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  #1  
Old 10-12-2003, 08:06 AM
Frame Dragging Frame Dragging is offline
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Why do my clocks lose time? Bad electricity?

I have three digital-read out clocks in my house. All three clocks are about 2 minutes slow after one month. These are not battery-powered clocks but plugged into the 110VAC outlets in my home. All are on the same circuit in the house. My computer however gains about 30 seconds a day on its system time. The computer is plugged into a surge protector/UPS. What is going on here?
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  #2  
Old 10-12-2003, 08:19 AM
Desmostylus Desmostylus is offline
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a) Where are you? In most places the mains frequency is controlled so that over the space of a week or so, a simple electric clock that derives its timing from the mains will agree with the reference clock (usually an atomic clock) that the electricity utility uses.

b) The clock in your computer derives its time from an internal crystal oscillator. It doesn't make any difference whether the computer is plugged into the mains or a UPS.
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  #3  
Old 10-12-2003, 08:26 AM
Frame Dragging Frame Dragging is offline
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I'm in a small town in Minnesota USA. Utility company is run by the city. They buy power from the grid and distribute through the town. Thanks for the post.
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Old 10-12-2003, 09:04 AM
Desmostylus Desmostylus is offline
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Ok, you're connected to the main US grid.

Since making my earlier post, I've looked at the latest NERC standards. The requirement for matching mains frequency to the atomic clock seems to have been dropped. I don't know why.
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Old 10-12-2003, 09:29 AM
Q.E.D. Q.E.D. is offline
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Quote:
Originally posted by Desmostylus
The requirement for matching mains frequency to the atomic clock seems to have been dropped. I don't know why.
I'd guess it's because these days, most digital clocks rely on a quartz crystal oscillator.
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  #6  
Old 10-12-2003, 09:35 AM
Desmostylus Desmostylus is offline
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And I'd guess that the reason has absolutely nothing to do with your suggestion, but rather federal/state regulatory issues.
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  #7  
Old 10-12-2003, 09:39 AM
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Does the quartz crystal oscillator expect an exact 60 cycles/second source or does a variation in the cycles have no impact on its ability to keep time accurately?
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Old 10-12-2003, 09:41 AM
Desmostylus Desmostylus is offline
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No. A crystal oscillator doesn't care what the mains frequency is.
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  #9  
Old 10-12-2003, 09:42 AM
Q.E.D. Q.E.D. is offline
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Why else would they have bothered to control AC mains frequency that tightly, assuming they ever did?
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Old 10-12-2003, 09:48 AM
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Does the quartz crystal oscillator expect an exact 60 cycles/second source or does a variation in the cycles have no impact on its ability to keep time accurately?
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  #11  
Old 10-12-2003, 10:13 AM
Desmostylus Desmostylus is offline
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Quote:
Originally posted by Q.E.D.
Why else would they have bothered to control AC mains frequency that tightly, assuming they ever did?
That's the sole reason, AFAIK.
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  #12  
Old 10-12-2003, 10:33 AM
Wendell Wagner Wendell Wagner is offline
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If you want perfect time, get rid of your clocks and get ones that are reset each day by radio. There are clocks (and even wristwatches) that are reset each day by radio from the atomic clock in Colorado. This works anywhere in the U.S. except for Alaska and Hawaii (and it also works in parts of Canada and Mexico).
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  #13  
Old 10-12-2003, 10:41 AM
KP KP is offline
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A Frequency (and phase) match is very useful to minimize losses wehn sharing or bridging sections of the grid. Of course, in practice, it is impossible to maintain a strict lock under varying and somewhat unpredictable loads, but it's a good idea to try.
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  #14  
Old 10-12-2003, 11:22 AM
Desmostylus Desmostylus is offline
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Quote:
Originally posted by KP
A Frequency (and phase) match is very useful to minimize losses wehn sharing or bridging sections of the grid.
Frequency match between "sections of the grid" is a bit more than useful. It's an inherent property. It has nothing to do with minimising losses.

I'm not sure what you mean by phase matching. At a given instant, the phase angles at various parts of the grid will all be different.

