1. Technical Field
This invention relates generally to instrumentation for mass measurement and more particularly to a suspension system for an oscillating element microbalance which effectively decouples the microbalance from its environment and improves the instrument's mass resolution.
2. Background Information
An extremely sensitive microbalance employing an oscillating tapered element and capable of accurately measuring the mass of very fine particles and other matter, has previously been developed. This instrument employs a tapered elongate elastic element having a first end which is adapted to support a specimen and a second larger end which is anchored so that the first end and specimen carried thereby are free to oscillate. The elongate element is excited into oscillation at a resonant frequency. The resonant frequency of the oscillating element varies in accordance with the mass loading and accordingly can be monitored and measured to determine the mass of the specimen supported by the oscillating element. Details of construction and the operating principles of such an oscillating tapered element microbalance are described in U.S. Pat. No. 3,926,271, issued Dec. 16, 1975 to H. Patashnick. An improvement which facilitates use of the microbalance for the measurement of the mass of particulate or other forms of matter contained within a medium such as air or other fluids is described in U.S. Pat. No. 4,391,338 issued on July 5, 1983 to H. Patashnick and G. Rupprecht. The contents of these two patents are incorporated by reference herein.
In practice the oscillating tapered element microbalance has proven to be a highly accurate instrument which permits on-line, real-time direct measurement of particulate mass with great sensitivity and reliability. The instrument has been successfully employed in the evaluation of diesel exhaust, dust concentration and smoke measurement and is applicable to many other situations in which particles or other extremely fine forms of matter need to be detected and weighed.
The tapered element in the above-described microbalance vibrates in a clamped/free mode. This means that at the clamped end, where the tapered element meets the housing, two problems have to be confronted. First the vibration of the tapered element causes a strain on the support system, with the result that energy flows out from the tapered element through the housing into the support system, lowering the mechanical quality factor, Q, of the tapered element. Secondly, mechanical perturbations from the environment enter through the support system and housing, changing the phase and amplitude of the tapered element vibration, and thereby degrading the accuracy of the frequency determination and the mass resolution of the instrument. These two problems are not entirely unrelated because a decrease in the Q also has the effect of allowing a broader spectrum of environmental mechanical noise to enter the microbalance.
One approach for coping with these problems has been to attach the housing of the microbalance to a larger rigid distributed mass which is then decoupled by standard means such as a foam rubber cushion from the noisy environment. Although a three-dimensional decoupling of the tapered element from environmental noise can thus be achieved, this approach is rather cumbersome and not fit for certain applications. Further, the results are far from ideal, since it has been found that decoupling from environmental noise is not simultaneously achieved with a high Q and therefore compromises are necessary.
A need thus exists for an arrangement that simultaneously solves the two problems described above, thereby allowing the full potential of the microbalance to be realized.