Pendulum clock runs *fast* in hot weather

On 03/08/2018 11:26, John Entwistle wrote:

That's a *very* interesting point. Wood seems to have an expansion coefficient of around 3 x 10^-6, the first reference I checked gave brass as 10 to 19 * 10^-6 (both per degree C).
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The pendulum on this clock supported at the bottom like that. Goodness knows how the length is adjusted, because the weight doesn't move freely up and down the rod, resting on a nut that you can wind up and down a threaded rod. It looks as if you have to rotate the whole weight to move it up and down the screw, but if that's how you adjust it, it's very stiff and I'm not about to try it in case I break something. I just correct it each morning (and maybe in between if I notice it's wrong) so it remains no more than a couple of minutes fast.
+/- 5 seconds a day is pretty bloody amazing for a clockwork clock. It's good even for a quartz clock: the real time clock on my PC loses or gains more than that, but then gets corrected by scheduled process syncing with time.windows.com (or whatever) every 24 hours; I think the worst I've seen it (compared with time.is) is about 90 seconds out.
How long did it take to get it that accurate, given that you have to make a small adjustment to the pendulum length and then wait ages to see how much time it has lost over maybe 24 hours to work out how much further you need to adjust the screw, iteratively homing in on the correct setting. I suppose if you know the gearing of the clock (pendulum ticks per minute of hand movement) you can time a few ticks and extrapolate from there - but the more ticks you count, the less measurement error there will be.
Actually, it's uncanny. When I went to check the pendulum fixing just now, I found that the clock had stopped literally a minute or so ago, after I'd forgotten to wind it this morning. What is the chance of it stopping exactly at the time that I check the clock?
This is a very old full-height grandfather clock that has been in the family for years. It's named Francis after a distant ancestor who was a clock-maker. It would have been incredible to have found one of his own clocks in an antique shop, but my parents did the next best thing and bought one from a clockmaker who had lived in the same part of the country at about the same time - which I think was early 19th century. I presume the mechanism is original - in design, even if some parts have been replaced like-for-like over the years.
You'd tend to expect a big clock to have a deep sonorous chime, but this one has a little bell about the pitch of a bell used on a shop counter for attracting the assistant's attention - a bit weedy, as if its voice hasn't broken. The chiming mechanism is a bit uneven: I presume it has a wind vane that crudely regulates the interval between the chimes, and sometimes for no apparent reason you get a few chimes with a bit of time between them and then a couple of hurried ones at the end, as if it's saying "Sod it. Twelve o'clock is a lot of hard work. I'm nearly there, try and get the rest over quickly." But that just gives it character ;-)
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On Fri, 03 Aug 2018 14:31:01 +0100, NY wrote:

Didn't Harrisons watch (which had to go to sea) far exceed that. In the 1700s ?
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On 03/08/2018 14:39, Jethro_uk wrote:

Harrison's H4 was pretty incredible, but in fact the French were close behind
https://en.wikipedia.org/wiki/Marine_chronometer
Can't immediately spot the accuracy of the H4, but this link has an unreferenced statement suggesting that mechanical marine chronometers got to 0.1 seconds per day, giving an accuracy of 1 - 2 miles after a sea voyage of a month.
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On 03/08/2018 14:48, newshound wrote:

Here's a better one
https://www.theguardian.com/science/2015/apr/19/clockmaker-john-harrison-vindicated-250-years-absurd-claims
0.6 seconds over 100 days, and that was a pendulum clock to Harrison's design (though not, of course, at sea on a sailing ship).
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Yes but it was probably a far more elaborate mechanism with far more checks and counter checks that a normal clock mechanism. To make something that keeps consistent time, in a range of temperatures and on board a heaving ship, was a fantastic achievement. And it must have taken ages to adjust the clock, little by little, until it was keeping the correct time, even if once it *had* been set, it remained accurate. I presume it was a rotating balance wheel as opposed to a pendulum, so it would be a matter of tweaking the length of the spring. And what would they have used as a reference against which to measure how far fast or slow the Harrison watch was, while adjusting it. I suppose for a fixed location, the movement of the stars is known so you can tell the time fairly accurately using those and compare the watch against that.
Come to think of it, I wonder why my PC clock is so bad. Quartz watches keep better time and don't need much adjustment back to correct time. I wonder if when the PC is on, it uses some other time source than the quartz crystal.
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On 03/08/2018 14:57, NY wrote:

PC clocks were never particularly accurate because they didn't need to be. Doesn't everything get time from the internet these days?
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On Fri, 03 Aug 2018 15:04:24 +0100, newshound wrote:

