Topics

Clock or anemometer?


Bepi
 

I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends.  But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:


The clock period shown here is averaged over the 5 period impulsing interval.

The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.

The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.

I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.

During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.

 

The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.


The relationship with atmospheric pressure is shown below: 




I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.

--
Bepi


neil
 

Hi Bepi,
                Its good to hear from you again, and that you are well. My wife & I are also in lockdown, and staying home 24/7.
Fortunately i am abale to continue work on my clock. i received a 1m length of Quartz glass 10 days ago and have dismantled the clock, in preparation for replacing the invar pendulum with quartz galss, and a below the bob aluminum temperature compensator, as described in Matthys's book "the accurate pendulum".
At the moment I have a 14lb bob hanging on the glass, to test the means of attachment, which is not easy.  I was intending to drill a hole thru the rod and use a pin, but because of the lockdown, I am unable to access a diamond drill, so will probably use expoxy or epoxy/carbide dust as an adhesive. Not as good, but needs must.
neil 

On 4/04/2020 11:35 am, Bepi wrote:

I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends.  But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:


The clock period shown here is averaged over the 5 period impulsing interval.

The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.

The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.

I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.

During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.

 

The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.


The relationship with atmospheric pressure is shown below: 




I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.

--
Bepi


Eric Scace
 



On 2020 Apr 03, at 22:08 , Eric Scace <read@...> wrote:

Neil —

   I would suggest attaching the bob to the glass with modern epoxy or even Gorilla Glue. Not only do you avoid the complexities of drilling glass (and introducing micro-fractures at stress concentration points), the joint will be less likely to move than a pin.

— Eric

On 2020 Apr 03, at 19:09 , neil <njepsen@...> wrote:

Hi Bepi,
                Its good to hear from you again, and that you are well. My wife & I are also in lockdown, and staying home 24/7.
Fortunately i am abale to continue work on my clock. i received a 1m length of Quartz glass 10 days ago and have dismantled the clock, in preparation for replacing the invar pendulum with quartz galss, and a below the bob aluminum temperature compensator, as described in Matthys's book "the accurate pendulum".
At the moment I have a 14lb bob hanging on the glass, to test the means of attachment, which is not easy.  I was intending to drill a hole thru the rod and use a pin, but because of the lockdown, I am unable to access a diamond drill, so will probably use expoxy or epoxy/carbide dust as an adhesive. Not as good, but needs must.
neil  



neil
 

I agree Eric, but the problem with epoxy is the TC. As i recall the TC of epoxy is about 10x that of invar.
Ive been thinking about this a lot, and it is very very difficult to arrange the joint so that movement of the epoxy (with temperature) doesnt change the pendulum length. This is the advantage of the pin through the rod method.
With epoxy joint (at the suspension end)i think it must be arranged so there is clearance between the end of the rod and the suspension body. That way, when the epoxy changes length, an equal amount of movement occurs above and below the centre of pressure of the joint.
Ill do a drwg if this is not clear from my convoluted description.
The same would apply at the bob end. 
Interedted in your view.
Neil


On Sat, 4 Apr 2020, 5:09 PM Eric Scace <eric@...> wrote:


On 2020 Apr 03, at 22:08 , Eric Scace <read@...> wrote:

Neil —

   I would suggest attaching the bob to the glass with modern epoxy or even Gorilla Glue. Not only do you avoid the complexities of drilling glass (and introducing micro-fractures at stress concentration points), the joint will be less likely to move than a pin.

— Eric

On 2020 Apr 03, at 19:09 , neil <njepsen@...> wrote:

Hi Bepi,
                Its good to hear from you again, and that you are well. My wife & I are also in lockdown, and staying home 24/7.
Fortunately i am abale to continue work on my clock. i received a 1m length of Quartz glass 10 days ago and have dismantled the clock, in preparation for replacing the invar pendulum with quartz galss, and a below the bob aluminum temperature compensator, as described in Matthys's book "the accurate pendulum".
At the moment I have a 14lb bob hanging on the glass, to test the means of attachment, which is not easy.  I was intending to drill a hole thru the rod and use a pin, but because of the lockdown, I am unable to access a diamond drill, so will probably use expoxy or epoxy/carbide dust as an adhesive. Not as good, but needs must.
neil  



John Haine
 

Ni Neil, David Walter uses FQ rods in his (D)W5 clock.  This pdf from the HJ may be useful.


Andrew Nahum
 

The association with wind speed is fascinating. I wonder if the wind could in some way be pressurising your house?  Do you have a barograph actually next to your clock or is your pressure data that for  the Venice area taken from the public weather service?

