Caliber 8500 timekeeping question...

DoctorEvil · Aug 2, 2025

Greetings all. Hoping that someone knowledgeable in this area can shed some light on this issue.

I have a Seamaster Aqua Terra with the Caliber 8500 movement that currently keeps excellent time when I wear it during the day and rest it dial up for about 8 hours overnight, gaining only about 0.5-1.0 seconds a day. However, when I put it on a watch winder it loses about 6.0 seconds a day. In the watch winder, the watch dial sits at roughly a 45 degree angle. I decided to do a little experiment and see how much time it was gaining or losing over 24 hours in different positions at full wind. These were the numbers I got:

Dial up: +2 seconds per day
Dial down: 0 seconds per day
12 o'clock up: -1 seconds per day
12 o'clock down: -8 seconds per day
Crown down: -10 seconds per day
Crown up: -10 seconds per day

Naturally I was surprised to see how big some of these numbers were. Is someone able to tell me if this would be within spec for this movement and for a COSC certified chronometer? The watch was last serviced by Omega a bit over two years ago. Thanks in advance for your insights.

Knebo · Aug 2, 2025

COSC specifies a delta of 10 sec/day (largest difference in rates). So yours is a bit above that limits (+2 and -10 as the largest difference gives a delta of 12).
But if you're real life average is 0.5-1, I'd be happy about it and ask no further questions ;-)

502 to right · Aug 2, 2025

Archer undoubtedly can tell you what the standard is for an 8500.

By comparison, I have an Aqua Terra with the 8500 that also was serviced by Omega 2.5 years ago and I'm getting much better results. I have a positional delta of just 2 seconds, and if you only include the 5 COSC positions the delta is zero. The only position that deviates is crown right, which COSC does not include on the assumption that not many watches spend time in that position. Crown right is zero seconds per day and all other positions are +2.

That said, how a watch runs on your wrist is much more important than what any individual position gives you for timing.

RonP · Aug 2, 2025

According to ythe official omega workinstruction #28: for the 8500: Target is +3 sec / day, max deviation is 12 sec. between the positions fully wound and max 15 seconds after 24 hours.
Allowable daily rate is between 0 and 6 seconds.
So it looks like you are on the edge of what is acceptable. In the end what matters is how the watch performs in daily life which is as for your statement 0,5 - 1 seconds. So enjoy your watch and in the next service they can finetune the screws on the balance wheel to tweak the deviation between positions a little bit better maybe?

DoctorEvil · Aug 2, 2025

@Knebo @502 to right @RonP
Thanks for your responses. I'm very happy with the performance of my watch when I'm wearing it. Just a bit annoyed about losing time when it's on the watch winder. I'd rather have it run a few seconds fast each day so that when setting the time I can just stop the movement and let the atomic time catch up. I'll leave it as is and see what can be done at the next service.

Archer · Aug 4, 2025

DoctorEvil
Greetings all. Hoping that someone knowledgeable in this area can shed some light on this issue.

I have a Seamaster Aqua Terra with the Caliber 8500 movement that currently keeps excellent time when I wear it during the day and rest it dial up for about 8 hours overnight, gaining only about 0.5-1.0 seconds a day. However, when I put it on a watch winder it loses about 6.0 seconds a day. In the watch winder, the watch dial sits at roughly a 45 degree angle. I decided to do a little experiment and see how much time it was gaining or losing over 24 hours in different positions at full wind. These were the numbers I got:

Dial up: +2 seconds per day
Dial down: 0 seconds per day
12 o'clock up: -1 seconds per day
12 o'clock down: -8 seconds per day
Crown down: -10 seconds per day
Crown up: -10 seconds per day

Naturally I was surprised to see how big some of these numbers were. Is someone able to tell me if this would be within spec for this movement and for a COSC certified chronometer? The watch was last serviced by Omega a bit over two years ago. Thanks in advance for your insights.

First off, you need to eliminate the 12 o'clock up position, as this is a COSC watch and not a Master Chronometer. So the -1 seconds per day measurement would be eliminated for the purposes of determining if this falls into spec or not.

Second, these tests should have been done at full wind - if not they can;t be used. So if you want to do this correctly, you should fully wind the watch using the crown, let the watch sit for at least 30, but not more then 90 minutes, and then run the tests.

As already noted, the difference between the fastest and slowest position should be no more than 12 seconds, which is right on where you are, at the max. for full wind. In terms of average daily rate, it should be between 0 and +6 seconds, and your is -5.2, so is definitely out of spec.

Since pretty much all of your vertical positions (the last 4 you list) are slow, it's not at all surprising that when you have it on the winder, it runs slow. If this was still under warranty, it could be regulated to speed it up. If you have a watchmaker that is trained on co-axial, then they could do that also - if someone is not familiar how to handle the balance in a co-axial, I would not recommend letting them do this.

DoctorEvil · Aug 4, 2025

@Archer
Thanks for your response. Sadly the service warranty expired in April 🙁
I might wait until the timekeeping "on the wrist" starts to go off and then take it back to Omega for a service.