We have one clockwork clock. All the rest get their time via NTP, or from Anthorn.
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A clock only needs to be as accurate as the gain or loss that you can tolerate in the interval until it is next synchronised with a master time source.
Windows defaults to syncing once a week. After I found my PCs drifting by several minutes, I changed that to once every day. It's a shame that it requires a registry change, and isn't changeable from a menu built into Windows.
As I understand it, the Raspberry Pi doesn't have a battery-backed real time clock, and has to set itself afresh every time it is booted and can talk to an NTP server. I presume it resyncs periodically while it is running.
I'm cautious of clocks that have no free-running capability, which depend entirely on a radio source. My wife bought a Rugby clock/radio years ago and towards the end it started displaying bizarre times, probably if it lost the radio signal, because it couldn't keep going (even if with a slight error) until the next time it got a valid time. Not good when it's the alarm clock that wakes you to go to work :-(
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<snip> >As I understand it, the Raspberry Pi doesn't have a battery-backed real time

Wouldn't all that be down to the OS running on it, so not the RPi itself?
On one ESP32 project I built it makes a WiFi connection, connects to an NTP server, logs the values of a temperature sensor to an SD card and then goes to sleep for n time.
On my Arduino projects I have an external RTC.
Cheers, T i m
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On 03/08/2018 15:25, NY wrote:

Some get their time from GPS if they want greater precision.

It only *needs* to be that accurate but there is real clockmakers kudos in having something that drifts by only a miniscule amount over the period where it needs to be rewound. 200 day clocks for instance.
Marine chronometers and observatory clocks at the beginning of the last century represented the peak performance of classic mechanical clocks. They also included cunning mechanisms that prevent rewinding interfering with the accuracy and amplitude of the pendulum beat.
The Shortt-Synchronome was the ultimate in precision mechanical clocks - good enough to detect seasonal variations in the Earth's rotation back in the 1920's - long before atomic clocks.
http://clockvault.com/heritage/

That is mainly because the designers CBA to load and trim the crystal properly unlike the ones in cheap watches which are usually done right.

The original PC design didn't have an RTC either - you had to manually set time and date every time you booted it (pre internet).

Domestic "atomic clocks" usually do have a local RTC that is disciplined by the incoming signal to trim it when free running to fractions of a ppm. Simpler ones just free run at whatever the crystal inside happens to oscillate at.
My guess at the design of the pendulum that is running fast would be an invar main pendulum bar with an iron weight that is about 1/10 its length supported from below. This is approximately tempco = 0ppm. Trouble is invar used to vary from batch to batch.
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On 04/08/2018 09:15, Martin Brown wrote:

Fascinating. This states that variations in atmospheric pressure are enough to affect high accuracy pendulum clocks.
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On 05/08/2018 13:32, newshound wrote:

Not exactly high accuracy if the pressure affects it. Maybe a suitable bellows mechanism is need in the pendulum?
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On 05/08/2018 15:11, dennis@home wrote:

I'm still interested in the physical mechanism. The thread is about why someone's clock runs fast (rather than slow) in hot weather, and there have been suggestions that it might be the fact that atmospheric pressure is consistently high (rather than that the temperature is hot) which is the explanation. But high atmospheric pressure would cause more drag on the pendulum, and thus should slow the clock down.
To my mind one of the more plausible explanations is that the C of G of the metal "bob" might go up, shortening the period. But reduced drag in lubricants is another possibility.
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On 05/08/2018 20:59, newshound wrote:

Well I don't believe anyone has mentioned the special thermally compensated pendulums that consist of several lengths of different wire running up and down the pendulum so its centre of mass stays in same place when the temperature rises so its probably the length of the pendulum changing.
If it has been copied for cosmetic reasons rather than using the correct alloys anything could happen.
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On 05/08/2018 23:05, dennis@home wrote:

The OP specifically said it was *not* a compensated pendulum. Apparently it is a wooden rod with a metal (brass, lead?) fixed at the bottom (so that the CofG may move upwards with increasing temperature)

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Metal, not wooden, rod. Looks like iron or brass, but dark, not silvery (iron) or yellow (brass), so very tarnished with age.
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On 06/08/2018 13:08, NY wrote:

Sorry, my mistake; but another poster *does* have a wooden one (and this does have a surprisingly low expansion coefficient)
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On Monday, 6 August 2018 17:07:27 UTC+1, newshound wrote:

His name wasn't Pinokio was it ;-)
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wrote:

But, but...
CoG would be a first-order effect, and drag and viscosity of air or lubricant an second order-effect?
Thomas Prufer
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