The empty city must be quite wonderful in a way!  Andrew Nahum 


I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends.  But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:


The clock period shown here is averaged over the 5 period impulsing interval.

The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.

The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.

I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.

During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.

 

The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.


The relationship with atmospheric pressure is shown below: 




I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.

--
Bepi


Chris
 

Never underestimate the power of the wind [and water waves] to produce infrasonics. i.e. Sub-20Hz frequencies.

I have experimented for decades with large, audio subwoofers with responses well below 10Hz at levels well above 110dB[C].
So I can attest to the fact that most building structures will readily vibrate in sympathy with low frequency sound.

By its very nature infrasonics are often completely inaudible to most humans. Except when something rattles in sympathy. Anywhere in the building!
Audio enthusiasts, with a flair for the dramatic, often relate their solid concrete floors having shown visible surface waves when LF frequencies are reproduced at very high levels.
Measuring [or even detecting] infrasonics requires a suitable microphone with an extended LF audio response and audio frequency response, graphing software.

It is quite possible that the clock case is being rapidly and cyclically pressurized and evacuated much like an aneroid barometer capsule or drum. 
Or even that the apparently solid, clock wall is vibrating. No doubt water waves are similarly responsible for exciting building structures.

Both water and air are likely simultaneous VLF energy sources in Bepi's particular environment.
Windows and doors provide almost no protection from VLF audio waves.
They will usually vibrate in sympathy and instantly re-transmit the airborne "noise" on their exteriors directly into the interiors of a building.
YouTube is well furnished with videos of doors and windows vibrating over an inch out of true when subjected to infrasonics.
Rooms, themselves, can often amplify [or exaggerate] certain VLF if the dimensions of standing waves between opposite walls match a particular audio frequency.


On 04/04/2020 14:33, Andrew Nahum wrote:
The association with wind speed is fascinating. I wonder if the wind could in some way be pressurising your house?  Do you have a barograph actually next to your clock or is your pressure data that for  the Venice area taken from the public weather service?

The empty city must be quite wonderful in a way!  Andrew Nahum 


I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends.  But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:


The clock period shown here is averaged over the 5 period impulsing interval.

The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.

The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.

I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.

During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.

 

The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.


The relationship with atmospheric pressure is shown below: 




I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.

--
Bepi


John Haine
 

As the pendulum Q is rather high the important components of infrasonic frequency would be up to 2 Hz at most probably.  Bepi, I think the baro sensor you are using may be able to sample fast enough to sense this (not sure whether it's sensitive enough though).  Would be an interesting experiment.  Though the period graph shows a symmetrical noise waveform it would be interesting to know what its spectrum is and how it integrates over time into time error.


Andrew Nahum
 

Of course, the wind could also depressurise the house if any permeable areas are in the right ‘wind shadow’ zone. Few buildings are truly airtight - except bio security labs. 



On 4 Apr 2020, at 13:33, Andrew Nahum via groups.io <andrew.nahum@...> wrote:

The association with wind speed is fascinating. I wonder if the wind could in some way be pressurising your house?  Do you have a barograph actually next to your clock or is your pressure data that for  the Venice area taken from the public weather service?

The empty city must be quite wonderful in a way!  Andrew Nahum 


I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends.  But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:


The clock period shown here is averaged over the 5 period impulsing interval.

The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.

The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.

I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.

During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.

 

The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.


The relationship with atmospheric pressure is shown below: 




I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.

--
Bepi


Bepi
 

hi  Neil, all fine here thanks, i live by myself in a very large house with a great garden, not much has changed because of the lockdown for me. Impressive how the surroundings wildlife, in our case birds and fish, has moved in as soon as we have retired inside.

About your pendulum my recent experience is that a more aerodynamic and higher density bob made a huge difference in terms of Q, hence on all of the intrinsic noise, and thermal expansion can be easily compensated between rod and bob, at least as far as the slow time scale is concerned. I discovered that it was already so with the cast iron original bob, but nothing prevents from doing the same with lead by changing the bob aspect ratio.