Phase matching is only important at the instant of connecting something to the grid. E.g. a generator that's just been started can be connected to the grid once its frequency (roughly) matches that of the grid. You do that connection when the phase angle of the generator (roughly) matches the local phase angle of the grid. Thereafter, the frequency of the generator is locked with the frequency of the grid.
Quote:
Originally posted by KP
Of course, in practice, it is impossible to maintain a strict lock under varying and somewhat unpredictable loads, but it's a good idea to try.
I assume here that you're talking about a strict lock with an independent reference, like an atomic clock, rather than a strict lock with other "sections of the grid". It's impossible not to maintain a strict lock with other sections of the grid.
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  #15  
Old 10-13-2003, 10:46 PM
Meeko Meeko is offline
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:ucks::
Even a stopped clock is right two times a day.
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  #16  
Old 10-13-2003, 10:56 PM
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Thanks Meeko but these guys are not stopped - they are just SLOW. It's either cheaply made clocks bought for a cheap price by me at Walmart or something with the current in my house or my small Minnesota town. A real pisser for me, an anal Virgo (is that redundent ?), who likes everything perfect. And time should always be perfect.
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  #17  
Old 10-14-2003, 12:26 AM
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An anal Virgo or an anally probed Virgo? Perhaps you and your home were swept up and away at near light speed, probed, and then returned again at near light speed. That would explain the time difference.
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  #18  
Old 10-14-2003, 01:59 AM
octothorpe octothorpe is offline
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a1997xf11, if you have a voltmeter handy, check the voltage at a convenient outlet. Chances are that you are running slightly less than 120v. This could be caused by being at the end of a circuit, a voltage regulator that is either 'locked down' or improperly set, or it just may be that your municipality is running slightly undervoltage. My folks (also in rural Minn.) have the same problem occasionally.

Desmostylus is correct in his statement that it's impossible not to maintain a 'strict lock' on the grid as was recently demonstrated in the massive outage in Eastern US states. If enough generators (supply) are lost, the load (which has not lessened) will be such that the additional strain on the remaining generators will cause them to slow down. Relays in substations will detect this 'underfrequency' and trip out load until the frequency recovers. If the relays are set incorrectly, or if a relay otherwise fails to operate, the protective relays on the generators will open causing further loss of supply resulting in a 'cascading blackout'. The underfrequency relays are set such that x amount of load will be dropped at, say, 59.6hZ, y amount will open at 59.4hZ, z amount at 59.2, etc. This allows individual utilities to somewhat prioritize their loads leaving the most critical on for the longest time possible.

As for quartz clocks, the quartz crystal will vibrate at a 'regular' frequency if provided with a 'regular' voltage. If the quartz crystal is 'tuned' to 120 volts, any deviation in voltage will result in a higher or lower rate of vibration. As i believe it would not be cost effective to test each crystal to determine its exact frequency for a given voltage, i would expect that there is an allowable 'range' that is acceptable depending on the quality of the timepiece.

I am unfamiliar as to whether there is or was a 'standard' that mandated all generators run strictly to the atomic clock, as (here in the US) the frequency was agreed upon for economic reasons well before the advent of the Cesium Atomic Clock.

Curiously, railroads, whose signal lines operated at 25hZ, used to send a 'spike' down their lines once per day to 'reset' all their station clocks to the hour. In days past, the railroads were the 'official' timekeepers.

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  #19  
Old 10-14-2003, 08:46 AM
elfkin477 elfkin477 is offline
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Quote:
Originally posted by Meeko
:ucks::
Even a stopped clock is right two times a day.
Only an analog clock. A dark plastic square is never the right time.
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  #20  
Old 10-14-2003, 09:49 AM
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Quote:
Originally posted by octothorpe
In days past, the railroads were the 'official' timekeepers.
In fact, it was the need for a standardization in time in regards to delivering mail that prompted the railroads to develop and implement the time zone system that we use today.
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  #21  
Old 10-14-2003, 10:59 AM
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I'd probably call my electric company (Im sure if the city runs it they have someone you can call) & ask them about this. They'd probably come out & look at things for me for free. They have before.
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  #22  
Old 10-14-2003, 05:15 PM
shelbo shelbo is offline
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Quote:
Originally posted by handy
I'd probably call my electric company (Im sure if the city runs it they have someone you can call) & ask them about this. They'd probably come out & look at things for me for free. They have before.
Ding ding ding!

This post has been rated 10.0 on the handy scale! Congratulations!
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  #23  
Old 10-14-2003, 07:39 PM
engineer_comp_geek engineer_comp_geek is online now
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The electric company does not guarantee you 120 volts. It may vary a bit by location, but typically they guarantee something like 120 +/- 10%, which is 108 to 132 volts. They used to guarantee the frequency in the long term so that clocks that ran off of synchronous motors (used in schools and other government buildings) were kept accurate. I don't know if this is still guaranteed or not.

