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Re: Solving the Floating-Point Goldilocks Problem!

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  Re: Solving the Floating-Point Goldilocks Problem! jmfbahciv <See.above@aol.com> - 2011-09-08 14:10 +0000
    Re: Solving the Floating-Point Goldilocks Problem! sidd <sidd@situ.com> - 2011-09-08 14:08 -0400
      Re: Solving the Floating-Point Goldilocks Problem! jmfbahciv <See.above@aol.com> - 2011-09-09 15:21 +0000
        Re: Solving the Floating-Point Goldilocks Problem! "Rod Speed" <rod.speed.aaa@gmail.com> - 2011-09-10 06:00 +1000

#3572 — Re: Solving the Floating-Point Goldilocks Problem!

sidd wrote:
> On Wednesday 07 September 2011 10:05, Huge wrote:
>
>> It had Nixie tubes, and I've wanted a Nixie tube clock ever
>> since.
>>
>> [Sighs nostalgically some more].
>>
>
> i once had a very,very,very sensitive ohmmeter with Nixie tube
> display, designed to measure large numbers of ohms. i could
> stand on a ladder to the head of the cryostat and run a probe past
> a rail of a hundred connections or so, and watch the display on the
> rack across the room and if i had any leakage at all, the whole
> display would flash...if the display remained dark i would laff
> like a maniac since that meant i would not have to warm up the
> cryostat and find and fix the short...which sometimes had the
> orrible habit of disappearing at room temperature...
>

What a kewl lab.  How did you find the disappearing shorts?

/BAH

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#3589

On Thursday 08 September 2011 10:10, jmfbahciv wrote:
 
> How did you find the disappearing shorts?

Depended on the specific fault. First, log the fault. Then find,
isolate and monitor and immediately set one or more recorders to 
monitor them. Back in those days, this might be a chart recorder, or a
grad student.Then you consulted the logbook, pored over heavily
modified schematic and illegible scrawlings. You had, of course,
measured resistances and  impedances of all legs of each circuit at
room temperature, when the cryostat was open, and you had
periodically measured many things as the cooldown proceeded. You had
data on the composition and behavior of the various bits of your
wires. From these and the currently measured values, you could often
make an intelligent guess at the location. Before you warmed up the 
cryostat you could do further testing...if for example if i push a 
current through the short, which bits of the cryostat warms up ? or if
a coax line, you might break out the storage scope and polaroid, and
do TDR off the line to get an idea of distance to the fault.
 

After you were done with any lo-T diagnosis, you warmed up the
cryostat watching the fault all the way to room-T. You have
thermometers and sensors all thru the cryostat, you might pause
along the route if you saw sumpn interesting...eg circuit
parameter changed when this bit of the circuit warmed through 
superconducting transition temperature...and such. Along the
warmup, you take advantage of the hysteresis, in that the innardz
of the cryostat cool the slowest and warm the slowest, so you are
applying different thermal (and thus mechanical) gradients to the
cryostat and you know which parts are being stressed in which order;
this order is different during cool down and warmup.

Disappearing faults disappear differently.The manner of their
vanishing often betrays whence they came. When all else fails,
you rip out and replace, an operation that (in some cases) can 
take months.

sidd

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#3594

sidd wrote:
> On Thursday 08 September 2011 10:10, jmfbahciv wrote:
>
>> How did you find the disappearing shorts?
>
> Depended on the specific fault. First, log the fault. Then find,
> isolate and monitor and immediately set one or more recorders to
> monitor them. Back in those days, this might be a chart recorder, or a
> grad student.Then you consulted the logbook, pored over heavily
> modified schematic and illegible scrawlings. You had, of course,
> measured resistances and  impedances of all legs of each circuit at
> room temperature, when the cryostat was open, and you had
> periodically measured many things as the cooldown proceeded. You had
> data on the composition and behavior of the various bits of your
> wires. From these and the currently measured values, you could often
> make an intelligent guess at the location. Before you warmed up the
> cryostat you could do further testing...if for example if i push a
> current through the short, which bits of the cryostat warms up ? or if
> a coax line, you might break out the storage scope and polaroid, and
> do TDR off the line to get an idea of distance to the fault.

Distance?!  Were there loops?

>
>
> After you were done with any lo-T diagnosis, you warmed up the
> cryostat watching the fault all the way to room-T. You have
> thermometers and sensors all thru the cryostat, you might pause
> along the route if you saw sumpn interesting...eg circuit
> parameter changed when this bit of the circuit warmed through
> superconducting transition temperature...and such. Along the
> warmup, you take advantage of the hysteresis, in that the innardz
> of the cryostat cool the slowest and warm the slowest, so you are
> applying different thermal (and thus mechanical) gradients to the
> cryostat and you know which parts are being stressed in which order;
> this order is different during cool down and warmup.
>
> Disappearing faults disappear differently.The manner of their
> vanishing often betrays whence they came.

Ah!  That would be useful.

> When all else fails,
> you rip out and replace, an operation that (in some cases) can
> take months.

Thanks.  No wonder physicists love their gadget (not to be
construed as a slight).

how does one learn all those tricks such as "disappearing
differently"?  Wouldn't the behaviour be unique to each
cryostat?  Or are they constructed so alike that working
on one trains you to be able to do the same diagnoses
on another.

/BAH

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#3597

jmfbahciv wrote
> sidd wrote
>> jmfbahciv wrote

>>> How did you find the disappearing shorts?

>> Depended on the specific fault. First, log the fault. Then find,
>> isolate and monitor and immediately set one or more recorders to
>> monitor them. Back in those days, this might be a chart recorder, or
>> a grad student.Then you consulted the logbook, pored over heavily
>> modified schematic and illegible scrawlings. You had, of course,
>> measured resistances and  impedances of all legs of each circuit at
>> room temperature, when the cryostat was open, and you had
>> periodically measured many things as the cooldown proceeded. You had
>> data on the composition and behavior of the various bits of your
>> wires. From these and the currently measured values, you could often
>> make an intelligent guess at the location. Before you warmed up the
>> cryostat you could do further testing...if for example if i push a
>> current through the short, which bits of the cryostat warms up ? or
>> if a coax line, you might break out the storage scope and polaroid,
>> and do TDR off the line to get an idea of distance to the fault.

> Distance?!  Were there loops?

>> After you were done with any lo-T diagnosis, you warmed up the
>> cryostat watching the fault all the way to room-T. You have
>> thermometers and sensors all thru the cryostat, you might pause
>> along the route if you saw sumpn interesting...eg circuit
>> parameter changed when this bit of the circuit warmed through
>> superconducting transition temperature...and such. Along the
>> warmup, you take advantage of the hysteresis, in that the innardz
>> of the cryostat cool the slowest and warm the slowest, so you are
>> applying different thermal (and thus mechanical) gradients to the
>> cryostat and you know which parts are being stressed in which order;
>> this order is different during cool down and warmup.

>> Disappearing faults disappear differently.The manner of their
>> vanishing often betrays whence they came.

> Ah!  That would be useful.

>> When all else fails, you rip out and replace, an operation
>> that (in some cases) can take months.

> Thanks.  No wonder physicists love their gadget (not to be
> construed as a slight).

> how does one learn all those tricks such as "disappearing differently"?

Same way you do any engineering, observe its behaviour carefully.

> Wouldn't the behaviour be unique to each cryostat?

You mean each copy or each basic technology ?

> Or are they constructed so alike that working on one trains
> you to be able to do the same diagnoses on another.

Not between technologys. 

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