Take care, b.-


Neville Michie
 

I have done a lot of work with barometric pressure with respect to caves.
Caves are often a large volume which equilibrates with surface pressure
with huge volumes of air flow.
When a wind is blowing, there are often enormous eddies, mixing air from
the surface with the convection ceiling often a kilometre or two from the
ground. These eddies cause fluctuations or gusting, with a period about
2 - 10 minutes in the velocity of the wind at ground level.
The variation of velocity is accompanied by variations of barometric pressure.
These are so obvious that they can be seen as continuous movement of the
needle of a low friction barometer or altimeter. Aircraft altimeters have
too much friction, but the instruments made by Wallace & Tiernan, if examined
with a magnifying glass, can be seen to be in continuous motion.
Digital barometers based on fairly insensitive IC chips do not show the
effect very well.
So the frequency band that you should look at has periods between 30
seconds and 5 minutes. The variations are also related to micro-seisms,
which are found in the ground, oceans and the air, due to coupling
of energy from the atmosphere to the ground and water.
cheers,
Neville Michie

On 5 Apr 2020, at 00:28, Chris <chris.b@smilemail.dk> wrote:

Never underestimate the power of the wind [and water waves] to produce infrasonics. i.e. Sub-20Hz frequencies.

I have experimented for decades with large, audio subwoofers with responses well below 10Hz at levels well above 110dB[C].
So I can attest to the fact that most building structures will readily vibrate in sympathy with low frequency sound.

By its very nature infrasonics are often completely inaudible to most humans. Except when something rattles in sympathy. Anywhere in the building!
Audio enthusiasts, with a flair for the dramatic, often relate their solid concrete floors having shown visible surface waves when LF frequencies are reproduced at very high levels.
Measuring [or even detecting] infrasonics requires a suitable microphone with an extended LF audio response and audio frequency response, graphing software.

It is quite possible that the clock case is being rapidly and cyclically pressurized and evacuated much like an aneroid barometer capsule or drum.
Or even that the apparently solid, clock wall is vibrating. No doubt water waves are similarly responsible for exciting building structures.

Both water and air are likely simultaneous VLF energy sources in Bepi's particular environment.
Windows and doors provide almost no protection from VLF audio waves.
They will usually vibrate in sympathy and instantly re-transmit the airborne "noise" on their exteriors directly into the interiors of a building.
YouTube is well furnished with videos of doors and windows vibrating over an inch out of true when subjected to infrasonics.
Rooms, themselves, can often amplify [or exaggerate] certain VLF if the dimensions of standing waves between opposite walls match a particular audio frequency.


On 04/04/2020 14:33, Andrew Nahum wrote:
The association with wind speed is fascinating. I wonder if the wind could in some way be pressurising your house? Do you have a barograph actually next to your clock or is your pressure data that for the Venice area taken from the public weather service?

The empty city must be quite wonderful in a way! Andrew Nahum


I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends. But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:
<Pasted Graphic 1.jpg>

The clock period shown here is averaged over the 5 period impulsing interval.
The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.
The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.
I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.
During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.

The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.
<Fluct vs wind.jpg>

The relationship with atmospheric pressure is shown below:

<fluct vs press.jpg>


I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.
--
Bepi


Bepi
 

Turbulence is certainly very broad band and the pendulum is sampling at 1/2 Hz, what I see is down to the sampling frequency and its SD looks like the wind speed, most of its power is up there, frequency wise,  where we see it by eye. Yes the 3 baro sensors I have in the case are fast enough, they are essentially low frequency microphones. I could cross correlate them to analyze a broad spectrum with exceptional sensitivity, but luckily they have been off for some time now, they were not yielding anything more useful than the city barometer which is more reliably calibrated. These days it's far less windy, I was hoping nobody would have suggested that measurement. I totally agree about the spectrum, its knowledge is essential to see if these fluctuations matter for standard long term accuracy but wouldn't be easier to check if accuracy correlates with windy days? Something else to keep in mind when checking for regular, long term, atmospheric pressure correlations. John, weren't you working on this already?

Chris, thanks for all that fun info on infrasonics but at the end of the day, if I could put my apartment under high vacuum, would I still see those fluctuations or not?

Andrew why you ask about depressurization? Pressure waves or vibrations aren't they all zero average waves?

--
Bepi


neil
 

Chris, thanks for all that fun info on infrasonics but at the end of the day, if I could put my apartment under high vacuum, would I still see those fluctuations or not?

Depends on what you mean by high vacuum Bepi. With "total vacuum", you would only see the infrasound transmitted by the building structure, and no airborne sound.
Neil
On 5/04/2020 09:41 am, Bepi wrote:


