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  1. #46

    Default Re: Bob temperature compensation

    To mark my 100th post, here is a graph from the test we did earlier (re.#38) of the validity of using the computer time service for monitoring clocks. The point "A" on the graph is the point at which the input to the system was swapped from my clock to the NTP/GPS derived 30 sec pulses.
    The blue line is the offset error as described in my previous post and is calculated as an 5 minute average of the raw data as collected every 30 secs
    The red line is ( as I understand it) the rate of change of the raw data ie. NTP/computer clock errors).
    For my purposes I think this is as good as is necessary for this type of clock. I do have an expanded scale version that shows that the blue error line is actually about 4 to 5 Milliseconds later (minus) than GMT due to the inherent delay in the network paths etc. We do wonder if anyone else has used NTP for this purpose..Roger


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  2. #47
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    Default Re: Bob temperature compensation (By: rogerj)

    Quote Originally Posted by rogerj View Post
    There is a fair degree of temp compensation (the subject of the original post) and the actual temperatures prevailing, with the central heating off (broken), are around 20C with no great variation night or day at present.
    Hi Roger. As was previously pointed out the rate of temperature equalisation between the bob, the invar rod and the (buried) steel rod section will not be the same due to the difference in relative location, geometry and thermal mass. Without careful measurement of rate versus temperature you cannot just dismiss this aspect as job done. For your configuration even a change in the rate of temperature change will produce different results. If your method of temperature compensation is good then all (invar rod) pendulums would already be designed this way.

    Quantify "fair degree" and " with no great variation"

    The rate change appears to be almost twice what it should be for 34 millibar change in barometric pressure.

    What was the barometric pressure at the end of the 7 days.

    A photo of you pendulum, bob and suspension arrangement would be useful. As previously said, pendulum behaviour is a complex subject. The usefulness of the comments you get will be directly proportional to the amount of information you provide.

    Plotting only the rate drift hides a multitude of issues that can be revealed by a boxcar average plot. Can you not download the raw data to Excel, then you would be able to graph the hell out of the data.

    Phil

  3. #48

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

    Hi Phil..With such a knowledgeable audience I'm a bit reluctant to post detailed pictures and information here about my DIY clock as it is definitely not a work of art..basically it's a hobby electronics project which came about after buying a copy of Electrical Timekeeping by Hope-Jones.
    I bought the book following the purchase of a MK2 Synchronome movement and its subsequent finishing off as a "marriage". After I retired I acquired 4 more 'nomes plus a PO36 and a Gents C7.
    However I have written a blog on the Chipp Toggle clock as I went along and it can be easily found by Googling "Hipp Toggle"
    The idea has never been more than a voyage of discovery rather than an attempt build a near perfect clock. I just hoped it would be as good a timekeeper as a Synchronome - and It has achieved that in spite of its shortcomings.
    Specific answers: temperature compensation is what it is..1 metre of invar plus 1.5 inches of steel plus plus about 3 inches of steel 4ba rating thread. The bottom supported steel bob is 8 inches tall. There is about 1.5 inches of brass in the top chops plus 3/8 inch suspension spring.
    I have no doubt the compensation is not right - but insufficient maths to calculate by how much.
    Since my heating failed a couple weeks ago, A max/min thermometer over the last week shows 16.7 to 20.6 and that would cover the period of the graph in #38
    I completely accept that the variations in rate are twice what one would expect for those barometric changes but I have no idea why.
    At the end of the period shown in #38 the pressure was 996Mb I think. As I write the pressure is showing a 1006 Mb and the clock is neither gaining or losing.
    As I've already said the clock is was not meant to be a "regulator" or anything similar. All this discussion has come about as a result of monitoring it with a computer Before that, but excluding the ridiculous period when I tried heating in the case, the clock was - and still is a reasonable timekeeper and has run for several weeks at a time before I need to press the buttons that add or subtract seconds to the count.
    Re Excel. my maths is pretty poor and I don't know how to use Excel. But my friend does and I just send him the data and that's exactly what he does with it..The graphs I get back are from Excel.
    This monitoring system is cheap to set up but not as comprehensive as the commercial equivalent. It would indeed be great if temp and pressure could also be monitored at the same time. Then the observations might be far easier to interpret. But is has been fun..Roger

  4. #49
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    Default Re: Bob temperature compensation (By: rogerj)

    Hi again Roger, The rate at the end of the run, with a pressure of 996mb, is slower than the apparent rate at 1025mb (this is the wrong way round to be explained by barometric changes). If your invar rod does not come with a certificate for coefficient of expansion then calculation is useless. For example several people (including Philip Woodward) have reported that their invar pendulums do not require any compensation, this list would now include me. Temperature compensation should not be buried inside the bob and the bob should be supported from its centre. Anything else and you could be chasing your tail for a life time.You can only determine the correct temperature compensation by actual measurement. With a temperature variation of 4 degrees C you are wasting your time considering rate variations related to barometric pressure, they will be swamped by the temperature issue.

    Good Luck

    Phil

  5. #50

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

    Phil..I have taken to heart your comments on temperature effects and will revisit that topic. I have ordered a temp logging thermometer with remote sensor and will install it in the clock.
    Re the central support of the bob in precision clocks (a la Woodward etc) I (and my friend) have long wondered about the compensation of a standard Synchronome master clock with its bottom supported bob. The thermal inertia of the cast 16lb bob must be considerable and we pondered on what, if any, heat treatment of the Invar might have been done by HJ.
    Any thoughts on that would be appreciated.. Roger
    BTW My invar did not come with any certificate...it was bought from a steel stochholder.

  6. #51
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    Default Re: Bob temperature compensation (By: rogerj)

    I think a synchronome is normally 1 order of magnitude less accurate than a precision regulator, so using the expansion of the bob to provide approximately compensation for the invar rod is good enough, but the pendulum regularity will always suffer from transient temperature effects. You can only achieve temperature compensation by calculation if you know the coefficient of expansion (certificate) of your particular rod and the other pendulum materials, otherwise you have to have a means of adjusting the compensation and do it by experiment. As I said earlier a number of people seem to be finding that modern invar requires no compensation (maybe). I wouldn't worry to much about annealing and dimensional stability yet. It is probable that your off-the -shelf invar is already annealed. If do you identify dimensional stability as an issue for the level of precision you are aiming for then use quartz. This raises the question of how far shall I go towards ultimate precision. Increased precision goes hand in hand with increased cost and degree of difficulty. It is a good idea to list all parameters that influence pendulum accuracy together with the degree of influence each has and what needs to be done to eliminate the influence. Then sort the list by degree of influence. You can then get an overview of how far you want/can afford to go.

    Who or what is HJ?

    Phil

  7. #52

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

    Apologies for my brevity..H.J is Frank Hope-Jones, the man who devoted 50 years in developing the Synchronome system. So far my clock appears to have equaled the timekeeping of a Synchronome and that's all I set out to achieve..Ultimate precision it will not be !..Roger

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