Paul (below) is correct that Altronics loads (and
Bird) are thin film resistive elements on a core,
with water flushing by them. There is not
requirement for super low conductance water in
them, as the RF voltage is across the hot side of
the coax to ground, and is shunted by a 50 ohm R.
Guess where the RF current will flow.?
Bringing cooling water across HV requires a
different water. DI is the best and is
recommended by tube manufacturers, although it IS
corrosive. But there is a difference in the
metal-scavenging power of the water depending on
its temperature, ph, and resistivity. Take a look
at the Eimac paper on cooling water, I forget the
title, but they said it all. RCA/Burle also has
similar advice in their papers. Ditto, E2V,
Thales, etc.
When we build high power systems that require
cooling, and the water crosses HV gradient, the
hose diameter, the length of the hose, the type
of piping, and so forth are all critical for the
proper design. I had assumed when I said to use
DI water, that all of this was accounted for by
the amplifier maker (Alpha?).
You don't use DI water with steel or iron piping.
Use stainless steel, brass, copper, or plastics.
Do not allow the resistivity to get up to maximum
(18 Mohm-cm) as it will become very corrosive to
metals, including the tube cooling jacket. That
is why it is good to try and keep it in the few
Mohm-cm if you can, and make the hoses long
enough, or the diameter small enough, that the
'resistance' of the water hoses (2 of them in
parallel remember) prevents appreciable current
from flowing. How much is this? Depends on your
system. For big tubes, a few Ma may be
acceptable, as the amount of metal which m
migrates is small enough that damage is minimal
for the lifetime of the tube (few years). In a
small system, it should probably be uA. The
larger liquid and vapor cooled (and multiphase or
hypervapotron cooled) tubes will have a
sacrificial metal anode (or is it cathode?) in
the water hose fittings at each end of each hose.
With this, the plug will slowly burn away,
leaving ions in your system. It is inspected
regularly, at least yearly, to determine the rate
it goes away. When it reaches the edge of the
metallic fitting, then it is replaced. This keeps
the DI water from taking away important metal
(like tube fittings, pipe fitting at ground,
etc.). This is only needed across the HV
boundary, not throughout the water circuit. One
more thing, oxygen is a bad thing to have in a DI
water system, so it needs to be removed (with
another Culligan style bottle).
The idea of hipot testing water can be done, if
you use a nonconductive pipe and want to measure
the leakage in a known volume of DI water. This
won't work for tap water, hence the need for DI.
I hipotted a tube in circuit last week, with 5
M-ohm-cm DI water. I had about 0.5 mA of leakage
in the hoses, so the high potter must be able to
handle that without tripping off. It worked fine,
and I discovered a failed plate blocking
capacitor without removing the tube. Not
impossible, nope just did this.
I have never used anything but DI water and most
other high power RF systems for acclerators and
for broadcast (Continental Electronics in Dallas
makes many) use the same. But one must be very
careful of the fittings, the resistivity, etc.
Probably beyond the scope of what we are talking
about for ham use in an amplifier, perhaps. There
are some excellent papers on this, from Argonne
National Lab, which I can refer folks to if they
are interested further.
John
At 2:32 PM -0500 2/6/07, amps-request@contesting.com wrote:
>Message: 7
>Date: Tue, 6 Feb 2007 14:16:14 -0500
>From: "Harold Mandel" <ka1xo@juno.com>
>Subject: Re: [Amps] WATER for vapor cooled amplifier
>To: "'Robert B. Bonner'" <rbonner@qro.com>, <fsiyfr@okeechobee.com>,
> <amps@contesting.com>
>Message-ID: <006001c74a23$47081010$630ca8c0@laptopHM>
>Content-Type: text/plain; charset="us-ascii"
>
>Okay, so let's back up one step and sorta tabulate our findings
>and observations:
>
>1. Someone found that Deionized water was too conductive.
>2. Altronics says never to use DI water in loads.
>3. Someone found distilled water too conductive.
>4. Altronics says distilled water is too reactive, also.
>5. Some people find Zephyr Hills distilled water
> not too conductive and okay.
>6. Altronics says clean, potable tap-water with less
> than 500 parts per million of debris is okay for
> RF loads.
>7. BIRD says distilled water for loads is best.
>8. Paul found one water source supposedly good, a
> dead short.
>
>My head is beginning to spin!
>
>The idea of putting the HiPot tester in the water seems
>the top winner today.
>
>Water, at it's best is supposed to be at pH 7.00, yes??
>Then, it's at 1.00 Specific Gravity, yes?
>That should mean it has a certain conductivity, yes?
>
>Why would potable water, not distilled or DI, have a different
>conductivity between brands?
>
>Discounting the DI story, why would distilled water be
>harmful? Why would it have differing conductivities
>from brand to brand?
>
>Lastly, Altronics says to include a "people-safe" antifungal
>agent in recirculating systems that does not include salts
>or any other caustic agents. Do any of you use antifungal
>materials in your recirculating or vapor-phase cooling rigs?
>
>Hal
>
>
>
>
>------------------------------
>
>Message: 8
>Date: Tue, 6 Feb 2007 14:32:29 -0500
>From: "Paul Christensen" <w9ac@arrl.net>
>Subject: Re: [Amps] WATER for vapor cooled amplifier
>To: "Harold Mandel" <ka1xo@juno.com>, "AMPS" <amps@contesting.com>
>Message-ID: <030f01c74a25$8a802dd0$f9153405@OfficeDell>
>Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> reply-type=original
>
>> "Okay, so let's back up one step and sorta tabulate our findings
>> and observations..."
>
>A significant difference in the use of water with RF loads is that it is not
>mixed with high-voltage and accordingly, probably no real reason to minimize
>leakage, but immunity to long-term corrosion is important. Typically, in a
>water-cooled load, water flows through a glass jacket which, surrounds a
>50-ohm resistive element.
>
>In my prior life, I used a 25KW Altronic water-cooled load on a Harris
>FM-20K commercial broadcast transmitter. We simply ran a hose from the 46th
>floor water tap and it remained that way for 15+ years without encountering
>any trouble. In all fairness though, I believe the building (downtown
>office complex) used a soft water system.
>
>On the other hand, since the vapor-cooled amps require good HV isolation, it
>would seem that the water requirements are much more restrictive with two
>overarching requirements: (i) low water conductivity; and (ii) low corrosive
>action. I am not sure what the right answer is but the Zephyhills product
>labeled as "distilled" seems to work well with my Alpha 70V.
>
>Paul, W9AC
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