A digital alarm clock is just a microcontroller and a display. The microcontroller gets its time base from the crystal oscillator that runs the microcontroller. For example, if the manufacturer chose to use a 4 MHz pic microcontroller, then all of the timing would be based off of dividing down the 4 MHz oscillator to generate a 1 second time tick. The main power is converted into DC for use by the clock's circuitry, so variations in both voltage and frequency on the AC side aren't going to matter. Cheaper crystals (like you'd expect in a cheaper clock) have worse manufacturing tolerances, so while crystals in general are fairly accurate, if you have a cheap clock then the one in yours may not be so great.

One of my early labs way back in college was to create a digital clock that used all TTL chips, and used the AC frequency as its time base (fed into the circuit via a schmidt triggered gate for any curious electronic hobbyists). It's a fairly typical EE student project, but I don't know of any commercial clock that uses this method.

Your computer has 2 methods of keeping time. While it's turned off, a real time clock chip keeps the time. When you turn the computer on, the operating system reads the time out of the clock chip. There is a thing in the computer called a timer tick interrupt. This is a periodic signal generated by a timer chip on the motherboard (a different chip than the real time clock chip). Among other things, the timer tick is used by the operating system to update what the operating system thinks is the correct time. Most operating systems also use the timer tick to run the operating system kernal since that guarantees that the kernal will run frequently. When you shut down the computer, the current time kept by the operating system will be written back into the real time clock chip. Since the real time clock and the timer chip use two different crystals, it's entirely possible for one or both of them to be inaccurate. You may find that your computer keeps better time when it's on than when it's off, or vise-versa. There are computer programs available which will periodically sync your computer clock to an atomic clock via the internet.
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  #24  
Old 10-14-2003, 11:12 PM
Desmostylus Desmostylus is offline
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Okay. It looks like the mains frequency control requirements haven't been dropped, just relaxed a bit and altered in form back in 1997.

Frequency Control Concerns In The North American Electric Power System (warning, .PDF, and also incredibly dull reading)

So a clock synchronised to the mains won't accumulate errors, even though it may be wrong by a few tens of seconds at any point during the day.

Such clocks are still quite common, I just looked at the schematics for my oven and for my microwave - they both operate that way.

Regarding your clocks, a1997xf11, I can't say what's going on. It could that they're all crystal controlled, and just happen to all be slightly slow. Or it could be that they're mains synchronised, and there's some problem, say momentary supply interruptions, that are causing them to run slow.
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  #25  
Old 10-15-2003, 12:59 AM
David Simmons David Simmons is offline
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Re: Why do my clocks lose time? Bad electricity?

Quote:
Originally posted by a1997xf11
I have three digital-read out clocks in my house. All three clocks are about 2 minutes slow after one month. These are not battery-powered clocks but plugged into the 110VAC outlets in my home. All are on the same circuit in the house. My computer however gains about 30 seconds a day on its system time. The computer is plugged into a surge protector/UPS. What is going on here?
If your clock has a backup battery to keep it going in case of utility power failure then the clock itself runs on dc and uses an oscillator as a time base which can be off from the time standard at the Naval Observatory (or wherever). The frequency of your electric power has nothing to do with it.

If you are off by 2 min. in a month that is a frequency error of about 46 parts per million shich seems to be reasonable for the inexpensive oscillators they put in clocks and watches.

Same thing for your computer clock.
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  #26  
Old 10-15-2003, 01:43 AM
octothorpe octothorpe is offline
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Quote:
Originally posted by engineer_comp_geek
The main power is converted into DC for use by the clock's circuitry, so variations in both voltage and frequency on the AC side aren't going to matter.
I'm a bit confused by this statement. I was taught that the oscillation was caused by the voltage; higher voltage = higher frequency, lower voltage = lower frequency. Your statement suggests that the frequency of oscillations is unrelated to the voltage (or, at least, as the voltage is DC, deviations from design voltage do not affect the oscillations). By this do you mean, regardless of the supply voltage, the rectification to DC will be unaffected by any deviations on the AC side? I would think the DC output would be dependent on the AC input.

As the original post by a1997xf11 mentioned that all 3 clocks were off by 2 minutes per month, and i am under the (possibly mistaken) impression that frequency of oscillations is voltage dependent, i assumed low system voltage from the particular municipality.

