The (air buoyancy) weighting correction is in principle too small to be significant for the small weight(s) (mass) weighted by the TGA thermobalance: about 1.5 parts per thousand at room temperature; decreasing on heating. Partial correction is typically possible by setting the initial sample weight (mass) to that of the sample weighted in air; then, the correction would only apply to the sample weight change, usually smaller than the initial sample weight, and should be neglectable.

It is usually also neglectable the weighting buoyancy correction for the N2. substitution for air.

For details (or correction formula) you may refer to "Vogel's Textbook of Quantitative Chemical Analysis", 5th ed., Longman Scientific & Technical, 1989, p. 78.

The first reasons which come to mind why a TG investigation is planned, are:

* to determine the stability and the content of the volatiles in a material;

* to study a material without interference of oxygen;

* to prepare an inert environment into which the active components of interest can be subsequently introduced.

With this diversity of possible applications, one cannot go much further with advice.

However, some precautions are likely to be of use:

* Even the highest vacuum is never free of molecules; actually, there are plenty of them. Achieving a non-oxidizing environment by evacuation is totally impossible due to the residual leaks and outgassing; the water vapor, always present in up to 500 layers of molecules on every practical surface, can oxidize even stronger than oxygen can. That includes the water vapor on the surfaces of the mechanism of the balance of the TG instrument.

Since the ideal vacuum is un-achievable, a more practical choice is admitting that fact, and working with an added residual bleeding of an inert gas trough the balance chamber, then through the area where the sample is. 1 Torr or a fraction of it can be much more stable in every respect than the "10-6" We need to remember that the vacuum gauge is measuring it's own environment, while the vacuum level at the other locations is anyone's guess.

* 1 ml of air weighs about 1000 micrograms, so evacuation of the moving parts of the balance plus the sample and its support produces a strong shift of the zero line. Still, some procedures allow achieving a ppm precision.

* Samples of the materials when thermally decomposing, produce condensable fumes which can destroy the TG instrument almost immediately.

And so on, and on, and on. A thermogravimetric experiment can produce rich information, but it requires a thorough preparation, to be meaningful. This is the area of conflicts between the user and the customer-support department of the instrument's manufacturer. They say: "The instrument seems to be working properly, as designed." - The user: "The instrument itself may work OK, but it does not work with my samples!" - "Sir, you certainly know much more about your materials than we possibly can; we are only responsible for the performance of the instrument." - "But I have paid so much money for the instrument to study my materials!" - "I recommend your checking Google if any other user had your problems." Please note this: "... your problems". No way around, TG researchers have to research how to use their instruments with their materials. They are never as straightforward as a tape measure.

In this respect the term "capibration" sounds to belong to the area of hopes.

If you are more specific about your experiments and measurements, a more focused help can be tried.

Jerry C.

What is the vacuum range? The performance of TG dramatically changes with the vacuum range. We are working at 1e-3 Pa using a suspension-type TG.

I am sorry for late reaction. The maximum vacuum that I would be capable of is 7 Pa (according to the specs of TA Instruments SDT Q600). We are looking at thermal decomposition (dehydration) in the area of space research. Oxydation is not that much of an issue (or interest). The prime interest is how the descriptive quantities of thermal decomposition change when the experiment is done under vacuum as compared under inert atmosphere.