Battery, capacitor or a bit of both

[large_ghostman]

Hi, I noticed something that got me thinking and I cant work out which is the most likely explanation. I have posted this pic before and again it isnt the exact vessel being used but they are very very similar.
Alot of the top parts that are stainless steel are actually sealed with a O ring, so they are in effect isolated electrically from the main top. The bottom collar is made from an insulating material (I dont know what the material is though), the one I am using also has a rubber collar on it and the stainless steel top sits on another airtight seal.
I am keeping these vessels in a light tight room, they are heated and contain organic material including some fruit. I wanted one of them in the chain to be slightly more insulated than the others after inoculating it, I placed some aluminum foil around the glass part of the body to do this.
While general faffing about trying to get a TDS probe working that sits in the vessel I noticed on my DMM that if I touch the central stirring mechanism with one probe and the foil with the other I get a voltage reading (800mV).
The contents are buffered to pH 4.5-5, my first thoughts were it was acting like a capacitor, but the voltage held for a around 15mins before I had to go and do something else. So now I am unsure if its creating a voltage or is indeed acting like a capacitor or maybe both? It dosnt matter so much in regards to the experiment, however when I measured it today the voltage had dropped to 40mV and didnt hold long, there has been biological changes to the contents so now I am wondering if this effect is worth looking into as a way of monitoring what is going on inside the reactor.
Like I say it isnt massively important to me but I would like some input on what people think I am seeing, if its behaving like a battery then it might be useful to me as another non invasive way to measure activity, if its simply a capacitor then it probably isnt as useful and it has pH probes etc inside and therefore I can see various ways a charge would build up.

So to cap
Stainless steel core isolated from the other parts, a organic acidic medium of thickish consistency, then a glass wall and aluminum layer on the outside.

Hmm might be a kind of battery as the foil was touching another part of the top and now it isnt so maybe now its more like a cap

 

[alec_t]

Could the stirring action be generating static electricity?

 

[Les Jones]

As I suspect all the metal part immersed in the liquid are the same metal (Stainless steel.) then I don't see that there should be any voltage difference between them. If the aluminium foil was not in contact with the main parts of the metalwork but was close to it and a drop of liquid bridged the gap then this would form a cell and would help to explain the readings. Another possible explanation is the voltage is from an external source. Possibly some leakage from the supply that drives the stirrer motor.

Les.

 

[large_ghostman]

Could the stirring action be generating static electricity?

I guess it could but I was measuring with the stirring off. I dont want to waste time looking into it if its a dead end, then again it might be useful. The more I think about it the more I am convinced its more like a capacitor, there are pH probes and all sorts in this reactor so I am assuming a small amount of current is put through the reactor, mainly from the TDS probe. I have built a jam jar capacitor a long time ago so it would fit with that, the only thing that makes me unsure is the length of time it took to discharge the first time I measured it.
I guess there is no harm in using one of the loggers ADC points and just logging the voltage on the inlet wash tube (this was the isolated metal part I touched with the probe) and the foil outer layer. Its hard to know what is going on with this particular reactor as its not been behaving the same as another identical one I have running. I have seen sudden regular drops in pH and increases in TDS at points where I wouldnt expect them, the other reactor that is set up the same way is behaving as the prediction says it should.
Its starting to bug me because i can see no obvious real difference in them, all cultures I have plated up have shown identical organisms in both reactors, although one has a slightly higher level of Archea (not enough effect this). Apart from blips in pH they seem within limits as far as chemical composition goes, I am fighting the urge to take it apart and clean it out! It produces less methane but only by around 1.7%, when I do a chi squared probability test on it this difference is within reasonable variance. I might leave this reactor in place and monitor it, it isnt going to harm the experiment so I might as well.

BTW I didnt think about static, it might be worth me grounding them in future

 

[large_ghostman]

As I suspect all the metal part immersed in the liquid are the same metal (Stainless steel.) then I don't see that there should be any voltage difference between them. If the aluminium foil was not in contact with the main parts of the metalwork but was close to it and a drop of liquid bridged the gap then this would form a cell and would help to explain the readings. Another possible explanation is the voltage is from an external source. Possibly some leakage from the supply that drives the stirrer motor.

Les.

You might be onto something with the foil and liquid, it cant be a voltage leak from the motor as the mount from the motor shaft ends in a wooden dowel with a cut in it to turn the shaft, so there is zero electrical contact. As for different metals..... I will double check because this reactor uses parts from other reactors, I had better double check there is only stainless steel inside next time I open it up.
There is a tiny chance I have a aluminum paddle on the stainless steel shaft, I doubt it but will double check. A current leakage from the TDS probe might be possible and the same with the redox probe, actually the redox probe has a platinum wire on the outside. This is the kind of redox probe used for measuring stuff like ozone levels injected into water, I am not measuring ozone but the probes are the same

 

[large_ghostman]

Just an update, in this case it turned out mainly to be the redox probe causing the problem. I guess when I saw the reading the probe was being read by the monitoring system, I dont know much about the probe and unit as it was a pre built system. I am kind of disappointed in a way