Topics

Fused quartz pendulum - temperature coef


neil
 

Matthys in his book "accurate Clock Pendulums" provides the following:

Pendulum Materials:
F Quartz                       0.55 x 10-6
Regular Invar             1.5 x 10-6
Super Invar                 0.63 x 10-6
Carbon Fibre             <0.5 x 10-6 but the glue is 5.0 x 10-6
Steel                           10.5 x 10-6
Walnut                         6.6 x 10-6   + bad moisture effects
Beech                          2.6 x 10-6   + bad moisture effects

Suspension Materials
Stainless steel              10.8       bad strength properties
NiSpan                            7.6 - 8.1   good
Berylium copper            17.5        good
Phosphor Bronze          17.8        good
Al Si bronze                 18              best
Si Bronze                          18         some thermal hysteresis
Ni Sn Bronze                 18.5         bad termal hysteresis

                        
Neil 
On 5/04/2020 12:19 am, John Haine wrote:

I'm glad it's useful Neil.  As a matter of interest, what is the tempco of fused silica, how does it compare with carbon fibre?

John.


John Haine
 

Thanks Neil.  Are the suspension numbers to be also x 10-6?

Interesting number for CF.  Someone has reported a measurement something like what you give, I think it was in HJ, measured for the composite but similar number to just the fibre.  


Bepi
 

On the subject of temperature sensitivity when I changed over to a lead bob I noticed that the original gray cast iron bob had, in principle, just the right coefficient of expansion (1.08 10-5 ºC-1) and height to compensate for an invar rod (1.21 10-6 ºC-1) when the two are rigidly connected at the bottom of the bob. Does anybody know if this design feature is typical of all synchronomes?
In principle for a given bob weight, one can always compensate exactly for the pendulum temperature coefficient with the right choice of materials and bob aspect ratio, so that the problem changes from finding the smallest coefficient material to the best known coefficient material. I would also be curious to know if anybody has attempted to measure thermal expansion experimentally with decent precision other than from period vs natural ambient temperature fluctuations.
--
Bepi


John Haine
 

Indeed they have.  For one, the team building replicas of the Harrison RAS Regulator in the UK have measured expansion of modern brass and steel for making the "gridiron" pendulum.  I'll try to find the reference.


neil
 

Yes Bepi - Matthus  reports the results of what would have been a very lengthy series of tests in his book. He used a FQ pendulum with a brass bob and did a number of tests with different compensators and different bob support positions. He heated the entire clock by wrapping with an electric blanket, waiting a day or 2 then removing the heat and measuring the step change in rate with a set change of temperature.
A few months ago I attempted to calculate the TC of my entire cast iron bob, invar pendulum and phosphor bronze dual suspension, and compensate with an aluminum spacer, but it was difficult to do in the middle of summer with the daily high ambient temperatures in my shop. I have since then dismantled the clock and purchased several lengths of fused quartz tube ( the type used to make heater elements). I'm right now in the process of building a new suspension with soldered in place chops. I may do a You-tube video if the motivation strikes.  I will support the bob ( cast iron) in the middle, use a glass spacer and set the compensator outside of the bob.
 Matthus also makes the same comment as you about supporting a cast iron bob at the bottom, and not needing any further compensation, but of course the suspension has as much if not more TC that the entire pendulum rod, so getting it right is easier said than done.
neil

On 8/04/2020 02:00 am, Bepi wrote:
On the subject of temperature sensitivity when I changed over to a lead bob I noticed that the original gray cast iron bob had, in principle, just the right coefficient of expansion (1.08 10-5 ºC-1) and height to compensate for an invar rod (1.21 10-6 ºC-1) when the two are rigidly connected at the bottom of the bob. Does anybody know if this design feature is typical of all synchronomes?
In principle for a given bob weight, one can always compensate exactly for the pendulum temperature coefficient with the right choice of materials and bob aspect ratio, so that the problem changes from finding the smallest coefficient material to the best known coefficient material. I would also be curious to know if anybody has attempted to measure thermal expansion experimentally with decent precision other than from period vs natural ambient temperature fluctuations.
--
Bepi


Harvey Moseley
 

Has anyone used quartz as the suspension flexure?  It is very strong, and if thin enough, can be kept well below the brittle fracture limit. 

