Sartorius Precision Balance "repair" . Any balance techs out there?

[mematyi]

Hi again, dear EEVblog community!
I hope you're doing well during this Covid crisis 
Interesting question for you. I hope some of you are experienced in using/fixing precision scales, or should I call balances. 
I have a Sartorius CP324S balance, that I bought used.

The problem with it is that while it seem to work flawlessly; repeatability, accuracy, sensitivity is very much OK, it drifts badly with off-center loading.
Its a 320g capacity balance, with d=0.1mg, with internal calibration.
I tested it with a 50g weight, but it changes as much as 0.0400g if I place the weight on different places on the pan. With a 1g weight, it changes zero..

It has a "monolithic" force restoration-type load cell, which has two screws for adjusting this phenomenon, but the manual calls for +-0.0003g of off-center drift. I am not sure if 0.0300 can be fixed with them.

I leveled the balance multiple times, properly preheated is, the pan is not bent, nothing seems to be loose, binding, etc. Everything seems to be in order, except the off-center thing.


Considering I bought it for about 70$, it is still a bargain for me, but I hope somehow it can be fixed.

Have a nice day and thanks for your attention! 

[wraper]

Not that I repaired that many lab scales, but should be a few dozens and have seen this problem a few times. Most likely it can be adjusted, however do not expect it will necessarily stay adjusted in long term, YMMV, and quite likely likely will drift again. IMHO this happens because balance at some time experienced large overload/mechanical damage.

[HighVoltage]

I have repaired similar Mettler Toledo scales and such behavior usually is a mechanical one.

One scale that needed re-seating of the load cell with the correct torque on the screws and it fixed such problem.

Can you show some pictures from the inside of the scale, especially the load cell.
 

[mematyi]

Thanks for your kind replies!

The adjustment points are the two brass slotted screws pointing upwards.
The case has been opened before, but I did not see any apparent mechanical damage, stripped screws, etc.

I adjusted one screw about half a turn, but it did not change meaningfully, so I reset it to the "original" position.
The thing is, if I place the weight to the Exact same position, anywhere on the plate, it reads wonderfully the same every time, but more than 2-3mm of off-centricity and it starts to wander some mg-s.

[Hamelec]

check this Document: p18 ff
https://www.dataweigh.com/media/3637/wcp5001-e02072.pdf
Regards

[mematyi]

Thanks!
I got this manual!
The problem is that it suggests adjusting a few milligrams, but mine have 40milligrams of error.

[HighVoltage]

Thanks for the pictures, that looks like a really nice scale.
It looks like all important hinges are good, just amazing metal work.

Maybe you have some pre-stress in the load cell metal arms from a previous overloading.

I would carefully loosen all load cell and adjacent arm screws and tighten them again correctly.
This is one way to relieve some stress.
Smallest movement and stress in the metal can make a huge difference.

After that you need to do a complete new calibration.
This might not fix it right away but this way you can notice changes and try again until it fits.
I got one 0.1 mg Mettler Toledo scale fixed this way.

[Hamelec]

what is inside the bore of the two brass slotted screws pointing upwards?
another srew / countersrew / setsrew?

[mematyi]

Indeed, there is another screw, however it cannot be turned, or at least not easily.

I tried to turn the bigger brass screw, it surely drifts the tare point, but it does not change the weight it senses, nor the eccentricity.

[Kleinstein]

From the picture it looks like the center screw inside the adjustment ones goes all the way though and is locked with a nut from the underside.
So one may have to loosen that nut.  For fine adjust the srews one can turn likely have slightly different threads on the inside and outside and thus work as a different thread with very little movement per turn.

It looks like the whole hinge mechanism is machined out of a single block.

As I understood it they don't use DMS and a classic load cell, but electromagnetic force compensation.  The weights and the lifting mechnism is there to do an internal calibraion or at least a check inside.

[mematyi]

Yes, indeed you are correct.
I removed the weighing cell from the machine, carefully inspected it, but I could not find any visible damage, loose screws, etc.

I tried to adjust both turnscrews back and forth, and indeed it changes the zero position, you have to re-tare is, but after that it reads with the same corner load differences.

I guess it is what it is, i am sure way more skilled people tried to repair it before selling the balance off.

[wraper]

Did try adjusting outer screws?

[DenCraw]

You defenetly have to cover the scale with a glass bell or cover.
You have to correctly level the scale in a very firm base, not a simple four leg table. Avoid any vibration.
Any vibration or air flow may affect dramatically your measures.
Pay attention to the ripple in the power supply.

[mematyi]

Thanks guys!
Yes, I did try everything you mentioned.

I guess it is beyond repair, that is why it was left to sell so cheaply.


I did made a stopper for my measuring cup, so I can use it for comparative measurements at least.