Crystal oven theory...

[bitrex]

Here\'s a paper on the theory of crystal ovens:

<http://www.karlquist.com/oven.pdf>

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

I picked up an otherwise really nice HP 5334b frequency counter with
both the 1.3 GHz input and OCXO options for a song. Unfortunately
there\'s a fault somewhere inside the OCXO module and it outputs about
1.8 MHz instead of 10...seems to be maybe a bad cap somewhere around the
Colpitts section and the crystal and heater sections are OK.

The heater transistors screws to the internal mass have to be re-torqued
to spec if you mess with them, I didn\'t mess with \'em.

 

[Phil Hobbs]

bitrex wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

I picked up an otherwise really nice HP 5334b frequency counter with
both the 1.3 GHz input and OCXO options for a song. Unfortunately
there\'s a fault somewhere inside the OCXO module and it outputs about
1.8 MHz instead of 10...seems to be maybe a bad cap somewhere around the
Colpitts section and the crystal and heater sections are OK.

The heater transistors screws to the internal mass have to be re-torqued
to spec if you mess with them, I didn\'t mess with \'em.

Rick Karlquist used to post here from time to time--a very smart guy.
(I know him slightly from a side gig some years back, before he retired.)

He also did some interesting ham radio work on direct frequency
synthesizers based on cheap 455 kHz ceramic IF filters, and got pretty
impressive results.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Gerhard Hoffmann]

Am 22.05.22 um 03:46 schrieb Phil Hobbs:

Rick Karlquist used to post here from time to time--a very smart guy. (I
know him slightly from a side gig some years back, before he retired.)

He also did some interesting ham radio work on direct frequency
synthesizers based on cheap 455 kHz ceramic IF filters, and got pretty
impressive results.

10.7 MHz filters IIRC.
He is a regular in the Time Nuts email group on febo.com.


> Cheers

Gerhard

 

[server]

On Sat, 21 May 2022 20:50:12 -0400, bitrex <user@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

I picked up an otherwise really nice HP 5334b frequency counter with
both the 1.3 GHz input and OCXO options for a song. Unfortunately
there\'s a fault somewhere inside the OCXO module and it outputs about
1.8 MHz instead of 10...seems to be maybe a bad cap somewhere around the
Colpitts section and the crystal and heater sections are OK.

The heater transistors screws to the internal mass have to be re-torqued
to spec if you mess with them, I didn\'t mess with \'em.

Here\'s my EO modulator oven, with the big top cover removed.

https://www.dropbox.com/s/f6h8tfyq0xkqx1q/Oven_Cables_pub.jpg?raw=1

There are six mosfets on the big blue board on the bottom of the box
as heaters.

The temp sensors are thermistor wheatstone bridges on the small blue
board on the bottom of the modulator, with a 24-bit ADC. The modulator
is mounted on spacers inside the main oven block, not very thermally
conductive, so thermally it\'s a second-order system. It\'s stable to
way better than a millikelvin.

Note the SMA feedthrus and long squiggly cables, which reduce cold
sneaking into the EOM over the coaxes.



--

Anybody can count to one.

- Robert Widlar

 

[Anthony William Sloman]

On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:

On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

The heater transistors screws to the internal mass have to be re-torqued
to spec if you mess with them, I didn\'t mess with \'em.

Here\'s my EO modulator oven, with the big top cover removed.

https://www.dropbox.com/s/f6h8tfyq0xkqx1q/Oven_Cables_pub.jpg?raw=1

There are six mosfets on the big blue board on the bottom of the box
as heaters.

The temp sensors are thermistor wheatstone bridges on the small blue
board on the bottom of the modulator, with a 24-bit ADC. The modulator
is mounted on spacers inside the main oven block, not very thermally
conductive, so thermally it\'s a second-order system. It\'s stable to
way better than a millikelvin.

People have been managing that since the 1960\'s. It\'s not easy, though good inter-changeable thermistors and 20-bit A/D converter chips have made it easier.

> Note the SMA feedthrus and long squiggly cables, which reduce cold sneaking into the EOM over the coaxes.

Probably more helpful to say that they increase the thermal time constant of modulator whose temperature is being controlled.

--
Bill Sloman, Sydney

 

[Phil Hobbs]

Gerhard Hoffmann wrote:

Am 22.05.22 um 03:46 schrieb Phil Hobbs:

Rick Karlquist used to post here from time to time--a very smart guy.
(I know him slightly from a side gig some years back, before he retired.)