Neville Michie
 

The Littlemore clock, by E.T.Hall is documented in Derek Roberts’ book:
Precision Pendulum Clocks: France, Germany, America and recent advancements.
In this article the author blames a chestnut tree for causing fluctuations
to his clock. He shows in his data, the effects of wind on time keeping.
However, it is difficult to separate the air pressure effects from the seismic
interference with the pendulum through the ambient gravity. And the chestnut tree may
have been innocent as the effect may have been ground seismic fluctuations on a larger scale.
The turbulent eddies in the atmosphere involve masses of air in the order of cubic kilometres,
with temperature contrasts that cause density differences and consequent mass movements.
There are not many good barometric sensors, but even fewer sensitive gravity
sensors.
I saw an article, years ago, about a down borehole gravity meter using a vibrating wire
supporting a weight. I have thought of constructing a gravity meter using a tungsten wire
supporting a tungsten weight, vibration of the wire being sustained electrostatically.
The bottom line, though, is that these are all oscillations about a mean, and the mean
stays quite constant over a long period of time.
Cheers,
Neville Michie

On 5 Apr 2020, at 07:48, neil <njepsen@gmail.com> wrote:

Chris, thanks for all that fun info on infrasonics but at the end of the day, if I could put my apartment under high vacuum, would I still see those fluctuations or not?

Depends on what you mean by high vacuum Bepi. With "total vacuum", you would only see the infrasound transmitted by the building structure, and no airborne sound.

Neil
On 5/04/2020 09:41 am, Bepi wrote:



<njepsen.vcf>


Andrew Nahum
 

I should introduce myself as a newcomer here. I have a fairly long term interest in synchronomes - though not as long as you guys - and I see that I bought my first one over twenty years ago since when it has run pretty perfectly as the house clock in our main room, subject to periodic cleaning and adjustment. I have often wondered what the limits are of the clock and what the sources of error are. In a way, it should be better.  But I have not tried optical sensing and better impulsing and love to hear what this group is doing. I also run a Brillie in the same room and my one, anyway, is definitely less stable than the Synchronome. 

Bepi - I asked about where the barometric data came from because I had thought the local aerodynamic effects of wind on the building could cause a divergence inside the house which might last for a considerable period. ie. that the effects you see were a secondary effect of wind speed and perhaps not driven by wind speed per se.  I wasn’t thinking about waves, so much as a relatively continuous pressure change of either sign away from the external barometric pressure that could last for hours or maybe a day. But I gather now that you are fully instrumented for pressure at the clock itself, so my speculation would not be valid. A

On 4 Apr 2020, at 22:41, Bepi <pepicima@...> wrote:



Turbulence is certainly very broad band and the pendulum is sampling at 1/2 Hz, what I see is down to the sampling frequency and its SD looks like the wind speed, most of its power is up there, frequency wise,  where we see it by eye. Yes the 3 baro sensors I have in the case are fast enough, they are essentially low frequency microphones. I could cross correlate them to analyze a broad spectrum with exceptional sensitivity, but luckily they have been off for some time now, they were not yielding anything more useful than the city barometer which is more reliably calibrated. These days it's far less windy, I was hoping nobody would have suggested that measurement. I totally agree about the spectrum, its knowledge is essential to see if these fluctuations matter for standard long term accuracy but wouldn't be easier to check if accuracy correlates with windy days? Something else to keep in mind when checking for regular, long term, atmospheric pressure correlations. John, weren't you working on this already?

Chris, thanks for all that fun info on infrasonics but at the end of the day, if I could put my apartment under high vacuum, would I still see those fluctuations or not?

Andrew why you ask about depressurization? Pressure waves or vibrations aren't they all zero average waves?

--
Bepi


Harvey Moseley
 

Very interesting.  Do you have a barometer with rapid readout to see if the house is pressurizing and depressurizing.  I am almost sure that is it.   If it were building motion, you would expect a significant dependence on wind direction; and perhaps even for internal pressure differences.  Would be interesting.


On Fri, Apr 3, 2020 at 6:35 PM Bepi <pepicima@...> wrote:

I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends.  But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:


The clock period shown here is averaged over the 5 period impulsing interval.

The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.

The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.

I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.

During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.

 

The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.


The relationship with atmospheric pressure is shown below: 




I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.

--
Bepi


Chris
 

I fear that all the fun could go out of clock watching if Bepi's entire apartment was evacuated. ;-)
Another thought: Entire canal systems could become cyclic or even resonant in nature.
Such a hydraulic "circuit" could make domestic, water hammer look like very small beer.
Just imagine the energy that vast mass of water would put into the ground and buildings!
Bepi is just going to have to move to somewhere more sensible.


On 05/04/2020 03:32, Harvey Moseley wrote:
Very interesting.  Do you have a barometer with rapid readout to see if the house is pressurizing and depressurizing.  I am almost sure that is it.   If it were building motion, you would expect a significant dependence on wind direction; and perhaps even for internal pressure differences.  Would be interesting.