Harvey

On Tue, Apr 7, 2020 at 5:38 PM neil <njepsen@...> wrote:
Yes Bepi - Matthus  reports the results of what would have been a very lengthy series of tests in his book. He used a FQ pendulum with a brass bob and did a number of tests with different compensators and different bob support positions. He heated the entire clock by wrapping with an electric blanket, waiting a day or 2 then removing the heat and measuring the step change in rate with a set change of temperature.
A few months ago I attempted to calculate the TC of my entire cast iron bob, invar pendulum and phosphor bronze dual suspension, and compensate with an aluminum spacer, but it was difficult to do in the middle of summer with the daily high ambient temperatures in my shop. I have since then dismantled the clock and purchased several lengths of fused quartz tube ( the type used to make heater elements). I'm right now in the process of building a new suspension with soldered in place chops. I may do a You-tube video if the motivation strikes.  I will support the bob ( cast iron) in the middle, use a glass spacer and set the compensator outside of the bob.
 Matthus also makes the same comment as you about supporting a cast iron bob at the bottom, and not needing any further compensation, but of course the suspension has as much if not more TC that the entire pendulum rod, so getting it right is easier said than done.
neil

On 8/04/2020 02:00 am, Bepi wrote:
On the subject of temperature sensitivity when I changed over to a lead bob I noticed that the original gray cast iron bob had, in principle, just the right coefficient of expansion (1.08 10-5 ºC-1) and height to compensate for an invar rod (1.21 10-6 ºC-1) when the two are rigidly connected at the bottom of the bob. Does anybody know if this design feature is typical of all synchronomes?
In principle for a given bob weight, one can always compensate exactly for the pendulum temperature coefficient with the right choice of materials and bob aspect ratio, so that the problem changes from finding the smallest coefficient material to the best known coefficient material. I would also be curious to know if anybody has attempted to measure thermal expansion experimentally with decent precision other than from period vs natural ambient temperature fluctuations.
--
Bepi


Darren Conway
 

Or maybe mica.   Not as good a coefficient as quartz but still a lot better than SS.

Sheet mica is readily available as thin sheet used for insulating electronic components from a heat sink.


Regards

Darren Conway

Lower Hutt
New Zealand


On 8.04.20 10:03 pm, Harvey Moseley wrote:
Has anyone used quartz as the suspension flexure?  It is very strong, and if thin enough, can be kept well below the brittle fracture limit. 

Harvey


Virus-free. www.avast.com


Bepi
 

Neil, yes Matthys says in his book (pag 3) that it's better ( and easier) to hang a bob from the bottom edge.  Where di you get the description of his experimets though?
What I was saying is that while designing the new bob for my pendulum I realized that the Synchronome original one seems to have been designed for exact compensation, within my ability of determining the gray cast iron and the invar TC.

Two reasons for pointing this out: historical on the Synchronome genesis, it would be nice to know if my guess is right, James Meaton who worked for them might know.

Second, to figure out if it's worth doing so in general.  The only way to understand it would be to measure the effects of putting efforts at this kind of temperature compensation, it's not obvious to me that exact temp comp of the rod and bob thermal expansion leads to a proportional reduction in temperature sensitivity of the period at the level of precision we are interested in. A couple of months ago while I was modifying the bob I exploited the occasion to install 15W of LED strip lights inside the case which, with the inside 4 thermometers, should provide a reasonable chance of controlling the clock temperature boundary conditions. My intuition tells me that the ability to modulate the temperature would be more useful, and easier to implement, than to heavily insulate the case both for sensitivity and to separate time constants. No time to use the system yet though.

--
Bepi


James Meaton
 

Hi Neil and others
From what I remember from about 50 years ago the bob weight was supported by a brass knurled, threaded nut at the bottom of the Invar pendulum.  Although this facility obviously provided some adjustment for timing, customers could also order a set of precision weights that could be placed on to of the bob weight for more precise adjustments.  From what I remember the majority of the master clocks were hidden away in cellars, lofts and old cupboards; in those days the main priority of a Synchronome system was that all the clocks in the building(s) were at the same time, rather than being spot on for GMT.  I always regret getting rid of the master clock case I had for many years, but to be honest this was well before the times when they were considered of monitory or historical value.
Some of you may be aware of the Watchman’s Tell Tale System that could be linked to a master clock system. It consisted of a 6” dia aluminium tube that rotated once over twenty four hours.  A paper chart was fixed to the drum and up to 24 PO type 3000 relays with a “pricking” angled pin were fixed to the base of the machine.  As the night watchmen (women) carried out their patrols they would operate key-switches at important locations indicating by the pin-prick in the paper chart that they had made this check and at a given time.  (I converted a couple of short lengths of reject tubing into ceiling lights and bongo shells?!) 