He also did some interesting ham radio work on direct frequency
synthesizers based on cheap 455 kHz ceramic IF filters, and got pretty
impressive results.

10.7 MHz filters IIRC.

Right, thanks. Cool gizmo, anyway.


> He is a regular in the Time Nuts email group on febo.com.

Cheers

Phil Hobbsn

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[bitrex]

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:

On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.

 

[bitrex]

On 5/22/2022 1:24 PM, bitrex wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10,
jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm

Equivalent to 6mm of air, rather.

 

[server]

On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.

In a reasonably close-fitting box, without a lot of space for
convection, air is a better thermal insulator than foam or fiberglass.



--

Anybody can count to one.

- Robert Widlar

 

[Phil Hobbs]

jlarkin@highlandsniptechnology.com wrote:

On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.


In a reasonably close-fitting box, without a lot of space for
convection, air is a better thermal insulator than foam or fiberglass.

That depends very much on the thermal emissivity of the surfaces and on
the thickness of the gap. The JWST doesn\'t have to deal with a lot of
convection, but its sun shield has many layers on account of the
radiation issue.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Anthony William Sloman]

On Monday, May 23, 2022 at 5:14:27 AM UTC+10, Phil Hobbs wrote:

jla...@highlandsniptechnology.com wrote:
On Sun, 22 May 2022 13:24:02 -0400, bitrex <us...@example.net> wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.

In a reasonably close-fitting box, without a lot of space for
convection, air is a better thermal insulator than foam or fiberglass.

You add foam or fibre-glass to stop air convecting.Both the walls of the foam cells and the glass in the glass-fibre have much higher thermal conductivities than air, but you don\'t put in enough to short-circuit the air.

Convection does depend on the space available. 6mm isn\'t much.

https://en.wikipedia.org/wiki/Rayleigh_number

If it is less than 500 you don\'t get significant heat transfer by convection. If it\'s more than 100,000 you get turbulent convection

That depends very much on the thermal emissivity of the surfaces and on
the thickness of the gap. The JWST doesn\'t have to deal with a lot of
convection, but its sun shield has many layers on account of the
radiation issue.

Radiation goes as the fourth power of temperature, and the sun is at 6000K. It\'s not really a relevant example.

--
Bill Sloman. Sydney

 

[bitrex]

On 5/22/2022 2:21 PM, jlarkin@highlandsniptechnology.com wrote:

On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.


In a reasonably close-fitting box, without a lot of space for
convection, air is a better thermal insulator than foam or fiberglass.

Here\'s a glamour shot of the internals:

<https://imgur.com/a/W4hmKt4>

Don\'t know what the foam is. It doesn\'t seem to conduct heat too good.

 

[Mike Coon]

In article <bt9j8h5plkg05fmbgr8ra4fj9rqktuup2v@4ax.com>,
jlarkin@highlandsniptechnology.com says...

On Sat, 21 May 2022 20:50:12 -0400, bitrex <user@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

I picked up an otherwise really nice HP 5334b frequency counter with
both the 1.3 GHz input and OCXO options for a song. Unfortunately
there\'s a fault somewhere inside the OCXO module and it outputs about
1.8 MHz instead of 10...seems to be maybe a bad cap somewhere around the
Colpitts section and the crystal and heater sections are OK.

The heater transistors screws to the internal mass have to be re-torqued
to spec if you mess with them, I didn\'t mess with \'em.

Here\'s my EO modulator oven, with the big top cover removed.

https://www.dropbox.com/s/f6h8tfyq0xkqx1q/Oven_Cables_pub.jpg?raw=1

There are six mosfets on the big blue board on the bottom of the box
as heaters.

The temp sensors are thermistor wheatstone bridges on the small blue
board on the bottom of the modulator, with a 24-bit ADC. The modulator
is mounted on spacers inside the main oven block, not very thermally
conductive, so thermally it\'s a second-order system. It\'s stable to
way better than a millikelvin.

Note the SMA feedthrus and long squiggly cables, which reduce cold
sneaking into the EOM over the coaxes.

My design for a foyer quartz clock back in the early 1960s included a
crystal in a thermostatic oven made from Perspex. I have a sketch and
isometric of the oven and a note that there were five thermistors in a
bridge, but no schematic. I do remember that on first testing the
control circuit initially ran at full power heating, then cut off, then
gradually settled on steady warmth!

