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  1. #16
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    Default Re: Bob temperature compensation (By: Phil Burman)

    The ideas is as Phil shows. You want the compensater to be
    about 1/10 the length of the invar if steel. The problem is these
    numbers show more digits than the +- of these materials actually
    have. The steel can vary quite a bit. The optimal would be to have it
    with 2 nuts. One on the inside of the bob to adjust the percent of
    the bob used for compensation and a second to set the relative
    length position of the bob for the rate.
    Roger's plot shows hours so I'd suspect that differential thermal inertia
    wasn't the issue. Stratified air in the clock enclosure was more likely the
    problem.
    Tinker Dwight

  2. #17
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    Default Re: Bob temperature compensation (By: Phil Burman)

    My calculation was inverted. The bob would be raised by the expansion so the length should be reduced and the period would also be reduced. But the delta would be the same.
    As for the expansion of invar. It is very dependent on the actual composition of the alloy. Also I have seen reports where the invar rod is heat cycled before use to stabilize its thermal expansion. In the case of Philip Woodward's W5 clock he found that there was perfect zero expansion of his invar pendulum - though it was much shorter than a 1 second pendulum.
    I was wanting to show that simply by supporting the bob at the underside instead of the center the period of the pendulum is affected by a quite small temperature change. The effect of expansion is very marked.
    The optimum material is fused quartz.

  3. #18
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    Default Re: Bob temperature compensation

    Tok you were right the first time, the OP reported a 4 deg C drop when he switched off the heater, the fan and opened the door. With regard to invar coefficient, I'm begining to think you may be right. I currently have zero compensation on my pendulum and that is starting to look like the correct setting for zero temperature effect. The calculated length of the compensation tube (303 stainless) was around 140mm but that showed to be way over-compensated. I also read where Woodward reported that his W5 required zero compensation, but he didn't have an explanation as to why. The Microset timer software shows just how complicate things get, with what looks like several effects all going on at the same time. I've given up trying to understand the theory and have finally taken on-board Tinkers advice to run it for 24 hours with stable conditions, then adjust the ambient temperature, wait for stabilisation and then run for another 24 hours. Ignore all the short term noise and use only the stabilised rates at the two temperatures.

    Phil

  4. #19
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    Default Re: Bob temperature compensation (By: Phil Burman)

    Are you calculating the increase in the rotational inertia of the bob,
    when you say zero compensation? Even at the center of the bob, the
    bob grows and adds a small amount of slowing, just because it is bigger
    at a higher temperature.
    This usually means it will under compensate, though. This is the main
    issue with the mercury compensator but then mercury expands quite
    a bit. Most only look at the change in CG.
    Tinker Dwight

  5. #20
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    Default Re: Bob temperature compensation (By: Tinker Dwight)

    Hi tinker, yes I had considered that but I didn't think the effect could overwhelm the expected increase in length of the 1 metre of invar rod, in any case as you say a rise in temperature would cause the bob to get larger which would increase the MoI due to the L squared effect and this would slow the pendulum down. However I do keep meaning to do the calculation in order to check. At one time I planned to make a spreadsheet taking account of all issues but then realised that the uncertainty about expansion coefficient values would make this into an even more complicated hair pulling exercise. All the books I read recommend to do it by experimentation, then what you get is right regardless of the theory. It looks like I may now have to devise an short, adjustable, reverse compensation arrangement, with the bob hanging from the compensation tube rather than sitting on it!

    Phil

  6. #21

    Default Re: Bob temperature compensation (By: Phil Burman)

    Hi..I've only just realised that the conversation had gone to page 2..Still reading and learning..I've started a trial with no heat and will report back in 3 days if a pattern emerges. It seems that you are all saying that temp comp which changes with expansion, air density and flotation is very complicated. I do try reading up on this stuff but so far have only found these issues explained separately and nothing about how they operate at the same time. I hope the trial will indicate what I need to do.

  7. #22

    Default Re: Bob temperature compensation (By: rogerj)

    Quote Originally Posted by rogerj View Post
    It seems that you are all saying that temp comp which changes with expansion, air density and flotation is very complicated.
    *Very* much so... Analysis/theory/opinion about this has been raging on and off for several centuries, and still goes on today among some of the best minds in horology and physics. There still seems to be no absolute agreement. In my thinking, it seems that in the sub-parts-per-million scale, chaos enters into events, with no way to predict the absolute outcome. Woodward calls this "random walk", but even with his rigorous analysis, not everyone agrees that it is purely random. In some ways, it's unfortunate for us precision junkies that we can measure our pendulum timekeeping so accurately that we can see every little fluctuation.

