From: tiq@fennel.cc.uwa.oz.au
Subject: Re: Heat pulses in P&F cells
Organization: University of Western Australia
Date: Fri, 10 Apr 1992 06:13:59 GMT

In article <920409175943.208004be@FNALD.FNAL.GOV>, ames!FNALD.FNAL.GOV!DROEGE writes:


> A constant pressure servo continuously measures the cell gas volume.  
> We interpret an increase in gas volume as being due to absorption of 
> deuterium by the cathode leaving behind un-recombined oxygen to be 
> measured.
> 
> We have been running for the last thirty days with a current saw 
> tooth which starts at 40 ma per sq cm, ramps to 600 ma per sq cm 
> over a half hour, then returns to 40 ma per sq cm.
> 
> We observe with McKubre et. al. that cells are noisy.  He says (Como 
> p-436) "For both electrodes there was a 'high-frequency' fluctuation 
> of the excess power with periods of aprox. 0.1 - 1h which correlated 
> with cell pressure variations, characteristic of partially 
> intermittent operation of the catalytic recombiner."
> 
> While some measurements are consistent with intermittent 
> recombination, others indicate to us that gas pulses in and out of 
> the cathode.  We note that the gas volume stays constant during the 
> first half of the ramp.  This indicates that at low gas evolution 
> rates the catalyst keeps up with the gas production.  At the higher 
> rates, there are both sudden decreases (larger) and increases in gas 
> volume.  In our naive view, we would think that the catalyst would 
> work better at the high rates, since it is hotter.  The cell 
> contains at least 10x the catalyst area needed for the current.  
> Physical observation seems to confirm the pulsing.  Storms seems to 
> have observed similar behavior. We would welcome comments by 
> experts. 
> 
> Now for an event of the type that puzzles me.  During the higher 
> current portion of the ramp, we occasionally observe a sudden 
> decrease in volume of typically 2 cc, or 100 relative volumes of 
> deuterium (2/.037 *2). On the same one minute sample time the cell 
> temperature increases by .15 C and the catalyst temperature 
> increases.  There is a temporary change in heat balance but the long 
> term heat balance is consistent with zero to the present calibration 
> - about 4 mw.  

This is more of a general comment on recombiners and loading measurements
than an explanation of your strange results, but here goes.
Firstly, a better way of measuring the loading might be to fill the cell
headspace initially with deuterium gas. Then, the excess oxygen will recombine
with deuterium and there will be a volume decrease which can be related to the
loading. Actually, I prefer to do keep the thing at a constant volume and 
look at the pressure drop but the methods are equivalent. The rationale for
filling the cell with deuterium is that the catalyst is always kept in a 
reducing enviroment and is kept in its most active state. In a cell where
there is an oxygen rich enviroment (ie in your cell) you run  the risk of
partially oxidising the Pt catalyst and killings its activity. 
The effect is quite dramatic, and the catalytic activity can drop by more
than an order of magnitude  for fairly moderate amounts of oxidation of the
catalyst. Maybe this effect can be related to your observations, Tom.
Recombination is certainly intermittent in any case. In the few closed cells 
I've run, it was fairly common to see the calorimeter output plunge below
the calibration and then shoot above it briefly before settling. This was
consistent with the catalyst effeciency dropping below 100% and then 
"catching up" again. One cell would do this about every two hours, regular as
clockwork.

--
Todd Green 

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