 

[server]

On Sun, 22 May 2022 15:14:12 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.


In a reasonably close-fitting box, without a lot of space for
convection, air is a better thermal insulator than foam or fiberglass.

That depends very much on the thermal emissivity of the surfaces and on
the thickness of the gap. The JWST doesn\'t have to deal with a lot of
convection, but its sun shield has many layers on account of the
radiation issue.

JSTW is dealing with a huge delta-t, and vacuum is a poor thermal
conductor. I\'ve experimented with heated gadgets inside a deep-drawn
aluminum box, which has a low emissivity. My testing usually showed
that I got less heater power dissipation with air, as opposed to
styrafoam or fiberglas.

It\'s not a difficult experiment. I think most people just assume that
insulation always helps.



--

Anybody can count to one.

- Robert Widlar

 

[server]

On Sun, 22 May 2022 16:46:44 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 2:21 PM, jlarkin@highlandsniptechnology.com wrote:
On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.


In a reasonably close-fitting box, without a lot of space for
convection, air is a better thermal insulator than foam or fiberglass.


Here\'s a glamour shot of the internals:

https://imgur.com/a/W4hmKt4

Don\'t know what the foam is. It doesn\'t seem to conduct heat too good.

The interesting test would be to measure heater power with and without
the foam.



--

Anybody can count to one.

- Robert Widlar

 

[Phil Hobbs]

jlarkin@highlandsniptechnology.com wrote:

On Sun, 22 May 2022 15:14:12 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

jlarkin@highlandsniptechnology.com wrote:
On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.


In a reasonably close-fitting box, without a lot of space for
convection, air is a better thermal insulator than foam or fiberglass.

That depends very much on the thermal emissivity of the surfaces and on
the thickness of the gap. The JWST doesn\'t have to deal with a lot of
convection, but its sun shield has many layers on account of the
radiation issue.

JSTW is dealing with a huge delta-t, and vacuum is a poor thermal
conductor.

The delta-T is only huge in the first gap. After that it\'s much much
smaller. The temperature of the first shield is near the subsolar
temperature, but the amount it radiates into the first gap is much lower
than a black body\'s, on account of the very low emissivity (i. e. high
reflectivity) of its surfaces.

The subsolar temperature of a flat plate at a radial distance r from a
star of radius a is

T_ss = T_star * sqrt(a/( r sqrt(2) ) ),

i.e. the temperature at which the thermal emission of the star (sigma
T_star**4 * a**2) balances that of a notional sphere of unit emissivity
(sigma T_ss**4 * r**2). (The factor of sqrt(2) is there because the
plate radiates from both sides--for a spherical planet, whose area is 4
pi R**2, it\'s a full factor of 2.)

The temperature of the solar photosphere is around 6000K, and its radius
is 7e5 km. The mean radius of the Earth\'s orbit is 1.5e8 km. Thus the
subsolar temperature for a flat plate is

T_ss = 6000 K * sqrt(7e5 km / 1.5e8 km / sqrt(2)) = 334 K.

(For a spherical object such as the Earth and Moon, it would be
sqrt(sqrt(2)) less than that, i.e. 281 K.)

There are two main contributors to the effective thermal resistance of a
multilayer stack of spaced reflectors. One is just breaking up the gap
into N subgaps, which does what you\'d expect--M layers gets you about M
times the thermal resistance. (The effect is bigger at larger deltaT,
where the emission from inner layers is much less.)

The other contributor is that, since the layers of JWST\'s heat shield
are spaced wide apart, the great majority of the thermal emission of
each surface bounces around until it escapes into space. Of course the
ambient radiation comes in as well, but since its temperature is very
low, that\'s not a big worry.

I\'ve experimented with heated gadgets inside a deep-drawn
aluminum box, which has a low emissivity. My testing usually showed
that I got less heater power dissipation with air, as opposed to
styrafoam or fiberglas.

It\'s not a difficult experiment. I think most people just assume that
insulation always helps.

Plain air is better than insulation for narrow spaces and small delta-T,
and insulation is better otherwise. Just where the break-even point
happens depends a lot on the situation.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[whit3rd]

On Sunday, May 22, 2022 at 4:57:25 PM UTC-7, jla...@highlandsniptechnology.com wrote:

... I\'ve experimented with heated gadgets inside a deep-drawn
aluminum box, which has a low emissivity. My testing usually showed
that I got less heater power dissipation with air, as opposed to
styrafoam or fiberglas.

