From: Paul
Date: Sat, 06 Jul 2002 15:00:13 +0100
Subject: Re: [TSSP] Re: Top V Probe Design
Terry wrote: > http://hot-streamer.com/temp/Jennings-JP-325/Diagram.jpg Thanks, your annotations make things a lot clearer. Are you planning to try this unit inside a TC? Wonder how conservative Jennings are with their ratings. By way of a recap on all this 1/ We want to know how much charge flows into/out of the streamer load on each cycle of the beat, and how much energy is dissipated in the process. We want to see how much, and how sharply this limits the topvolts rise below the value predicted without breakout. We also wish to see just what the breakout voltages are in order to see if a particular surface field strength threshold is involved. 2/ All of the above can be obtained quite accurately by simultaneously recording the top voltage and the base current, through a dual trace scope into a data file. 3/ We must ensure that we record a signal proportional to the top voltage rather than the top charge, otherwise we discard the very effects we want to examine. 4/ The top voltage pickup must not be sensitive to coil charge. 5/ The top voltage, capacitance, and total charge are related by Q = C V We have to treat all three quantities as functions of time, so the differential is dQ/dt = C dV/dt + V dC/dt We get at dQ/dt by noting that the coil base current is the sum of the displacement currents dQ/dt of the topload, and Ces dV/dt of the coil itself, Ib = dQ/dt + Ces dV/dt The Ces dV/dt term is calculated from the V trace and subtracted from the Ib trace to obtain dQ/dt. So far the only approximation involved is to assume Ces is constant with time. 6/ Then, since dQ/dt, dV/dt, and V are all measured, dQ/dt = C dV/dt + V dC/dt becomes a differential equation in C which we can solve. This would involve a least-squares curve fit and the error introduced here will depend on how clean the waveform capture is. This gives us C as a function of time. 7/ Applying Q = CV, and taking C(t) to be Ctop + Cstreamer(t), we can then calculate Cstreamer and its charge as a function of time during the whole firing event. 8/ There are three sources of error, a) Absolute error of the V and Ib measurements. b) Approximation of Ces as constant. c) Trace noise and limited information content (10K points by 8 bits). (a) can be calibrated out, say to 1% (b) can be estimated by computing Ces for a range of estimated streamer loads, and revised once a real streamer load is measured. (c) we have to live with. We cannot expect to average multiple shots because the fine detail of the streamer loading will not be coherent from one shot to the next. 9/ The modeling software is only involved as far as calculating values of Ces and Ctop. Well, there's some notes to be thinking about. Anyone spot any problems? It seems to me that once we crack this top-V measurement problem, it opens up quite a pandoras box. -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.