[Amps] Dummy load. Can distilled water be used instead of Oil ?

[Jim Thomson]

Date: Mon, 12 Aug 2013 15:15:39 -0400
From: "Roger (K8RI)" <k8ri at rogerhalstead.com>
To: amps at contesting.com
Subject: Re: [Amps] Dummy load. Can distilled water be used instead of
Message-ID: <5209345B.8060304 at rogerhalstead.com>
Content-Type: text/plain; charset=UTF-8; format=flowed

On 8/12/2013 12:24 PM, Jim Thomson wrote:
> Date: Sun, 11 Aug 2013 18:29:57 +0200
> From: peter chadwick <g8on at fsmail.net>
> To: Chris Wilson <chris at chriswilson.tv>, amps at contesting.com
> Subject: Re: [Amps] Dummy load. Can distilled water be used instead of
> oil ?
>
> One problem is that water has a dielectric constant of about 78, as opposed to around 3 for things like liquid paraffin. The resistors, assuming that you have thin wall tube, will be in a transmission line of about 66 ohms. This will drop by the square root of the dielectric constant, so each resistor will be in a transmission line of around 7.5 ohms. I figure that by the time you get to 28MHz, you're going to see a noticeable SWR and even more so at 50MHz.
>
> I once had a 50 ohm resistor in a suitable copper tube to make 50 ohms, but discovered that water screwed it up at the top of the HF range.
> In my apprentice days, we had water cooled 50 watt resistors as loads for 30kW HF transmitters, but the dimensions of the container were adjusted to get the SWR down. Quite a few gallons per minute were used, but at 30kW PEP, it took less than 5 seconds  to burn the resistor if the water flow failed.
>

Add compensation.  It remains the same across the HF bands and is often 
required even with the 4 large glowbar resistors.

Contrary to some beliefs, water, distilled or otherwise is the most 
effective "normal" cooling medium, but the dielectric constant does need 
to be taken into consideration.

73
Roger (K8RI)


> 73
>
> Peter G3RZP

###  I just thought of something.  IF tap water is used, wont the tap water short out the input connection ?   One could have as much as 700-1 kv peak sitting across the input to the load.  Distilled water would probably be
fine, even circulated through an external rad-fan setup. But if tap was used, and fed into the load, and out the other end, into a drain,  I would think that tap water would conduct the .7 – 1 kv ??   In my case, the way  the load
is already constructed, it would be pretty tough to keep water away from the 7-16 din connector.   Each of the 1 inch diam  x 12 inch long resistors sits inside a 2.875 inch OD x sched 40   AL  pipe.  All 4 x al pipes are also
12 inch long..and all 4 are heliarc welded together.   The base ends of the resistors are bonded to the bottoms of the AL pipes.  The top ends of the resistors are floating via insulators from the tops of the al pipes.  The tops of the
4 x resistors  are strapped to the hot side of the 7-16 din.  Tops of the 4 x al pipes are bonded to the cold side of the 7-16 din.   The V breakdown of these chassis 7-16 dins is aprx 2900 v peak..in air.   I may have just painted myself
into a corner.   I just can’t see this working with tap water.   OK,  I just checked a clean glass of tap water with my B+K 875B....and its 14.5 K ohms, with the leads .25 inch apart.   Increases  to 17.5 k ohms with leads 2 inchs apart.
The hot ends of the resistors are aprx .72 inchs away from the tops of the AL pipes.    But I only have .25 inch between hot side of 7-16 din  pin and top lid.   Flash, just tried distilled water,and its 60K with .25 inch spacing....and 
increases to to 200 k  with  1.25 inch spacing.    I have a few inchs between top of resistors and  7-16 din.  I could put any outlet fitting on the side of the 7 gal can.... just above the resistors, that way circulated water never gets 
near the 7-16 din.   I just might have a solution. 

Jim  VE7RF