DoctorEvil · Aug 19, 2025

I was wearing the AT at work today and had a spare hour. So, I decided to try taking it to the OB in town for a check and re-regulation if needed. The watch tech put it on the timegrapher and these were the readings:

The numbers seemed totally at odds with what I had obtained in my initial post on this thread. The tech said that this was within specs, so he didn't re-regulate my watch.

@Archer
Do you mind perusing these numbers please? It's all Greek to me and I don't know if the tech is being truthful or he's just giving me the run around. Thanks in advance for your help.

Archer · Aug 20, 2025

Yes, those results indicate that watch is well within Omega specs, so I'll talk about the readings that Omega has specs for...

I am assuming these were done at full wind - there would be another set of the same numbers required at 24 hours after full wind, but I wouldn't expect them to test that when you are on a quick visit.

You have each individual position reading with rate, beat error, and amplitude. Omega require that the beat errors must all be below 0.8 ms, so all are fine.

Down below you have X - that average rate, beat error, and amplitude of the above - Omega only looks at the average rate, and requires this to be from 0 to +6 for this movement and yours is 1.1 so well within that spec.

D is the Delta, so the difference between the largest and smallest of the individual readings, for rate, beat error, and amplitude. The only one Omega has a spec for is the rate Delta, which has to be less than 12 - yours is 4.2, so again well within spec.

Why is this so different from what you had? I can only guess, but either the machine you were using isn't picking up the co-axial escapement well, or the watch was in a different state of wind, or...something else...

ErichKeane · Aug 20, 2025

Archer
Yes, those results indicate that watch is well within Omega specs, so I'll talk about the readings that Omega has specs for...

I am assuming these were done at full wind - there would be another set of the same numbers required at 24 hours after full wind, but I wouldn't expect them to test that when you are on a quick visit.

You have each individual position reading with rate, beat error, and amplitude. Omega require that the beat errors must all be below 0.8 ms, so all are fine.

Down below you have X - that average rate, beat error, and amplitude of the above - Omega only looks at the average rate, and requires this to be from 0 to +6 for this movement and yours is 1.1 so well within that spec.

D is the Delta, so the difference between the largest and smallest of the individual readings, for rate, beat error, and amplitude. The only one Omega has a spec for is the rate Delta, which has to be less than 12 - yours is 4.2, so again well within spec.

Why is this so different from what you had? I can only guess, but either the machine you were using isn't picking up the co-axial escapement well, or the watch was in a different state of wind, or...something else...

My read of the initial post is he actually set the watch at that position for 24 hrs and then compared it to his phone. So he had crown up/down at -10s/day after a day sitting in that position.

No mention of wind-amount though.

DoctorEvil · Aug 20, 2025

Archer
Yes, those results indicate that watch is well within Omega specs, so I'll talk about the readings that Omega has specs for...

I am assuming these were done at full wind - there would be another set of the same numbers required at 24 hours after full wind, but I wouldn't expect them to test that when you are on a quick visit.

You have each individual position reading with rate, beat error, and amplitude. Omega require that the beat errors must all be below 0.8 ms, so all are fine.

Down below you have X - that average rate, beat error, and amplitude of the above - Omega only looks at the average rate, and requires this to be from 0 to +6 for this movement and yours is 1.1 so well within that spec.

D is the Delta, so the difference between the largest and smallest of the individual readings, for rate, beat error, and amplitude. The only one Omega has a spec for is the rate Delta, which has to be less than 12 - yours is 4.2, so again well within spec.

Why is this so different from what you had? I can only guess, but either the machine you were using isn't picking up the co-axial escapement well, or the watch was in a different state of wind, or...something else...

Hi Archer. Thanks for looking at this. Reassuring to know.

The numbers I obtained in my initial post were done by fully winding up the watch, putting it in the relevant position, leaving it for 24 hours, and then reading the time difference against a reference (ntp time). I made sure that I fully wound up the watch prior to each 24 hour period. I don't have a timegrapher.

BTW are you able to tell me what DVH and Di stands for on the timegrapher printout? Thanks 😊

DoctorEvil · Aug 20, 2025

ErichKeane
My read of the initial post is he actually set the watch at that position for 24 hrs and then compared it to his phone. So he had crown up/down at -10s/day after a day sitting in that position.

No mention of wind-amount though.

Thanks for following. I made sure that the watch was fully wound up before putting it in each position.

Archer · Aug 20, 2025

DoctorEvil
Hi Archer. Thanks for looking at this. Reassuring to know.
The numbers I obtained in my initial post were done by fully winding up the watch, putting it in the relevant position, leaving it for 24 hours, and then reading the time difference against a reference (ntp time). I made sure that I fully wound up the watch prior to each 24 hour period.

Unless the watch has received some type of shock or something, the only other option I can think of (provided your time source is good) is that there is enough isochronism to account for the differences in rates as the mainsprings wind down. That would be unusual for that much deviation though.