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  1. #21
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    Default Re: Digital chronometer

    ANy modern quartz clock movement is at least as accurate as an old time chronometer; ones with a digital display probably more so than analogue displays (no mechanical bits to accumulate dirt).

    These days, "chronometer" is pretty much an obsolete term. ANY modern solid state timekeeper will keep time far more accurately and reliably than an old time chronometer will. Check it against a reliable time signal every so often to get the rate, and you'll exceed chronometer accuracy quite easily. Rolex still trade on their watches being certified as chronometers; what they don't tell you is that the certification is redundant for any modern non-mechanical time-piece!

  2. #22
    Join Date
    Jul 2008
    Location
    East coast UK. Mostly. Sometimes the Philippines
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    Default Re: Digital chronometer

    Not that easily.

    For amusement I rated a couple of modern quartz clocks described as chronometers; a Weems and Plath and a Rigel. Both were OK but both wandered gently off as time went on and eventually accumulated errors of several minutes.
    I suspect that their ability to withstand changes in temperature might be their weak point.

    Both would be quite OK for navigation, as you would correct them from the time signal. But let’s not run away with the idea that quartz clocks and watches are perfect.

    Nobody in their right mind would put a chronometer on a yacht or power boat. The motion would make the detent escapement skip. Which is why, before modern navigation, small naval vessels were issued with deck watches, accurate watches with a lever escapement, which lived in a box.

    Here are a couple - the quartz Weems and Plath chronometer top left is away with the fairies after a couple of months, the WW2 Ulysse Nardin RN deck watch top right is gaining too fast but gaining at a reasonably steady rate and the USAAF WW2 Hamilton GCT aircraft navigation watch bottom right is actually holding a chronometer level of accuracy at +1.5 seconds a day.

    Last edited by Minn; 08-07-19 at 08:09.

  3. #23
    Join Date
    May 2007
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    37,245

    Default Re: Digital chronometer

    Quote Originally Posted by AntarcticPilot View Post
    ANy modern quartz clock movement is at least as accurate as an old time chronometer; ones with a digital display probably more so than analogue displays (no mechanical bits to accumulate dirt).

    These days, "chronometer" is pretty much an obsolete term. ANY modern solid state timekeeper will keep time far more accurately and reliably than an old time chronometer will. Check it against a reliable time signal every so often to get the rate, and you'll exceed chronometer accuracy quite easily. Rolex still trade on their watches being certified as chronometers; what they don't tell you is that the certification is redundant for any modern non-mechanical time-piece!
    The least accurate clock I know of is the dashboard clock on my motorbike.
    It gains so much (10 to 20 minutes a month?) it's useless and annoying. I leave it set 5 or 6 hours out so I don't start riding like I'm late. I've added a little clock, like an analogue wristwatch. Cost about 7 quid and works despite the V-twin vibration.

  4. #24
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    May 2007
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    Cambridge, UK
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    6,039

    Default Re: Digital chronometer

    Quote Originally Posted by lw395 View Post
    The least accurate clock I know of is the dashboard clock on my motorbike.
    It gains so much (10 to 20 minutes a month?) it's useless and annoying. I leave it set 5 or 6 hours out so I don't start riding like I'm late. I've added a little clock, like an analogue wristwatch. Cost about 7 quid and works despite the V-twin vibration.
    A mechanical chronometer would probably fare even worse on a motorbike!

  5. #25
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    May 2007
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    37,245

    Default Re: Digital chronometer

    Quote Originally Posted by AntarcticPilot View Post
    A mechanical chronometer would probably fare even worse on a motorbike!
    But 'Chronometric' Speedo's are much sought after...
    With accuracy of a few %.
    One second per day is about 12ppm. To get that from a quartz crystal, you have to treat it right.

  6. #26
    Join Date
    May 2005
    Location
    up on the moors.
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    33,277

    Default Re: Digital chronometer

    I seek certainty !

    All the three battery "chronometers (<£25) I have tried have a variable rate probably bcs of temperature. They can just about be relied on to boil an egg properly.

    My Casio ProTrek sailing watch has gained 4min 37 secs since 9 Aug 2018. And that is a mixture of being worn, and sitting in a chart drawer; but the rate is constant.