It\'s not a difficult experiment. I think most people just assume that
insulation always helps.

Insulation in the sense of low-conductivity material is NOT the intended function of
fiberglass; it is used to reduce air convection, which (thermal convection is
delta-T squared heat transfer) is a tiny effect in a low-temperature-gradient system.
In a house-on-fire, fiberglass in the walls is not tiny, but a big help in getting out alive.

In a very real sense, fiberglass filled cavities are air-insulated.

 

[Jan Panteltje]

On a sunny day (Sun, 22 May 2022 22:51:09 +0100) it happened Mike Coon
<gravity@mjcoon.plus.com> wrote in
<MPG.3cf4c0956a87c90d9896be@usenet.plus.net>:

My design for a foyer quartz clock back in the early 1960s included a
crystal in a thermostatic oven made from Perspex. I have a sketch and
isometric of the oven and a note that there were five thermistors in a
bridge, but no schematic. I do remember that on first testing the
control circuit initially ran at full power heating, then cut off, then
gradually settled on steady warmth!

Yes that is overshot, maybe over-compensated

This was pretty good too:
http://panteltje.com/panteltje/tri_pic/

Little box inside big box...

temperature within a few ADC steps over many years..
Took about half an hour to get the PID working correctly in PIC asm ..

For the rest the 1 ppm TCXOs as in my RTL-SDR sticks are very good and very small.

 

[Phil Hobbs]

jlarkin@highlandsniptechnology.com wrote:

On Sun, 22 May 2022 16:46:44 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 2:21 PM, jlarkin@highlandsniptechnology.com wrote:
On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote:

On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote:

Here\'s a paper on the theory of crystal ovens:

http://www.karlquist.com/oven.pdf

He didn\'t survey the literature all that well.

There\'s a lot more to it than just slapping a crystal & heater in a
metal box and calling it a day! There are probably some pathologically
bad geometries even a really fast control loop can never stabilize very
well.

The part about the outer can acting like a Faraday shield is
interesting, shunting ambient gradients around the core. Does anyone
know how they make the negative space look like a linear tehrmal
resistance over a wide range? There\'s some kind of insulating foam in
there, is main heat thermal transfer radiative or conduction?

Conduction. Foam pretty much stops convection.

It was mentioned on another thread that the thermal resistance of
radiation between two concentric blackbodies at 300K is about 6mm,
interestingly that\'s very close to the same thickness of foam-filled
negative space between the outer and inner shells of the OCXO module on
my 5334B.

Don\'t know if that\'s a coincidence or not, obviously the two aren\'t at
exactly 300K. The interior I think runs about 355K, while the exterior
(I haven\'t measured it) seems cool enough to keep your hand on a while.


In a reasonably close-fitting box, without a lot of space for
convection, air is a better thermal insulator than foam or fiberglass.


Here\'s a glamour shot of the internals:

https://imgur.com/a/W4hmKt4

Don\'t know what the foam is. It doesn\'t seem to conduct heat too good.



The interesting test would be to measure heater power with and without
the foam.

Air at room temperature has an alpha of about 0.026 W/m/K. High-density
styrofoam (the kind that crunches when you poke it with your finger) is
about 0.040, and low-density styrofoam (the kind that squeaks) is around
0.030.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

Jan Panteltje wrote:

On a sunny day (Sun, 22 May 2022 22:51:09 +0100) it happened Mike Coon
gravity@mjcoon.plus.com> wrote in
MPG.3cf4c0956a87c90d9896be@usenet.plus.net>:

My design for a foyer quartz clock back in the early 1960s included a
crystal in a thermostatic oven made from Perspex. I have a sketch and
isometric of the oven and a note that there were five thermistors in a
bridge, but no schematic. I do remember that on first testing the
control circuit initially ran at full power heating, then cut off, then
gradually settled on steady warmth!

Yes that is overshot, maybe over-compensated

Or else windup, which is pretty likely if the sensors and heaters aren\'t
well coupled.

This was pretty good too:
http://panteltje.com/panteltje/tri_pic/

Little box inside big box...

temperature within a few ADC steps over many years..
Took about half an hour to get the PID working correctly in PIC asm ..

For the rest the 1 ppm TCXOs as in my RTL-SDR sticks are very good and very small.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com