Amongst my Synchronome bits and pieces I have a piece of ancient equipment that was fitted in one of the old battery charging cases.  I am afraid it’s horrendous condition is due to a leaking roof from many years ago.
I thought it was some sort of device for comparing mains 50 cycle synchronous time to that outputted by a master clock, but looking at the connections I am not so sure. 
If anyone is interested in this old piece of history please contact me. Please see attachments below.



James Meaton

On 11 Apr 2020, at 14:40, Bepi <pepicima@...> wrote:

Neil, yes Matthys says in his book (pag 3) that it's better ( and easier) to hang a bob from the bottom edge.  Where di you get the description of his experimets though?
What I was saying is that while designing the new bob for my pendulum I realized that the Synchronome original one seems to have been designed for exact compensation, within my ability of determining the gray cast iron and the invar TC.

Two reasons for pointing this out: historical on the Synchronome genesis, it would be nice to know if my guess is right, James Meaton who worked for them might know.

Second, to figure out if it's worth doing so in general.  The only way to understand it would be to measure the effects of putting efforts at this kind of temperature compensation, it's not obvious to me that exact temp comp of the rod and bob thermal expansion leads to a proportional reduction in temperature sensitivity of the period at the level of precision we are interested in. A couple of months ago while I was modifying the bob I exploited the occasion to install 15W of LED strip lights inside the case which, with the inside 4 thermometers, should provide a reasonable chance of controlling the clock temperature boundary conditions. My intuition tells me that the ability to modulate the temperature would be more useful, and easier to implement, than to heavily insulate the case both for sensitivity and to separate time constants. No time to use the system yet though.

--
Bepi


Harvey Moseley
 

Hi All,

Just a brief comment on temperature compensation of the pendulum. Since we know accurately the coefficients of expansion of the suspension, pendulum rod, and pendulum bob, you can determine accurately where along the length of the bob to provide first order compensation for the expansion or contraction of the materials in the pendulum system.  If we provide accurate first order compensation, the variation of the pendulum should be parabolic in temperature around the design temp.  The breadth of the parabola should be proportional to the second order changes, which for most materials, are proportional to their coefficient of expansion.  Bottom line, we can design the system to have no first order changes in period at the reference temp.  The breath of the residual quadratic variation will be determined by the coefficients of the suspension and bob.  Keep them all small, and the residual will be small.   This ignores any thermal effects from the air, which I think is of higher order, and thus of lower importance.  Hope this is helpful to those thinking about these system.

Best Regards,
Harvey


On Sat, Apr 11, 2020 at 10:25 AM James Meaton via groups.io <soundhutch=blueyonder.co.uk@groups.io> wrote:
Hi Neil and others
From what I remember from about 50 years ago the bob weight was supported by a brass knurled, threaded nut at the bottom of the Invar pendulum.  Although this facility obviously provided some adjustment for timing, customers could also order a set of precision weights that could be placed on to of the bob weight for more precise adjustments.  From what I remember the majority of the master clocks were hidden away in cellars, lofts and old cupboards; in those days the main priority of a Synchronome system was that all the clocks in the building(s) were at the same time, rather than being spot on for GMT.  I always regret getting rid of the master clock case I had for many years, but to be honest this was well before the times when they were considered of monitory or historical value.
Some of you may be aware of the Watchman’s Tell Tale System that could be linked to a master clock system. It consisted of a 6” dia aluminium tube that rotated once over twenty four hours.  A paper chart was fixed to the drum and up to 24 PO type 3000 relays with a “pricking” angled pin were fixed to the base of the machine.  As the night watchmen (women) carried out their patrols they would operate key-switches at important locations indicating by the pin-prick in the paper chart that they had made this check and at a given time.  (I converted a couple of short lengths of reject tubing into ceiling lights and bongo shells?!) 

Amongst my Synchronome bits and pieces I have a piece of ancient equipment that was fitted in one of the old battery charging cases.  I am afraid it’s horrendous condition is due to a leaking roof from many years ago.
I thought it was some sort of device for comparing mains 50 cycle synchronous time to that outputted by a master clock, but looking at the connections I am not so sure. 
If anyone is interested in this old piece of history please contact me. Please see attachments below.



James Meaton

On 11 Apr 2020, at 14:40, Bepi <pepicima@...> wrote:

Neil, yes Matthys says in his book (pag 3) that it's better ( and easier) to hang a bob from the bottom edge.  Where di you get the description of his experimets though?
What I was saying is that while designing the new bob for my pendulum I realized that the Synchronome original one seems to have been designed for exact compensation, within my ability of determining the gray cast iron and the invar TC.