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  #27  
Old 10-15-2003, 03:23 AM
engineer_comp_geek engineer_comp_geek is online now
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Quote:
Originally posted by octothorpe
I would think the DC output would be dependent on the AC input.
Usually not. A voltage regulator is used to keep the output at a constant level even if the input varies, otherwise the microcontroller might not function properly. The oscillator circuitry is mostly built into the microcontroller (usually the only external part is the crystal) so it's going to receive a regulated voltage as well.

Even if it didn't, there are many types of oscillator circuits. What you are describing is basically called a voltage controlled oscillator. As you vary the voltage, the frequency also changes. Other types of oscillators (such as those typically used by crystal oscillators) take advantage of ressonance and feedback. In this case the frequency output of the oscillator is determined by the inductance and capacitance of the filter stage, and isn't going to be dependent on the operating voltage.

Quartz crystal is a natural bandpass filter, which makes it ideal to use in a resonant feedback circuit to produce oscillations.

You can think of it as similar to blowing air over a bottle. The sound frequency you get is going to depend on the size of the bottle (the dimensions of the bottle will determine the resonant frequency). If you blow harder, you just get a louder sound, but the frequency is going to stay the same.
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  #28  
Old 10-15-2003, 11:53 AM
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engineer_comp_geek, well, your explanation makes perfect sense, thanks for clearing that up.

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  #29  
Old 10-15-2003, 11:50 PM
DarrenS DarrenS is offline
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Since it's been so well answered...somewhat related question:

Is it possible, in theory, for a clock to keep perfect time?

I'm guessing that would violate some law of quantum physics (Uncertainty Principle?) but I don't quite see how.
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  #30  
Old 10-16-2003, 01:34 AM
Derleth Derleth is offline
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Quote:
octothorpe wrote:
a1997xf11, if you have a voltmeter handy, check the voltage at a convenient outlet.
Am I the only one who read this as volunteer?

"Hey, Ed, go check the voltage of them mains over there."
:: BZZT! ::
"Ahhh... two spasms and a twitch means 120 Volts."
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  #31  
Old 10-16-2003, 01:48 AM
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Quote:
Originally posted by DarrenS
Is it possible, in theory, for a clock to keep perfect time?
I think you can make arbitrarily accurate clocks. Consider a clock that uses as a reference a massive rotating ball in vacuum under zero gravity. The accuracy is limited by the residual atoms and photons that collide with the ball, but you can reduce the effect by improving the shielding and increasing the mass of the ball. You can't get literally perfect accuracy, but you can get as close to it as you want.

By the way using a rotating ball isn't so far-fetched. Currently the most accurate time references are pulsars, which are neutron stars that rotate fast and emit one pulse of radiation per rotation.
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  #32  
Old 10-16-2003, 07:55 AM
Wendell Wagner Wendell Wagner is offline
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In any case, we have atomic clocks now that are more accurate than the Earth is. They can measure time so precisely that they can tell by how much the Earth's rotation is slowing down. There have been several "leap seconds" added to recent years. That is, the rotation of the Earth has slowed down enough in the past few decades that it has been necessary to add an extra second at the end of some years for "official time" (according to the atomic clocks). Had this not been done, it would have screwed up our astronomic observations. Within a few decades, we would have started to notice that the stars seemed to be reaching their zenith points a few seconds late.
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  #33  
Old 10-16-2003, 11:04 AM
David Simmons David Simmons is offline
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Trick question? Aren't the atomic clocks that are used to keep Greenwich time and from which time signals are sent out in order to standardize all other clocks, by definition, "perfect time?"
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  #34  
Old 10-16-2003, 11:23 AM
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I believe Universal Coordinated Time is kept as an average of a whole whack of atomic clocks around the world.
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  #35  
Old 10-17-2003, 10:13 PM
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Quote:
Originally posted by scr4
I think you can make arbitrarily accurate clocks. Consider a clock that uses as a reference a massive rotating ball in vacuum under zero gravity. The accuracy is limited by the residual atoms and photons that collide with the ball, but you can reduce the effect by improving the shielding and increasing the mass of the ball. You can't get literally perfect accuracy, but you can get as close to it as you want.

By the way using a rotating ball isn't so far-fetched. Currently the most accurate time references are pulsars, which are neutron stars that rotate fast and emit one pulse of radiation per rotation.
No. Currently the best atomic clocks are limited to accuracies of about a part in 10^15. Making them better isn't just a matter of deciding to - there are real physical and engineering obstacles.

