Water Cooled Power Resistor Gets Rusty

"D from BC"

** All you need do is coat the terminals and any exposed metal that is submerged with something waterproof.

Use shrink on tubing, silicone adhesive and/or spray on lacquer to seal it up.

I had much the same problem with tubular WW dummy load resistors used to test power amplifiers but have managed to make them last many years.

Forget using oil - far too damn dangerous.

If it don't smoke and catch fire, it will spill sometime and burn YOU like hell.

....... Phil

Use 3M's "Fluorinert" dielectric fluid.

FC-40 first one on the list.

1500V per mil breakdown strength.

As soon as your water gets into contact with those metals, it becomes ionized water.

Should post to plumbing.sci. We don't repair umbrellas.

Stanislaw.

Surprise, surprise...not.

Not to mention messing with the resistance. When you want a serious (but not altogether stable itself) resistive test load, just stick a couple of copper electrodes in a copper sulfate solution - it's what we used to do at the lab for making BIG power resistors.

One of my least favorite memories of lab work would be working in marx generator tanks dripping with oil.

Should be fairly benign.

Nichrome from an old toaster or heater in a box (to keep you from touching the live wires) with alligator clips is a nice adjustable test load that works fine when air-cooled, so it's nice and clean and non-icky.

A few ordinary 25-50W air-cooled power resistors and some switches to configure the bank could also work, though the resistors are rather expensive if not found surplus.

--
Cats, coffee, chocolate...vices to live by

D from BC wrote in news: snipped-for-privacy@4ax.com:

most people cool their power resistors in mineral oil. That is also what my USAF cal lab(PMEL) kept their standard resistors in. It can get a bit messy,though.

--
Jim Yanik
jyanik
at
kua.net

The old Heath Cantenna, used mineral oil.

I made up some fixtures with screw on resistors and heat sinks, with the capability to use a fan.

greg

"Jim Yanik"

** Only if the load is for RF use and there is a safety temp cut out.

..... Phil

"G"

** A dummy 'antenna ' by name and use. ** Very expensive and tends to stink the room out when it gets hot & there is any dust on the parts.

A water submerged, tubular WW resistor never smells or catches fire, dissipates huge amounts of power & self temp limits to just over 100C - keeping the resistor value accurate.

...... Phil

Commercial liquid cooled loads, like Heathkit and MFJ, use oil. Supposedly transformer oil is best, followed by mineral oil. I knew of one person who used automatic transmission fluid. Antifreeze may or may not work. The ones I mentioned are built into unused gallon size metal paint cans. You should be able to find a lot of info if you do a search on Cantenna.

Tam/WB2TT

how about using mineral or transformer oil instead?

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

"Tam/WB2TT"

** Realise that is because you CANNOT use water for a radio frequency dummy load.

The OP is not restricted to using a bad and highly DANGEROUS heat absorber like oil.

........ Phil

How about peanut oil? About 50% higher dielectric constant than mineral oil, better temperature characteristics, IIRC, and available at any grocery store. But any oil is going to be vasty inferior to water in heat capacity (like 1/2 or 1/3 as good). Maybe just add a bit of radiator antifreeze to the water (it contains corrosion inhibitors).

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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I use Olive oil and Balsamic vinegar.

It's a heat transfer medium *and* a salad topping.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

It's the liquid breathing rats stuff!

It's going to be exxxxpennnsive.... D from BC

Reading all replies... (thanks all by the way..)

Water Vs Mineral Oil

Water Pros Off the tap.

100C bp is commonly known Non toxic liquid Non toxic steam Odorless Higher specific heat capacity than mineral oil.

Water Cons Corrosion of ferrous terminals Ion content increases with rusting and leads to electrolysis erosion due to exposed terminals at 100VDC.

Mineral Oil Pros No corrosion. Obtained at pharmacy. No electrolysis at 100VDC.

Cons Possibly more resistance error due to higher boiling point. bp ~360C??

Vapors can ignite with spark (flash point) around 145C to 193C??.

(See movie of a fire ball from an exploding mineral oil transformer on http://205.243.100.155/frames/longarc.htm#Blowup )

Lower specific heat capacity than water.

Oil vapor coating everything.. Yuck.

Oil smell.

Soooo.. I'm going to coat the terminals and stick with water. (as per Phil post). But.. The tap contact on the tube power resistor is ferrous and I can't coat that.. It'll rust but not erode due to electrolysis (the other terminals are coated.) So if electrolysis can't take place then maybe I can try out Vitamin C.. :P I'm no chemist but I'm dying to try Vitamin C as a corrosion inhibitor. It's got that "fun with chemistry" appeal.. My steaming power resistor may have a nice zippy smell too :)

But ..if the coating fails on the other terminals ...just one microscopic crack or hole and I think the electrolysis will start peeling the coating and lead to even more electrolysis.

Seeing Vitamin C getting electrolyzed will be new :P

The device under test is safe to overload..so if I get a liquid short out..it's ok...

D from BC

[100 ohm adjustable, 100W range]

You can use several solutions: exchange the water for a nonionizing fluid (mineral oil with high flashpoint is cheap, transformer oils and fluorinert are also available), or just lower the water's corrosivity (add rust inhibitor from the auto parts store to deionized water), or use resistors that are water-safe.

Every electric water heater, from tea kettles to home hotwater tanks, has such an immersible resistor. Some have several. One here in my junkbox is rated at 4500W at 240VAC - so it's 12.8 ohms. Eight of those, and you can pick resistances from 1.5 to 100 ohms with no rust.

Neat... I can picture myself filling a shopping cart full of used water heaters at the local thrift store.. :)

D from BC

You could coat the tap contact and the entire length of the exposed resistance wire the tap slides on with silicone grease without increasing contact resistance or losing adjustability. The silicone grease sold in auto stores as dielectric grease is best as it has no additives and therefore the lowest water absorption of any silicone grease, but pretty much any grease should do the job for short term testing.

At a paltry 100 watts your reason for not using an air cooled load must be a parts on hand issue. I have used air cooled loads up to

1250 kW, and they make them much bigger than that.