each weight measurement.

Until the 1950s only mechanical balances were commercially available and the visual operation to record data was slow, cumbersome and unsuitable for this type of application particularly where small samples and higher precision were needed. The advantage of using small samples (eg 1g or less), where data to microgram precision was needed, was lost if only a precision laboratory microbalance was available, not least due to the skill and patience required for its use.

One alternative suitable for some applications comprised a helical quartz spring having a vertical axis and from which the sample was suspended. While awkward to set up, it had the unique advantage that being only made of quartz, with a small mirror attached, it could operate in virtually any environment without being damaged or becoming inaccurate. An optical system comprising a beam of light reflected by the mirror was directed on to a scale. It was difficult to calibrate, or provide absolute sample weight data, but with care it could be used to record the very small weight changes which are often of more experimental significance.

The picture changed dramatically in the 1960s when the electronic microbalance appeared. It comprised a miniature beam from one end of which the sample was suspended. Deflection of the beam was counteracted by a current in a coil attached to it which was positioned in the field of a permanent magnet. This used the Faraday effect, (dating from 1831), whereby the current in the coil is directly proportional to the torque applied to the beam. To operate the early microbalances, which used this principle, it was necessary to adjust a calibrated potentiometer to provide a current which set the beam to a null position.

A research project in 1961 led to the first fully automatic design, subsequently to be perfected by C I Electronics. In this a “closed loop servo system” was established, using an optical detector attached to the beam which via an amplifier continually adjusted the current level in the coil causing it to maintain the beam in its equilibrium position. Changes in the current were accurately proportional to the weight of the suspended sample. Digital displays were becoming available at the end of that decade so that the weight could be displayed continuously and it could be recorded and printed out repetitively in synchronism with time and any of the other parameters being monitored.

While the basic design of the recording microbalance has changed little in over forty years, its versatility has. Up-to-date electronic circuitry and computer software now provide a range of operating modes, much improved precision and the means to record many other parameters in synchronism with the weight and time data. The balance head can be incorporated directly into an experimenter’s own systems, supplied as a free-standing instrument with a DISBAL control unit for input/output to a PC, or in kit form with associated furnace and control for thermo-gravimetric analysis or built into a dedicated instrument for water vapour sorption measurements. In this last case the ultimate weight readability can be as little as 0.1μg and the behaviour of powder samples of a few milligrams can be monitored v. time, temperature and relative humidity.

The ever-increasing range of uses in industry includes research & development and quality control functions, a major example being for pharmaceutical development and manufacture. Similarly applications in university faculties have included Chemistry, Materials, Physics, Biology, Engineering, Archaeology and nuclear power facilities. In these fields experimental measurements cover thermal analysis, catalytic activity, chemisorption, magnetic susceptibility, surface and inter-facial tension, gaseous absorption, corrosion and decomposition.

The wide range of electronic top pan balances, analytical balances and many types of scales are in general aimed at manual use where accuracy is more important than either the speed of data output or their physical size. The recording microbalance may be regarded as a transducer rather than as another type of balance. Its strength lies not in the measurement of the fixed weight of an object, but in the way its weight changes when subjected to specific external conditions.

In contrast, the recording microbalance is ideally compact, robust, tolerant of its surroundings and rapid in response, with optimum relative precision and stability. Sample capacity and linearity and calibrated absolute precision are less important in comparison with rapid response and the recording of percentage changes in sample weight. These attributes are also those of most transducers, whether they measure pressure, velocity, temperature, humidity, current, voltage, the presence of gas, fluid level or any of the other multitude of conditions for which such devices now exist.

In conclusion, for thousands of years the measurement of weight was considered an activity which involved taking an object to a balance or scale to be weighed. The recording microbalance despite its long name is versatile in its own quite different way. With other relevant transducers it is so far as possible integrated within an experimental system, providing a continuous stream of data throughout each test procedure.

By Tony Roberts. Tony has been Chairman of C.I.Electronics Limited since 1965.