On Fri, Apr 3, 2020 at 6:35 PM Bepi <pepicima@...> wrote:

I am collecting long-term clock data in this period and looking at them occasionally I was bothered by noise bursts I could not attribute to anything obvious. Since i live next to a canal I was aware that boat traffic could account for night/day noise differences, less intense during weekends.  But I was also getting random and large standard deviations of the period measurements I couldn't explain with instrumental or other effects. Here is an example, the time trace is in hours:


The clock period shown here is averaged over the 5 period impulsing interval.

The most common steady level is the one on the right of the above chart, SD around 10 μs, as opposed to the 10 times larger one at the center of the image.

The clock is my magnetically impulsed synchronome described here, where the pendulum is impulsed once every 5 periods.

I don't get to go out too much during these quarantine days, a centuries old tradition of Venice, my town, but I go for groceries and some basking in the sun of the now almost perfectly empty old town.

During these outdoor activities I noticed that there was a good relationship between wind strength and the standard deviation of the period fluctuations.

 

The following chart shows this for the above illustrated typical case of March 23rd, where wind speed, in the 10 to 20 m/s at the peaks and professionally monitored by a city weather station, is superimposed to the fluctuations SD against time in days.


The relationship with atmospheric pressure is shown below: 




I am aware that these high frequency fluctuations are not very relevant from the standpoint of the time measurement accuracy, in the range where time accuracy is normally most valued, but I find interesting that this effect is so large for a clock which is bolted down to the internal foundations of a building as sturdy and heavily built as mine, a former army barrack from the Augsburg times. Wind turbulence is doubly screened by tightly sealed apartment windows and the clock case. Is this the effect of foundation vibrations or of pressure fluctuations propagating indoor as inaudible sub-hertz sound waves? Does anybody else see a similar behavior? Any alternative explanation? I do have wind direction measurements to help with the interpretation but I didn't have time to check their correlation.

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Bepi


markotime
 

In your code the following is confusing:


      TCCR1B = _BV(CS10) | // start Timer 1, no prescaler

                          _BV(ICES1); // Input Capture Edge Select (1=Rising, 0=Falling)

is there a term missing? thanks /mark



John Haine
 

Thinking about this last night I realised that this kind of variation is not necessarily of the pendulum at all.  It is clear that there's a correlation between the period variance and wind speed, but another way to think about this is that the pendulum is actually providing a time reference which could be very stable, but what we are seeing is an artefact of the measurement system.  Essentially, vibration caused by the wind is shaking the sensor around its nominal position, but it sees this as a variation in the period because the bob is hardly affected.  The clock is perhaps working as a sensitive seismometer.

The plot  of my clock I showed over in another thread includes some odd, very rapid, variations in amplitude and period which seemed impossible for a high Q pendulum.  In fact, some further observations demonstrate that the variations are an artefact of the opto sensor, which had sunlight falling on it at a particular part of the day (more details to follow) (thanks to Tom van Baak and Bob Holmstrom for their analysis).


Bepi
 

John I share some of your concerns and I thought about the possibility of electrical noise, in similar way that stray sun light was affecting the optical sensor in your case. One can always miss things, maybe something similar is happening in my case.

About what Q allows or not in terms of omega-dot I don't see any problem, think of a sudden small fall of the bob along the rod, energy doesn't change but the frequency changes in one period.

About the clock being a sensitive seismometer, well the clock is a seismometer, we can't certainly ask a pendulum not to measure an acceleration.

The picture which makes more sense to me now is that the sensor and the pivot are linked together well and both are mechanically attached to the building structure which vibrates statically, with wavelengths much longer than the rod. What originates the signal is the inertia of the suspended bob, which is doing its job.

The other interpretation on the table, the one of the "infrasonics", which I said, obviously joking, we could rule out taking the air out of my apartment, can't stand if the pendulum air interaction is the same as the one we see in stationary state. Some time ago I evaluated empirically the clock pressure sensitivity at .45 μs/mbar, you showed us recently a similar estimate, if the interaction would be similar in this case, the same at 1/2 Hz as at 10-5 Hz, the corresponding pressure variation at a SD of 45 μs would pop my ears all the time when it's a very windy day.

Two more considerations: the detector/pendulum could be undersampling and I overestimated the SD

Second: I haven't understood yet if one can use the usual first order perturbation theory conclusions (Ayrie's expressions to be practical), when there is so much of high frequency perturbations. Same thing which was bothering me with the, not by chance, aerodynamics of the open/close door effect.
Harvey you are right one should check the consistency between my internal pressure measurements with respect to an external barometer, more work.
Neville, interesting what you say about the Littlemore clock I will try to find the reference. Any more mention of wind effects?

I might have easily said too much, don't ever think I really understand what I say.

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Bepi