GS5-3 Pressure Puzzle - An Open Challenge

Pressure transducers are often based on strain gauges. If you change the stiffness of that component, you could change its response to changes in pressure. Could the pressure transducer be reacting with hydrogen in such a way as to change its modulus of elasticity?

Here's my best guess, which is probably worthless coming from a non-chemist.

I think the pressure reading is real. Residual Li/LiH, Al203 and H are reacting to form Lithium Aluminate (and corroding your mullite tube). This reaction proceeds more rapidly at higher temperatures removing the Hydrogen from the cell to form a hard vacuum. This "helps" the vacuum pump along to achieve the remarkable vacuum readings.

Did you observe the very low vacuum measurements with the pump operating, or had you closed the valve to the pump? If the latter was true you may have observed getter action by the contents of the cell. Some metals will adsorb certain species of gas molecules quite efficiently. In fact,this phenomenon is used to obtain ultra high vacuum and to produce extremely high purity gasses.

Jeff

I recall it was reported that the steel inserts/fillers inside GS5.2 developed rust on the cold ends of the reactor tube, which could have formed from steam coming from the slow decomposition into SiO+H2O of the silica fraction (SiO2) of the mullite ceramic tube in a H2 atmosphere at high temperature. Steam could have then interacted with the metal, forming oxides and hydrogen in the process mixed with residual water.

I wonder if a possible getter for hydrogen could be the water-forming reduction of Fe2O3 (and probably other oxides) under vacuum+heat conditions in a residual hydrogen atmosphere.

EDIT: I guess this would be possible only with the pump continuously operating, however.

Presure drop is nothing unusall. There is heat left, which allows for many possible endothermic reactions to happen. Usally reductions involving H!

From experience, you should never trust a Pirani gauge. Any gas could condensate at the gauge and bring false values. My Pirani gauge is just good to say the vacuum has been applied.
From CVG101 Worker Bee™ user manual:

Quote

Other contaminants
If your gases condense, coat, or corrode the sensor, the gauge calibration and response to different gases will change.
Generally, if the gauge can be "calibrated" ("zero" and "span" settings), these changes are small enough to be ignored. If
you can’t set zero and span, the gauge should be replaced or return to factory for evaluation and possible cleaning.

If you want an accurate vacuum pressure measurement, it is preferable to use active sensors with a diaphragm like CERAVAC-Transmitters from Leybold. It is quite expansive to compare with a Pirani jauge. It has limited range of measurements (4 orders of magnitude only). But this type of gauge is insensible to the type of gas or corrosive environment as it is with Lithium gas.

I may have something: dissociation of H2 >> 2 H increases the thermal conductivity.

"Nernst in 1904 had developed the theory of heat conduction in a dissociating gas and had shown that dissociation results in a great increase in the heat conductivity. The dissociation products diffuse from the hot portions of the gas into the cold portions and there, by recombining, give up the large energy of the reaction. This suggested, as mentioned earlier that the abnormal heat conductivity of hydrogen at high temperatures was due to dissociation of the hydrogen into atoms according to the reaction H2 → 2H."

http://www.astrosa.com/n06/77_Langmuir.pdf

Wouldn't that lead to a different correction factor for Hydrogen depending on the degree of dissociation?

I wonder if the low viscosity of hot hydrogen gas is just easier to pump, effectively making the pump seem more efficient.