    There are some very neat aircraft chronometers, especially the Davtron one.

    https://www.aircraftspruce.com/menus...onometers.html

    and the one whose specification I like is this

    https://www.davtron.com/product-detail.php?M880A-24

    Seems to me that a GA aircraft clock will be designed to cope with temp, vibration and pressure variations ?

    It would be good to have one piece of kit on board (apart from the compass and lead line (and I know about electrical storms, and magnetic anomalies over degaussing cables) ) that gives 100% reliable data without me having to challenge it against another piece of kit, or against my highly risk averse eidetic memory.

    All I want is to be able to make a mark on a paper chart or on a plotter by dropping a waypoint, so that I can say at hh:mmss) I was here, so that if some kerfuffle happens I have a good reliable starting position on which to base my DR.
    I think, therefore I am. I am, therefore I sail.

  7. #27
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    May 2007
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    Default Re: Digital chronometer

    Realistically, the scientist's way forwards is to understand that your time is not precise, and understand how that affects your position.
    If your time is a minute out, then at 6 knots, that's a cable. How exact do you need to be?
    I suppose a good approach would be to look at a typical EP and look at the uncertainties due to
    Estimating the tide
    Averaging your speed and course steered
    Leeway

    If the time is a minute wrong at the start, and wrong by 1 minute 5 seconds at the end of 6 hours DR, it's generally only the delta of 5seconds that will appear as an error. Hence 'synchronise watches', it's more important to be consistent than absolutely right half the time. 5 seconds at 6 knots is going to be between 1 and 2 boat lengths...

    Of course the full minute's error will get in, if you set your watch to R4 time halfway through.

    So long as the time-derived error is smaller than the others, it's unlikely to matter.

    When a standard crystal isn't good enough, we look to OCXOs. Ovenised crysal oscillators.
    That removes the temperature variation, but there is still long term drift, which might be 10ppm in the first year and another 10 over the next few years.

  8. #28
    Join Date
    May 2005
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    up on the moors.
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    Default Re: Digital chronometer

    I agree, seconds don't matter if one is setting up a DR from a good 'known' position over say 24 hours at 6 knots.

    However, for sextant work.... and (worst case) if one has to enter a harbour in fog and leaving a known buoy, it should be possible to use 30 seconds at 6kts is 100 yards. Again, a chronometer is not essential at that speed scale. The prudent mariner might decide to anchor using the ultra certain lead line, though.
    I think, therefore I am. I am, therefore I sail.

  9. #29
    Join Date
    Jul 2008
    Location
    East coast UK. Mostly. Sometimes the Philippines
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    Default Re: Digital chronometer

    Quote Originally Posted by sarabande View Post
    I agree, seconds don't matter if one is setting up a DR from a good 'known' position over say 24 hours at 6 knots.

    However, for sextant work.... and (worst case) if one has to enter a harbour in fog and leaving a known buoy, it should be possible to use 30 seconds at 6kts is 100 yards. Again, a chronometer is not essential at that speed scale. The prudent mariner might decide to anchor using the ultra certain lead line, though.
    FWIW, my Hamilton GCT (post 22) - a WW2 bomber’s on board navigation watch, has a “hack” function so it can be synchronised and a good one costs about the same as the Davtron GA chronometer but doesn’t need 28 volts!

  10. #30
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    May 2005
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    up on the moors.
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    Default Re: Digital chronometer

    The main advantage of the Davtron is the size and brightness of the digits, especially useful for evening sights, and for sailors with dodgy eyesight Plus the alarms (variable power) and count down and count up elapsed time functions.

    I've had an email back from Davtron. Their aircraft chronometer is powered by a temperature compensated DS3232 chip which has an accuracy of +- 3.5 ppm. So in a million seconds (11.5 days) it could be in the worst case that I lose or gain 1 second every 3 days. Eat your heart out John Harrison !

    Tks for the info about the Hamilton. They make serious money in the Sates
    https://www.ebay.com/sch/i.html_from...eter&_osacat=0


    Now if I can find someone to put a D3232 chip into a Raspberry Pi....
    I think, therefore I am. I am, therefore I sail.

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