Two reasons for pointing this out: historical on the Synchronome genesis, it would be nice to know if my guess is right, James Meaton who worked for them might know.

Second, to figure out if it's worth doing so in general.  The only way to understand it would be to measure the effects of putting efforts at this kind of temperature compensation, it's not obvious to me that exact temp comp of the rod and bob thermal expansion leads to a proportional reduction in temperature sensitivity of the period at the level of precision we are interested in. A couple of months ago while I was modifying the bob I exploited the occasion to install 15W of LED strip lights inside the case which, with the inside 4 thermometers, should provide a reasonable chance of controlling the clock temperature boundary conditions. My intuition tells me that the ability to modulate the temperature would be more useful, and easier to implement, than to heavily insulate the case both for sensitivity and to separate time constants. No time to use the system yet though.

--
Bepi


Andrew Nahum
 

Hi James,

Yes - of course you are right. The Synchronome was ideal for factories, schools and lunatic asylums for setting a local coherent time all around the premises, so there was a slave clock in every room, before accurate time was universally available. 

I remember as a cowering schoolboy about 10 years old waking up early in a shivering cold dormitory- I think it was called Dotheboys Hall - and seeing the minute hand on the clock advancing regularly in 30 second jumps as I watched till it had gained an hour. Of course it was the day the clocks in UK ‘went forward ‘.  But it was quite surreal and an Alice in Wonderland experience. 

I only realised years later that it was a master-clock system and there must have been someone in a cellar, somewhere we never went,  re-setting the time. 

Andrew 

On 11 Apr 2020, at 15:25, James Meaton via groups.io <soundhutch@...> wrote:

Hi Neil and others
From what I remember from about 50 years ago the bob weight was supported by a brass knurled, threaded nut at the bottom of the Invar pendulum.  Although this facility obviously provided some adjustment for timing, customers could also order a set of precision weights that could be placed on to of the bob weight for more precise adjustments.  From what I remember the majority of the master clocks were hidden away in cellars, lofts and old cupboards; in those days the main priority of a Synchronome system was that all the clocks in the building(s) were at the same time, rather than being spot on for GMT.  I always regret getting rid of the master clock case I had for many years, but to be honest this was well before the times when they were considered of monitory or historical value.
Some of you may be aware of the Watchman’s Tell Tale System that could be linked to a master clock system. It consisted of a 6” dia aluminium tube that rotated once over twenty four hours.  A paper chart was fixed to the drum and up to 24 PO type 3000 relays with a “pricking” angled pin were fixed to the base of the machine.  As the night watchmen (women) carried out their patrols they would operate key-switches at important locations indicating by the pin-prick in the paper chart that they had made this check and at a given time.  (I converted a couple of short lengths of reject tubing into ceiling lights and bongo shells?!) 

Amongst my Synchronome bits and pieces I have a piece of ancient equipment that was fitted in one of the old battery charging cases.  I am afraid it’s horrendous condition is due to a leaking roof from many years ago.
I thought it was some sort of device for comparing mains 50 cycle synchronous time to that outputted by a master clock, but looking at the connections I am not so sure. 
If anyone is interested in this old piece of history please contact me. Please see attachments below.



James Meaton

On 11 Apr 2020, at 14:40, Bepi <pepicima@...> wrote:

Neil, yes Matthys says in his book (pag 3) that it's better ( and easier) to hang a bob from the bottom edge.  Where di you get the description of his experimets though?
What I was saying is that while designing the new bob for my pendulum I realized that the Synchronome original one seems to have been designed for exact compensation, within my ability of determining the gray cast iron and the invar TC.

Two reasons for pointing this out: historical on the Synchronome genesis, it would be nice to know if my guess is right, James Meaton who worked for them might know.

Second, to figure out if it's worth doing so in general.  The only way to understand it would be to measure the effects of putting efforts at this kind of temperature compensation, it's not obvious to me that exact temp comp of the rod and bob thermal expansion leads to a proportional reduction in temperature sensitivity of the period at the level of precision we are interested in. A couple of months ago while I was modifying the bob I exploited the occasion to install 15W of LED strip lights inside the case which, with the inside 4 thermometers, should provide a reasonable chance of controlling the clock temperature boundary conditions. My intuition tells me that the ability to modulate the temperature would be more useful, and easier to implement, than to heavily insulate the case both for sensitivity and to separate time constants. No time to use the system yet though.

--
Bepi