Capacitive Coupling / Parasitic Capacitance Questions 2

[Buckshott00]

Hello All,

I have (2) questions regarding Capacitive Coupling. I am pretty out of my league here as I am ME, and been a long time since I've had a circuits class. Any help is appreciated

I am designing a handle for monopolar surgical instrument. Now the handle itself has quite the irregular shape; additionally, there is an electrode running through parts of it to charge the tool for the desired electrical cutting and cauterization. The handle is also metal, as I mentioned it is well insulated against direct coupling, or arcing, but as I understand it there is no way to avoid some level parasitic capacitance or induction charging on that metal handle. Additionally, the longer the device is used the more charge it builds potentially discharging in unintended places.

1st question: Since I have the CAD model and geometry of the components, the material properties of the handle, including the dielectric properties of the insulator components and I know the power settings of the electrode, is it possible to estimate the capacitance of the handle? We don't want surgeons accidentally zapping themselves.

2nd question: Is there anyway to accurately measure that? I mean without discharging? So if I put an electro-generator on a prototype is there some sort of probe or dmm I could use to accurately and precisely measure the capacitance, and I could record it over time?

Thank you!

 

[IRstuff]

I think that providing a contactable metal surface will make things worse, because, as I mentioned above, you've now provided a conductive electrode for the capacitance, allowing the entire handle to participate, rather than just one tiny part.

In any case, for your measurement, the case is one electrode, and either of the two wires would be the other.


TTFN
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[Compositepro]

The operating principles of this type of surgical instrument is similar to corona treaters which are described here

http://www.enerconind.com/mediaLib/stml/techPaper/Enercon-corona-treating-overview.pdf?ext=.pdf

The "arc" in these instruments is a corona discharge where the air at the tip of the instrument is ionized by a very high electric field created by the sharp tip and high voltage. C

The manual that was linked to above said the instrument operated at 50 kHz. That is a long way from a MHz.

 

[Buckshott00]

"The 'arc' in these instruments is a corona discharge where the air at the tip of the instrument is ionized by a very high electric field created by the sharp tip and high voltage. C

The manual that was linked to above said the instrument operated at 50 kHz. That is a long way from a MHz."

[ol 1][li] You want these to discharge at the tip and nowhere else, this discussion was about capacitive coupling, and discharge through insulation materials. In this case, the handle is actually acting as a capacitor, and the capacitive charge is building. The insulation is not being damaged to the point of discharge.
[li] The majority of the tips are rounded. Regardless, in a perfect situation, the tip has the least resistance to the grounding pad placed on the other side of the patient, thus purposely creating an arc. However, as evidenced by Encision and numerous surgeons the instrument itself will act as a capacitor and discharge in zones 2 or 3 (or rarely zone 4) and arc through sections where the insulator is acting as part of the capacitor. [/li]
[li] You'll also notice in my above posts that I mentioned various ranges for the generator units. That particular unit is considered to be in the low range. However, as I also mentioned earlier Coag is the worst condition for this, and if you look at the manual, it states that Monopolar cut and coagulation occur at 390kHz, still far short of the 1MHz seen in the high range generators but more than 7X the number you're stating. Where did you get that? [/li]
[/li]
[/ol]

In the end it doesn't matter, regardless of my calculations nor the fact that for less than $5K we could prototype and build five to play with and know for sure vs. $80K for tooling for an alternative design, the design and 4+ months of work were scrapped.

 

[Buckshott00]

BTW Compositepro

Thank you for the article, it is very interesting. I once worked for a company where I designed a treatment chamber to apply a "chemical secret sauce" to various substrates to increase the surface tension chemically. It worked almost or as well as flame treatment.