Measuring minute quantities of weight is often a challenging task for various reasons. Limitations of the measuring equipment, effects on the experiment due to the surrounding conditions, measurement error, and repeatability are just some factors that may contribute to problems with accurately measuring minute quantities of weight.
Generally, measuring small amounts of weight entails use of sensitive equipment, typically scales with sensitivity in units of micro grams. Additionally, error may affect the accuracy of readings when making measurements because, regardless how small, error may constitute a considerable percentage of the weight being measured, especially minute quantities of weight. Hence, the error, which is generally related to equipment shortcomings, whether it be calibration error, human error, and/or repeatability error, and which may be outside the control of the user making the measurements, can severely limit the reliability of the measurements. Reducing error in any of these areas is believed to improve accuracy.
A typical instrument that may be used to measure changes in minute quantities of weight is a thermal gravimetric analyzer (“TGA”), as shown in FIG. 1. A TGA often includes a pan 10 for supporting the sample to be weighed, a hang-down wire 13 that attaches pan 10 to one end 14 of an arm 16, and a stir-up 12 that connects pan 10 to hang-down wire 13. The other end 18 of arm 16 has a known weight 20 to counter balance the sample.
As shown, pan 10 is suspended within heater 22. In this orientation, effects caused by the pan's surroundings may affect the measurement of the sample in pan 10. Typical effects may include any static electricity in heater 22, any unwanted chemical reaction between the sample and its surrounding, effects due to convection, and/or any pressure or temperature differentials within heater 22 or between the inside of heater 22 and the outside of heater 22.
Additionally, theoretical and measured data suggest that a changed in weight is dependent upon a buoyancy of the pan, or the tendency of the pan to float, or be supported, by the atmospheric gas beneath the pan. Buoyancy generally contributes to error, which is believed to negatively affect accurate weight measurements, because buoyancy possibly hinders an ability to obtain a true reading of weight since it is at least supported in part by the atmospheric gas.
Further, buoyancy was found to be affected by the type of gas in the chamber surrounding the weight and pan, or the carrier gas. The carrier gas used typically affects buoyancy because buoyancy depends, in part, upon the carrier gas'density. Measured data may suggest, as temperature increases, the density of certain carrier gases varies and this variance affects buoyancy, which typically causes unwanted changes in weight during these temperature changes.
In addition to buoyancy, theoretical data suggests that the sample weight is affected by a volume of the pan and/or hang-down wire. From Archimede's principle, and assuming negligible effects due to isothermal conditions and convection, the following relation may exist between the volume of the sample pan, volume of the hang-down wire, density of the pan and hang-down wire, and density of the carrier gas:W(T)=Actual Weight−Buoyancy W(T)=Vpρ−Vpρc(T)                 where W(T) is the measured weight at a given temperature.        Vp is volume of the sample pan plus the volume of the hang-down wire.        ρ is effective density of the sample pan plus the density of the hang-down wire.        ρc is the density of the carrier gas and is a function of temperature.        
Hence, a pan and/or hang-down wire that occupies a large volume often results in greater error.
U.S. Pat. No. 6,057,516 to Nakamura et al. appears to relate to an improved TGA for measuring variations in the weight of a sample caused by temperature variations of the sample. To achieve this end, a weight of the balance beam is often minimized. This commonly results in a disadvantage of having a beam with reduced structural integrity where the hang-down wire may move relative to the balance beam due to bending of the beam. This problem may be exacerbated when measuring larger loads. The invention provides a mechanism that may overcome this disadvantage by preventing a large load to be applied to the hang-down wire without increasing the strength of the hang-down wire. However, strengthening the hang-down wire, which may include thickening the wire or using a heavier duty wire, increases the wire's volume and appears to teach away from the theory of Archimede's principle.
U.S. Pat. No. 5,055,264 to Czamecki appears to relate to a system for measuring changes of weight by directing a flow of condensable gases away so that measurements of variations in weight may be obtained.
U.S. Pat. No. 6,279,387 to Kikuchi appears to relate to a device for measuring a moisture content. Helium is used as a carrier gas.
Both Czamecki and Kikuchi do not seem to reduce variations in the weight of the sample by addressing, as illustrated by Archimede's principle, the volume or the type of carrier gas chosen.
What is desired, therefore, is a measurement system for improving measurements of minute quantities of weight. What is also desired is a system for reducing the effects of the sample's surroundings on the sample's measurement. Another desire is to reduce error when measuring minute quantities of weight. What is also desired is a way for determining factors that negatively affect measurement accuracy and minimizing the effects of these factors.