GS5-3 Pressure Puzzle - An Open Challenge

  • At the tail end of the GS5-3 run, we did some additional tests suggested by Ecco. We pumped out the cell at ~500 C (core), reaching ~40 microns. This is the typical limit of the vacuum system and cell plumbing in many previous cycles. Then we added H2 from a bottle to 1 bar. After a 30 minute dwell, we pumped the cell out again and found it reached 5 microns of vacuum.


    This test was repeated several times, at 600 C and 700 C in the core, with the same result. A final test cycle at 850 C (core) reached a vacuum level of just 2 microns, far better than the vacuum system can achieve even with a closed manifold (typically 25 microns).


    I find these results among the most startling and unexpected in my 2 years of Glowstick experiments, and I can't think of a physical explanation for what we measured. I'd appreciate comments on how this can be explained by ordinary physical and chemical phenomena. Tom Clarke's insight would be especially welcome, since he has a demonstrated skill at suggesting plausible explanations for unusual measurements.


    The vacuum pump is a Welch Duoseal 4000 with a Pirani vacuum gauge mounted on the manifold. It's connected to the cell with about 40 cm of Tygon tubing with Swagelok valves. The vacuum gauge is battery powered and hasn't shown any unusual behavior in the past. The cell heater was powered from a Variac during these tests, so no EMI above 60 Hz was present.

  • 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. :D


    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.

  • The measurement is from a VG200 Pirani vacuum gauge, not a pressure gauge. These transducers use a thermistor heated by constant current. The thermister temperature is an inverse function of the gas thermal conductivity. Pirani gauges are typically calibrated for Nitrogen (air). A downward correction factor of 0.5 is needed for Hydrogen. See
    https://www.mpi-hd.mpg.de/gerd…uals/bg805178be_d_web.pdf


    Thus the reading of 5 microns would correspond to a H2 pressure of 2.5 microns for this type of gauge.

  • 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

  • @jeff Yes, possible. We're looking for something in the cell that can getter free Hydrogen. In theory, the Nickel is fully loaded with as much H as it can adsorb. But Hydrogen can combine with molten Lithium, forming LiH which is solid at these temperatures. At ~900 C it begins to decompose, again releasing the Hydrogen.


    Further, if the cell is then pumped out, when it cools, a surplus of elemental Lithium remains. The process is in theory repeatable if H2 is again added. And that is in fact what we did during these tests.


    We'll explore this further in the next incarnation of GS.

  • 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.

  • 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.

  • @Arnaud Yes, I'm aware of the limits of a Pirani-type gauge. It does offer the important advantage of being able to tolerate positive pressure without damage.For a roughing vacuum system like mine, It's a logical place to start.


    I do plan to add a better instrument, perhaps a capacitance gauge like the CEREVAC. These typically require careful protection against over pressure, though some can tolerate up to several bar.

  • 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?