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Discussion Starter · #1 ·
Hello my friends,

I was taking a look at standard ETA 2824-2 charts and I found these terms:

Daily rate +/-12s
Positional error +/-30s
Isochronism +/-20s

Could it be possible for somebody to explain to me how these work using an example? If for instance a watch gains every day between 5 and 7 seconds how this number is applied to the above terms?

Thank you in advance.
 

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The daily rate is a measure of how much a watch deviates from the exact time over a 24 hour period.

The positional error refers to the difference in instantaneous rate of a watch when it is placed in different orientations with respect to gravity.

Isochronism refers to the difference in the instantaneous rate of the watch depending on how much power is in the mainspring.

In your example, the +5 to +7 spd rating relates to the daily rate.
 

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Discussion Starter · #3 ·
The daily rate is a measure of how much a watch deviates from the exact time over a 24 hour period.

The positional error refers to the difference in instantaneous rate of a watch when it is placed in different orientations with respect to gravity.

Isochronism refers to the difference in the instantaneous rate of the watch depending on how much power is in the mainspring.

In your example, the +5 to +7 spd rating relates to the daily rate.
Thank you very much.
So if my watch gains every day "exactly" 7 seconds it is considered very precise isn't it?
 

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The daily rate is a measure of how much a watch deviates from the exact time over a 24 hour period.

The positional error refers to the difference in instantaneous rate of a watch when it is placed in different orientations with respect to gravity.

Isochronism refers to the difference in the instantaneous rate of the watch depending on how much power is in the mainspring.

In your example, the +5 to +7 spd rating relates to the daily rate.
How should we interpret these figures for general use though? Does it mean that we should expect 12 s/ day or 30 s/ day as 'normal'?
 

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Positional error of 30 spd is the worst case.

Positional variance is generally reported in 5 or 6 positions...flat with crown down, flat with crown up, vertical with 12, 3, 6, or 9 up. These change the direction in which the components are pulled by gravity. Some are more of a problem than others. Yes, you can tweak the movement to correct some of these...but getting it to run great in one position generally leads to loss of accuracy in others. 12 seconds per day is generally an average of the positions it tends to take.

On the other point: a consistent +7 seconds is termed stable. It's often considered preferable to have a watch with a slightly inferior daily error but that is stable, over an erratic movement that is nominally changing less each day. The former is predictable. 7 seconds a day? A minute a week, pretty much. Something you can readily turn into a habit. And it's more comfortable. +3 seconds, then -4 seconds, then +4, +4, -3...that tends to beg the question, why is it changing its performance so much? It's somewhat more unsettling.
 

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Thanks both! Damasko have told me the Da36, while elabore grade, is adjusted to 5 positions and chronometer performance. So I can expect about 4 seconds per day error max I now understand.
 

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There's a difference between "precision" and "accuracy" as well, and it's usually related to price. A watch that gains 4 seconds one day and loses 3 the next will be accurate in the sense it's within +/- 4 seconds. A watch which gains 3 seconds every day might be less "accurate" (in two days it's 6 seconds fast), but is more "precise." The latter will usually be higher quality and more expensive, and can usually be regulated to an even better level of "accuracy."

"Adjusted to X positions (up to six)" balances out the variations in rate that gangrel describes above. An unadjusted watch might run fast when worn, but run slow crown-down on the dresser overnight and break even in terms pf "accuracy." My Seiko does that. My Omega, a chronometer adjusted to 5 positions, varies far less whether I'm wearing it or not, and is thereby more "precise."

So, a less-expensive, unadjusted movement night be +4 sometimes, -5 sometimes, run fast on the wrist or dial up overnight, run slow crown up or down overnight, but generally be pretty close to correct. An expensive, more precise, one adjusted to 5 positions might run +3 every day, but always​ +3 regardless of being on the wrist, dial up overnight, or if you sleep hanging upside down like a bat in a cave.

I know my Seiko will be somewhere within a minute of the correct time after a week, up or down. It might even be spot-on, but I can't be sure. After seven days I know my Omega will be exactly 21 seconds fast.
 
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