I have no first hand knowledge one way or the other [this was not my assertion]. I believe Dr. McKubre to be an outstanding researcher and have no reason to believe this escaped his attention. Even if it did, it would only be a minor error and does not alter conclusions.
It will make a difference whether in constant current mode or constant voltage mode. Given the nature of the chemical cell having a "cell voltage", constant voltage mode would likely be worse because I think the bubbles would cause larger swings in real instantaneous power than if the cell were run in constant current mode. The problem exists to some extent with almost any real DAQ. If the current remains perfectly fixed, the voltage will change and there will still be real instantaneous power variation, only I think less real instantaneous power variation than in constant voltage mode. The smaller the real instantaneous power variation, the smaller the error introduced by using average current and average voltage over a discrete interval. Keep in mind, we are talking about small changes in resistance - well within the regulation circuits of most power supplies. Adding a capacitor across the supply just insures constant voltage mode - which I believe would be worse than constant current mode. The most practical solution is to reduce the sampling period by spending more on the DAQ. The error can be reduced via shortening the sampling period until the error is no longer a concern - at additional cost of the DAQ. Just like anything else, you can usually buy more accuracy. Bob On Mon, Oct 27, 2014 at 10:21 AM, David Roberson <dlrober...@aol.com> wrote: > If the internal current control feedback mechanism is slow to act, then > the output current might indeed change significantly. I have never put a > constant current supply under careful supervision before but assumed that > the guys designing them would go to lengths to ensure that they in fact > maintain the output DC current constant under varying loads. Have you > performed this measurement on a high quality constant current supply? > > We also would need to assume that McKubre was not aware of the possible > problems that have been pointed out. Perhaps we should get feedback from > him to answer that question. > > If the current does not remain fixed and DC, then there are many possible > errors to follow up on. On the other hand, if the source really does keep > the current constant with adequate feedback control then the input power > can be accurately determined by only taking into account the average DC > voltage appearing across the supply terminals. AC signal voltages > generated due to bubbles, etc. should not enter into the power input > measurement unless they force the supply to go into operation outside of > its normal range. > > Bob, I would be somewhat surprised to find that an expert of McKubre's > caliber would not have a good handle upon the input power and energy levels > after chasing that sort of problem for many years. Surely he would have > seen the significant variation in current flowing through his test system > at some time and attempted to rectify the situation with a better constant > current system. Perhaps something as simple as a large capacitor across > the supply output terminals would smooth out the current pulses. How > confident are you that he missed this issue? > > Dave
