[time-nuts] Cheap Rubidium (heatpipe cooling for)

[Bob Camp]

Hi

The tip it and listen to it slam test is a standard way of checking out a triple point of water cell for basically the same reason (you check the vacuum. Of course since a TWP cell is thin glass and not a nice metal pipe, you *may* break the seal by testing it ....

Bob


On Dec 27, 2009, at 9:33 AM, Joe Gwinn wrote:

> At 12:00 PM +0000 12/27/09, time-nuts-request at febo.com wrote:
>> 
>> Date: Sat, 26 Dec 2009 17:04:46 -0700
>> From: Robert Darlington <rdarlington at gmail.com>
>> Subject: Re: [time-nuts] Cheap Rubidium (heatpipe cooling for)
>> To: Discussion of precise time and frequency measurement
>> 	<time-nuts at febo.com>
>> 
>> My comments are in-line, below....
>> 
>> On Fri, Dec 25, 2009 at 4:38 PM, Joe Gwinn <joegwinn at comcast.net> wrote:
>> 
>>> At 12:45 AM +0000 12/25/09, time-nuts-request at febo.com wrote:
>>> 
>>>> 
>>>> Date: Thu, 24 Dec 2009 17:14:38 -0700
>>>> From: Robert Darlington <rdarlington at gmail.com>
>>>> Subject: Re: [time-nuts] Cheap Rubidium (heatpipe cooling for)
>>>> To: Discussion of precise time and frequency measurement
>>>>        <time-nuts at febo.com>
>>>> 
>>>> 
>>>> On Thu, Dec 24, 2009 at 1:32 PM, Bob Camp <lists at cq.nu> wrote:
>>>> 
>>>>   Hi
>>>>> 
>>>>>  A heat pipe might work if the fluid had a sufficiently low boiling
>>>>> point.
>>>>> 
>>>> 
>>>> 
>>>> The working fluid in a heat pipe will boil at every temperature above its
>>>> melting point.
>>>> 
>>> 
>>> Well, I've been thinking about this, and I used the term "heat pipe" too
>>> loosely.  Both the one- and two-pipe systems mentioned here have no wicks,
>>> and so technically are two-phase thermosyphons, which depend on gravity to
>>> circulate vapor and condensate.  A true heat pipe has a wick, and will work
>> > in zero gravity.
>>> 
>>> One gets significant heat transfer by phase change so long as the vapor
>>> pressure in the heat input end is high enough to generate enough vapor to
>>> carry the thermal power flow, and this makes the pipe isothermal.  However
>>> the temperature (although constant along the pipe) varies with the thermal
>>> power flow (in thermal watts) being carried.
>>> 
>>> What I'm looking for is related but different:  A device where the heat
>>> transfer capacity varies sharply with temperature, so that there is a range
>>> of heat transfer rates over which the input-end temperature will be
>>> substantially constant.  This is why I envision the fluid boiling (versus
>>> evaporating), which is actually out of the operating regime of a true heat
>>> pipe.
>>> 
>> >
>>>> I tend to use water because it's cheap, but have made them
>> >> with 3M "engineered fluids", Fluorinert, and denatured alcohol.
>> >
>>> Fluorinert.  I think that's what the expensive commercial CPU-cooling
>>> heatpipes use.
>>> 
>> $1000 a gallon!  Or $5 a drum when you buy it at a salvage auction.
> 
> That explains why low-end heatpipes use alcohol or acetone.
> 
> Actually, one ought to be able to use the freon intended for automobile air conditioners, for a whole lot less money, even new.
> 
> 
>> >>  I've found
>>>> that ordinary solder works just fine.  A trick to make these things easy
>>>> to build is to use a ball valve at the top (I'm assuming there is a top and
>>>> we're going with gravity return because it's simple).  I've got a few that
>>>> are still under vacuum for several years now in this configuration.  My
>>>> giant heat pipe of doom is a 10 foot stick of 1/2" copper with a ball valve
>>>> at one end and an end cap at the other.  There is perhaps 100ml water in
>>>> there total, and no air.  You can either boil the liquid until it builds up
>>>> a nice head of steam, or go the easy way and pull a vacuum with a pump and
>> >> just close the valve.
>> >
>>> I wouldn't have thought that an ordinary ball valve would be tight enough,
>>> allowing the water to escape and the air enter, slowly, although I suppose
>> > one can replace the water if it comes to that.
>> >
>> Mine have been running for a few years with no sign of needing to be pumped
>> down again.  They just work.
>> 
>> > But I think people want to build this exactly once, so I followed
>>> refrigeration practice.  A properly made hermetically sealed refrigeration
>>> system keeps its working fluid essentially forever.  I suppose one can use a
>>> refrigeration fill valve, say from an automobile air conditioning system,
>>> but these all leak to some degree.
>>> 
>>> Is the ball valve anything special?
>> >
>> Nope, just whatever was on the shelf at the local hardware store.
>> Stainless ball with brass valve body.  Teflon bearing surface.
> 
> Ahh.  A quarter-turn ball valve, used as a cutoff.  The term "ball valve" isn't quite precise in plumbing parlance.
> 
> These are very good, but still they are not hermetic, and will over decades (if not a few years) lose their working fluid.  I bet that while water will be contained, freon will diffuse right through the teflon seal of the ball valve.  So, there's the tradeoff.
> 
> 
>> >>  These things are incredible.  If you pack snow around
>>>> the end of this thing, the other end that is ten feet away gets cold almost
>>>> immediately.  They want to stay isothermal and the heat transfer is at the
>>>> speed of sound through the working fluid.  Delays are introduced because
>>>> you're dealing with a thermal mass of copper pipe that needs to change
>> >> temperature along with the working fluid so it's not quite instant, but
>>>> still about 10,000 times faster heat transfer than copper by itself.  They
>>>> are certainly handy for getting heat out of confined spaces.
>> >>
>>> 
>> > What is the purpose of the heatpipe of doom?  Education?
>> >
>> Education, fun, and then later a demonstration piece.  It's fun to rapidly
>> move the thing along its axis, upward and then stop.  The slug of water
>> moves up and then slams back down to the bottom and sounds like a steel ball
>> in the pipe.  It makes a satisfying clang sound.   A couple of years back
>> when I did a demo, people were convinced I had a metal part in there that
>> was loose.  I opened the valve and out came a 100ml water and nothing
>> else.   Too cool, and you can make them at home for next to nothing.
> 
> This definitely sounds like a good physics demo for school use.
> 
> 
>> Before
>> I started using vacuum pumps to pump them down, I'd use a blowtorch to boil
>> the water and use the valve to throttle the steam coming out.  Once the
>> steam is coming out really fast you basically just quickly close the valve
>> and remove from the heat source.   That's it!  For smaller diameter pipes I
>> use other methods and other working fluids because heating tends to just
>> eject the sometimes very expensive fluid.
> 
> What sizes, what fluids, what purposes?
> 
> 
> Joe
> 
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