# [TCML] Measuring spark gap resistance

Lau, Gary Gary.Lau at hp.com
Thu Sep 11 15:05:45 MDT 2008

Hi Bert,

Re-reading and discussing my post from 2000 where I measured tank circuit losses due to eddy currents induced in nearby things has gotten me interested in comparing and measuring gap losses between my sucker, vortex, RQ, and RSG's.  It should be relatively easy to record the tank ringdown waveform without the secondary in place.  The ringdown envelope will be a linear decrement so it will be largely independent of firing voltage, and easy to compare.  But to put the results in a truly useful form, suitable for use in simulation models, I would like to boil that down to a representative equivalent resistance.

I understand that arc resistance is a very dynamic thing, dependant on current.  In that post from 2000 I referenced a formula that I attributed to you, to convert the linear ringdown time and inductance into an equivalent resistance: R = 2L/t.  It only now dawned on me that this is probably misleading me.  Since the ringdown is linear, if Vbang is reduced by a factor of two, the calculated resistance will be increased by the same factor.  While this makes sense given the dynamic nature of arcs, it doesn't help me arrive at a useful simulation resistance.  It looks like the ringdown slope is the indicator of losses, rather than the calculated R.

I don't really understand the significance of the 2L/t value and how it might relate to an "average" resistance.  Can you suggest a way to leverage the linear ringdown measurement into a fixed resistance that can accurately model gap losses?  I fear the short answer is "No, it can't be made that simple", but I thought I'd ask!  If not, the ringdown times alone will give a clear and useful quantitative comparison of losses.

Regards, Gary Lau
MA, USA