Pulsars don't do this well, IIRC, and they have a problem in that they tend to spontaneously change their frequencies, which means you have to recharacterize them, and that takes time to do.
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Old 10-17-2003, 10:15 PM
swansont swansont is offline
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Quote:
Originally posted by David Simmons
Trick question? Aren't the atomic clocks that are used to keep Greenwich time and from which time signals are sent out in order to standardize all other clocks, by definition, "perfect time?"
No such thing as perfect time. Which is why it's a bit difficult to decide if an atmic clock is running a teensy bit fast or slow, except relative to another clock.

At some level, deciding what "perfect" time is, is arbitrary.
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  #37  
Old 10-17-2003, 10:33 PM
Q.E.D. Q.E.D. is offline
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I don't know about pulsars spontaneously changing frequency. I've never heard of such a phenomenon, unless you mean the relatively tiny glitches that can occur on occasion (Chronos?). What does make pulsars somewhat poor timekeepers ina the long term is the fact that in spewing out the great quantities of energy that they do, they slow down their rate of rotation. This happens very slowly, typically, so in the short term, at least, pulsars are very accurate clocks.

Here is some good information on pulsars.
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  #38  
Old 10-17-2003, 11:02 PM
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Atomic clocks (I'm somewhat familiar with the Cesium variety) don't take a Cesium atom and count its oscillations. If they did that, they would keep perfect time. Instead, the clocks have oscillators that use the Cesium resonance as feedback to keep them on track. They're not phase-locked, just tuned, so they are not perfectly accurate.
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  #39  
Old 10-18-2003, 06:22 AM
swansont swansont is offline
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Quote:
Originally posted by Q.E.D.
I don't know about pulsars spontaneously changing frequency. I've never heard of such a phenomenon, unless you mean the relatively tiny glitches that can occur on occasion (Chronos?). What does make pulsars somewhat poor timekeepers ina the long term is the fact that in spewing out the great quantities of energy that they do, they slow down their rate of rotation. This happens very slowly, typically, so in the short term, at least, pulsars are very accurate clocks.

Here is some good information on pulsars.
As I understand it, they "settle" - have a "star quake" - and the frequency changes in an unpredictable way, which is a bad thing for clocks.
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  #40  
Old 10-18-2003, 06:36 AM
swansont swansont is offline
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Quote:
Originally posted by CurtC
Atomic clocks (I'm somewhat familiar with the Cesium variety) don't take a Cesium atom and count its oscillations. If they did that, they would keep perfect time. Instead, the clocks have oscillators that use the Cesium resonance as feedback to keep them on track. They're not phase-locked, just tuned, so they are not perfectly accurate.
No, they wouldn't. The 9192631770 number defining the second is for a cesium atom that has no perturbations. In a cesium clock there are frequency shifts due to things like magnetic fields, blackbody radiation, collisions, etc., and you can't measure these effects exactly, so there is always some uncertainty in the frequency.
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  #41  
Old 10-18-2003, 10:27 AM
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Re: Why do my clocks lose time? Bad electricity?

Quote:
Originally posted by a1997xf11
My computer however gains about 30 seconds a day on its system time.
My new Dell computer gained time, I tried several updates and other things...

Then I found this simple little program that background synchronization keeps your PC in sync with U.S. government-maintained atomic clocks at intervals you specify and automatically resets the computer clock. (XP does the same thing but it never worked in mine)

Since I installed Rockettime (free version) my computer is acturate to the second.. all the time!
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  #42  
Old 10-18-2003, 11:29 AM
Q.E.D. Q.E.D. is offline
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Quote:
Originally posted by swansont
As I understand it, they "settle" - have a "star quake" - and the frequency changes in an unpredictable way, which is a bad thing for clocks.
Yes, those are the glitches talked about on the page I linked to. The change in frequency is only about 1 part in 1011, typically, and also these glitches occur only rarely, it seems. Apparently they've never been observed in high-rate pulsars, either.
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  #43  
Old 10-18-2003, 11:46 AM
swansont swansont is offline
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Quote:
Originally posted by Q.E.D.
Yes, those are the glitches talked about on the page I linked to. The change in frequency is only about 1 part in 1011, typically, and also these glitches occur only rarely, it seems. Apparently they've never been observed in high-rate pulsars, either.
And 10-11 is a big shift for a good clock.
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  #44  
Old 10-18-2003, 11:49 AM
Q.E.D. Q.E.D. is offline
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Beats my digital watches by a wide margin. Besides if you want to avoid the glitches, use young, high-rate pulsars. There's no shortage of them.
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  #45  
Old 10-18-2003, 11:53 AM
Q.E.D. Q.E.D. is offline
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And that's a difference of only .022 seconds over the lifetime of the average human.
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