Cooling and insulating fluid

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De-ionised water would work. I've seen that used to cool the floating anode of an X-ray tube running at that kind of voltage. It's got a higher heat capacity and a lower viscosity than most oils.

An alternative would be a heat-pipe, if you could guarantee that the fluid being evaporated could be spread evenly over all your zeners.

Plugging them all into a block of a alumia - which does have a respectable thermal conductivity - 29 W/m/K - could work. Aluminium is better - at 201 - as is copper at 385 but both conduct. Diamond is brilliant (pun intended) at 900, and it is a good insulator - a vapour- deposited layer might be worth the trouble and expense if you were really pushing the state of the art.

some sort of boiling liquid is going to do better than an unforced circulating liquid. build a heat-pipe.

Will they sell you a small quantity? That is always the problem with "cool stuff". Often it is easier to sweet talk a rep than to buy the stuff.

That said, I've always seen mineral oil used, but that doesn't mean FR3 isn't better. It is just a matter of availability.

I know zip about regulating 25kV, but meditating on such a string of zeners, the first thing that comes to mind is a string of devices just needs one device to fail for the whole string to fail. Is failure tolerable for your application. I means, if nothing catches fire or blows up, that might be OK. If a zener failing means you have a IED, I'd consider a back up safety scheme.

Failure is an option as long as lawyers don't get involved.

I saw the post about using water, but you might want to consider expansion. I don't know what it takes to boil water in a small tube. Well actually I could compute it, but well you know...that would be work.

How long would the water remain de-ionised when in contact with components, leads etc?

Depends on the metals involved, and whether the surfaces had been varnished/potted before being exposed to the water.

Dutch plumbers fill regular central heating systems with clean water, throw in an flexible expansion chamber to take thermal expansion and contraction, and leave them sealed, with no corrosion inhibitor. It works fine if you aren't silly enough to add aluminium radiators to the copper pipes and the normal pressed steel radiators.

This is going to be running at a high temperature, so perhaps something which is solid or waxy at room temperature would make sense from the mess/handling point of view?

As you're relying on density change with temperature to provide circulation, maybe this characteristic is the most important characteristic to consider. Does transformer oil circulate? You'll need some expansion room too.

Cheers

I wonder how well a diamond paste would work?

Cheers

Not long. The HV would force some current through even the cleanest water. The current will tear apart metal surfaces and fill the water with metal ions, and the process will quickly run away.

All that effort to make a custom vibrator ?>:-} ...Jim Thompson

Water CAN be kept clean, but it is work. I work with cyclotrons, and they have low conductivity water and ultra-low conductivity water. The ULCW stuff is used to cool the transmitting tubes that generate the accelerating RF, and they may have 50 - 100 kV on the anodes at the peak of the RF cycle. They usually use a coil of nylon tube to extend the length of the water line across the high voltage span, and have maybe 10 feet of line.

This stuff is circulated through a heat exchanger with a deionizing cartridge and a conductance monitor.

But, to remove tens of Watts of heat, an oil, or Fluorinert if cost is not a problem, is probably a lot easier to maintain.

Jon

That didn't seem to happen with the water-cooled floating-anode X-ray source that I was (marginally) invovled with at Nijmegen University, back in the late 1990's. there certainly wasn't enough current flowing to mechanically damage any of the surfaces involved. I'd have expected the charge carriers to be (hydrated) hydrogen and hydroxyl ions - which form spontaneously in water at any temperature about absolute zero, at a concentration of about 10^-7M at room temperature. CO2 diffusing in from the atmosphere will push this up a bit, and regular distilled water has a conductivity of the order of 1uS/cm due to the carbonic acid content from the CO2 in the air.

Darn don't mention beer at ~4:30 on Friday. I might have to go home and 'insulate' myself right now.

George H.

Less corrosive, too. When ultrapure water becomes less pure, by dissolving metals out of the things it contacts, those metals are... well, no longer doing their original jobs. The better a job your deionizing cartridge is doing, the greater of the tendency of the purified water to eat away at your components.

I agree, some form of light neutral oil is likely to be less of a maintenance issue in the long run, if it's going to be in contact with the components.

Distilled water was used to cool high power tube based transmitters. It was changed ionce a year. The RCA TTU-25B had 7 KV across parts of the cooling system, and used about 50 feet of heavy rubber pipe to isolate the sections. They had a meter to test the water purity. That transmitter needed better than 100 Megohm per cubic centimeter resistance to stay on the air. If it dropp, you had to drin the transmitter, disolve five pounds of Citric Acid in fresh distilled water nand run it through the cooling system. Flush it two more times, then refill it with distilled water and maybe 50% glycol antifreeze depending on the location & season.

ATSM1816 is a 0.100" gap. This is similar to Shell Diala AX which is about 32kv for ATSM 1816.

It will be fine, as long as you have the right spacings and creepage distances. You need a expansion thingy there as it will probably expand 8% or so from 25c to 40c. and make sure your seals are Buna N.

Oh, and water is only good for making resistors ;)

Cheers

Robert is proposing 25 KV over about 6 inches. That's 350 times the voltage gradient of the RCA transmitter, so assuming linear scaling he's have to flush it about every day. And I don't think he'd include a circulating system or purity testing. It wouldn't last long.

Maybe a DOT 5 brake fluid?

Yeah, with ten feet of looped hose running back from the anode (maybe less, but always some length, even if the DI is fresh).

At mA, inside a case, without pump, filter or replacement, you're nuts.

DI goes bad after a while, because there are always ions to dissolve, even from nonobvious sources. Analytical chemistry texts go into grotesque detail when ultrapure water is desired: even quartz glass apparatus contains enough traces of sodium to foul up a sensitive ICAA test. The solubility of silica alone becomes noticable.

As oil goes, how well it cools depends on how well convection works. In a tight case, even water will end up significantly worse than potting compound. A good epoxy or silicone potting compound, advertised for high thermal conductivity ("high" meaning ~1 W/(m.K) or thereabouts), will do better than stagnant water.

Tim