[time-nuts] Cheap Rubidium (heatpipe cooling for)
Bob Camp
lists at cq.nu
Sun Dec 27 19:17:48 UTC 2009
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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