Thermomolecular flow

Significant temperature gradients in the furnace chamber will cause gaseous flow from hot to cold, which may apply a spurious force to the specimen pan. This is a more severe effect for chambers under moderate vacuum ( thermomolecular flow [3]). Purge gas flow direction may be an important consideration, in order to avoid condensation of gaseous products on the hangdown wire, or along the balance beam, as the gas flows out of the hot zone of the furnace. 

At low gas pressures, below about 300 Pa, any temperature gradient within a balance suspension or support will result in a (thermomolecular) flow of the gas present, from hot to cold. This will exert a force on the balance components, registering as a mass. The effect can be counterbalanced by the use of a symmetrical balance, similarly heated on both sides. Alternatively, the necessary correction can be determined by blank experiments, or a small pressure of an inert gas (above 300 Pa) can be introduced to eliminate the effect. 

Let us consider the two connected chambers, I and II, both of which are connected to their respective material and heat reservoirs. We shall denote the forces needed to cause the flow from one of the reservoirs to the chamber I by and the ones from the chamber II to another reservoir by Further, let us denote the external flows to the chamber I and those from the chamber II by and Jf, respectively. We now assume that, out of the / flows, only n flows are permitted, as the heat flow in thermomolecular pressure difference... 

The buoyancy effect is most of the time a problem that is misunderstood in the TGA measurement or often hidden by the numerical treatment of the TGA curves. Buoyancy is the upward force exerted on an object when it is immersed, partially or fully, in a fluid (liquid or gas). Its value is equal to the weight of the fluid displaced by the object. In the case of the TGA experiment with a given crucible (the object), this results in an apparent increase of the mass when the sample is heated. The effect is observed on all conventional balances. In fact the term buoyancy includes different parameters the true buoyancy as described before, the convection currents, the gas flow drag effects, the gas velocity effects, the thermomolecular forces, the thermal effects on the balance mechanism. 

In analogy with the frost heave terminology we call the pressure exerted by the confining wall the overburden pressure . If it is greater than the maximum pressure in the film then flow will cease, but if it is less than die maximum, flow will persist. The thermomolecular pressure gradient is written as... 

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