[Amps] Distilled water in dummy load

[John Lyles]

Jim and all,
I've spent a lot of time for the past two years with high power liquid-cooled loads. The globar resistor assembly sounds neat.
The popular high power resistor loads that have water hose connections (Bird and Altronic at least) use a film resistor on ceramic core, not a bulk carborundum element. 
They run water through the center as well as around the outside of the element. Altronics makes many of the elements for the other mfrs. 

These resistors are made to run with tap water typically, as the R element is 50 ohms and it shunts the effective Z of the package with even slightly conductive water (tap water). 
Distilled or deionized water is tough on end contacts over time, so Altronics doesn't recommend that practice. I have a 150 kW water cooled load running with a pulsed amplifier, and it connected
to DI (deionized) loop out of necessity, not preference. I keep the flow very low through it when it isn't powered, so as to not leave the harsh water in it and then later flush this slug of water into my loop.
Same loop runs through the plate circuits of two tubes, so needs to be > 2 Megohm-cm resistivity to prevent > 0.5 mA through the fat water hoses. 

You should be able to cool your globar load with tap water, but a check with a network analyzer or antenna analyzer would be prudent. If you don't care about VHF and above, then it will be easier. 
 It also helps to check the Z when it is hot, which, in my case, was simulated with a New Mexico chili pot and a kW heating element along with a small circulating pump. 
Specific Heat of water is 0.997 BTU/lb/degF at 100F. Mineral oil is about 0.44. 50/50 Ethylene glycol/water is 0.855. 

For very high power, I cannot use resistor loads period and resort to water column loads, where the water is doped with a weak ionic solution to make it appear as a lossy medium, and the length 
of the pipe is such that the reflected power at chosen frequency is very low. These are used  for 3.125, 6.125 and 9.375 inch diameter coaxial loads. I've been testing 
QRO for months now at 210 kW average, 2.1 MW peak, pulsed, with about 70 gpm of ionic solution flowing through the column, 50 deg C outlet temperature!
I wish I could use some of this waste power to heat my home but its a long way from the work QTH.

73
John
K5PRO

> Date: Tue, 29 Nov 2011 06:05:34 -0800
> From: "Jim Thomson" <jim.thom at telus.net>
> Subject: [Amps] Can distilled water be used for a dummy load ??
> To: <amps at contesting.com>
> Message-ID: <60CEF2A432F54AA8AF17B9B978498671 at JimboPC>
> Content-Type: text/plain;	charset="iso-8859-1"
> 
> My  hb 50 ohm dummy load  consists of  4 x 200 ohm globars in parallel  [type SP, glass bodied
> and suitable for oil immersion].   Each resistor is 1" diam x 12"  long. [275 W CCS in air]
>  Each resistor  resides inside it's own  2.875" OD x sched 40  Aluminum tube. 
>  All 4 x thick walled  AL tubes are heliarc welded. 
> 
> I heard some place that instead of xfmr oil, that  distilled water can be used instead? 
> The same glass bodied type SP  resistors, [50 ohm, high wattage]  are used in  water cooled loads,
> the type that  has the garden hose on one end, etc. 
> 
> Seems to me that distilled water will conduct heat a lot better than xfmr oil.  The question is,
> will the use of distilled water in a 7 gallon metal container, with resistors immersed present any
> swr problems ?    If I remember correctly, swr was high on heath cantenna loads  if no oil used at all,
> and dropped to dead flat once the oil was poured in.   There was also a similar 50 ohm load in the
> old arrl books, same resistor as I'm using but it was a single 50 ohm unit, and not 4 x 200 ohms in parallel. 
> 
> I would prefer not to use xfmr oil if at all possible.   It's a little tougher to get a hold of these days..and not cheap.
> My main concern however is something leaks. Water on the concrete floor in the shop is one thing, but 7 x 
> gals of xfmr oil is a disaster.  Can you add  rust inhibitor's  or  glycol to the distilled water ?? 
> Do I even  have to use distilled water, or will  clean tap water work ? 
> 
> A 50/50  mix of water/glycol will result in a 265 deg F  boiling point..BUT the heat transfer is no
> where as good.   I'm using a new 7 x gal paint can for this project, the 4 x resistors and the welded AL
> tube assy, hang from the top lid. 
> 
> Thanks... Jim   VE7RF  
> 
> --