    Johnny

  8. #23

    Default Re: Bob temperature compensation (By: John MacArthur)

    As promised..here's a graph of the clock's performance without any heaters..It is much better than when the heating was running which is a lesson learned. However I haven't learned anything more because the pressure has been high throughout 1020 > 1030 and the ambient temp close to 20C..maybe dropped a little at night. The trial will continue to see where it leads..You can see that until midday Thursday it was very good indeed, then quickly started losing and adopted a slower losing rate until now when it is 0.14 sec slow..I can't see any reason for this or any correlation with any environmental changes..I will see how things are after a week. Roger

    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  9. #24
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    Default Re: Bob temperature compensation (By: rogerj)

    It looks more like there was a peak on Thurs. than a drop started Thurs.
    Overall, it looks like it is doing well.
    I'm not sure how you are driving the pendulum?
    You need to record humidity, pressure and temperature
    along with the rate to determine anything useful.
    Tinker Dwight

  10. #25
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    Default Re: Bob temperature compensation (By: Tinker Dwight)

    The science of clocks and watches by A L Rawlings page 71 gives the buoyancy error due to change in atmospheric pressure:

    cast iron @ 7 kg/litre = 0.0073 seconds per day per millibar
    brass @ 8.5 = 0.0060
    lead @ 11.3 = 0.0045

    You can prorate for materials of other densities.

    So for a barometric change from say 1020 millibar to 980 millibar with a lead bob would give a rate change of 0.18 seconds per day.

    Phil

  11. #26

    Default Re: Bob temperature compensation (By: Phil Burman)

    @ Tinker: The pendulum is being impulsed every 52 seconds in true Matthias Hipp style..ie an electro-magnet beneath the bob.
    I would really like to include temp and pressure reading in the recordings but I'm using a small program written for the Raspberry pi which was given me and I have no programming skills to alter it. I do have a weather station though that records and I'll experiment with combining the results even if only in Photoshop.
    @ Phil: Thank you for those figures.My bob is mild steel so close to cast iron..With the stable high that have predominated over they past week it proved to me that the variations are more temp related - or due to mechanical imperfections caused by the not very precision which I was able to use in the construction.
    Meantime, as you know, clock watching is a slow business, so I will certainly come back here when I have something definite to report..Roger

  12. #27
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    Default Re: Bob temperature compensation

    Are you mechanically firing the coil or using an opto-electronic
    interrupter?
    I don't think it effects things but it may.
    It could be that the number of swings changes. You might be right on the edge
    of changing the number of swings. That would show such erratic behavior.
    It might be a good time to learn some coding.
    I prefer the Arduinos look-a-likes, simply because they are so cheap and you
    can dedicate each to its own function. Humidity, temperature and pressure sensors
    are easy to connect and there is code already done to access these devices.
    The can also monitor the # or swings and what ever else you'd like.
    Tinker Dwight

  13. #28

    Default Re: Bob temperature compensation

    Quote Originally Posted by Tinker Dwight View Post
    Are you mechanically firing the coil or using an opto-electronic
    interrupter?
    I don't think it effects things but it may.
    It could be that the number of swings changes. You might be right on the edge
    of changing the number of swings. That would show such erratic behavior.
    Tinker Dwight
    I have written up the cHipp Toggle clock, Tinker. Google "Hipp Toggle" it will be obvious which link to follow..But do excuse the ramblings of an amateur if you go there..As far as the time between impulses is concerned I have never seen it change from 52 seconds when I occasionally time it. The exact swing amplitude can vary within a couple of millimetres (width of the "flag") can change within that 2 sec window I think.
    Quote Originally Posted by Tinker Dwight View Post
    I prefer the Arduinos look-a-likes, simply because they are so cheap and you
    can dedicate each to its own function. Humidity, temperature and pressure sensors
    are easy to connect and there is code already done to access these devices.
    They can also monitor the # or swings and what ever else you'd like.
    Tinker Dwight
    I did play with an Arduino before a friend offered me help with a Pi program..I have tried to get my son, who does know how to code, to give me a hand but so far without success !! I doubt I have the ability to learn coding at my age but don't tell me that 10 year olds can do it,,that hurts :-)
    Basically, I find monitoring a clock in this way opens a whole can of worms..Today's downward trend in the rate looks terrible...but in fact when I used to monitor by comparison with a radio controlled wall clock, I would never have noticed....and been pretty happy with it. lol
    I've added a graph of the performance up to today..Just short of a week's worth. The pressure remains generally high - 1024 yesterday and this morning it's up to 1036. The short term blips must be temp related but how much of the drift downward is due to a rating error and how much to pressure, time will tell. I'll leave it alone for another week at least and hope for a passing low !
    Roger
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  14. #29
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    Default Re: Bob temperature compensation (By: rogerj)

    Hi Roger, no passing low pressure for at least the next 4 days. This link is useful.

    http://www.metoffice.gov.uk/public/w...ime=1492819200

    Phil

  15. #30
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    Default Re: Bob temperature compensation (By: rogerj)

    What are you using as a time reference?
    As long as the drift rate is relatively constant it
    is doing well.
    I understand the way a Hipp toggle works but also
    know it can be adjusted to center on a particular
    swing count.
    Tinker Dwight

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