Setting sensitivity and zero point of a mechanical lab balance

[peter63]

Dear All,

I have a stone-age semi-automatic mechanical analytical balance.

Short description: internal weights can be put by five mechanical dials as 100g/10g/1g/0.1g/0.01g and the last two digits (X.Xmilligrams) can be read from an illuminated marked scale as the moving horizontal rod (scale language).
I compared it with a calibrated digital analytical lab balance and it is basically OK.

This balance can be used a milligram precision weighting basically, but I have two problems with it:

1. The zero point is not exactly in the middle of the illuminated window and
2. Its sensitivity for milligrams is too low. It means that if I (after zeroing) set the last (0.01g) dial from zero to 0.01, the optical scale whows only 9 milligrams instead of 10. In the comparison table it can be clearly seen that the sensitivity bias is higher if the last two digits of comparison weights end 7-8-9 or 10 mg, independently on their absolute weights..

Prior doing anything, I obviously checked the overall cleanness of the key balance parts (eg scale pan) and the central position of the built-in libella.

First I tried to shift zero point to the middle of the window by aligning screw 1 and 2 (it was successful)
Then I tried to increase the sensitivity by aligning screw 3.

First surprise: aligning sensitivity shifted also the zero point, too.
Second surprise: originally I supposed that turning screw 3 a bit counterclockwise shifts the center of gravity of the whole moving arm (seesaw) higher so its endpoints will be a little bit wider (up to 10 mg instead of 9). But surprisingly, it happened the opposite: turning screw 3 a bit counterclockwise, it reached 8 mg only.

Thereafter I reset the original positions of both screws as don't want to make the original status worse so the 9/10 of sensitivity still remained.

So any advice/proposal are kindly welcome )

Thanks in advance,

regards
Peter

 

[rwj]

Welcome to the forum! That is an impressive/complex mechanical device with which I regrettably have no experience nor expertise to share. Hopefully, other members do. It may be helpful to post the balance’s manufacturer name.

 

[JFrank]

@1066
Alan might be able to lend some insight on these.
It might take him a bit to respond as he is across the pond.

 

[peter63]

Hi rwj!
Thanks for your reply!
Yes, it is an impressive mechanical balance, I really like it.
The Polish manufacturer's name is "ZAKŁADY MECHANIKI PRECYZYJNEJ" (Precision Mechanics Work), Gdans, The date of manudacturing is 1972. This type of analytical balance was rather popular in laboratiries in Hungary and also in East Europen countries in that time.
Hre is the original Polish description

 

[peter63]

Personally I am (not mechanical but) chemical engineer. Originally I hoped it's a snap, but when I experienced that sensitivity reacts in the opposite when I rotated screw-3 counterclockwise (so it become lower instead of becoming higher), I got a bit uncertain.

 

[Murray Brook]

@sparker
This guy should be of help.

 

[RegionRat]

In the world of calibration with respect to mass balances and scales, one of the most important performance characteristics is called the uncertainty.

Some of these types of instruments worked remarkably well, and others were a calibration nightmare.

This scale comes from a place and time behind what we called The Iron Curtain so it is unlikely folks from the west would have any specific familiarity with it.

I have no experience with this particular model but inherited and ran many of these kinds of scales in my early career.

Before you spend too much time and resource on your scale, consider if your goal will match up with the uncertainty of this scale.

It is a cool reminder of the times before transducers and integrated circuits.

 

[danny]

Dear All,

I have a stone-age semi-automatic mechanical analytical balance.

Short description: internal weights can be put by five mechanical dials as 100g/10g/1g/0.1g/0.01g and the last two digits (X.Xmilligrams) can be read from an illuminated marked scale as the moving horizontal rod (scale language).
I compared it with a calibrated digital analytical lab balance and it is basically OK.

This balance can be used a milligram precision weighting basically, but I have two problems with it:

1. The zero point is not exactly in the middle of the illuminated window and
2. Its sensitivity for milligrams is too low. It means that if I (after zeroing) set the last (0.01g) dial from zero to 0.01, the optical scale whows only 9 milligrams instead of 10. In the comparison table it can be clearly seen that the sensitivity bias is higher if the last two digits of comparison weights end 7-8-9 or 10 mg, independently on their absolute weights..

Prior doing anything, I obviously checked the overall cleanness of the key balance parts (eg scale pan) and the central position of the built-in libella.

First I tried to shift zero point to the middle of the window by aligning screw 1 and 2 (it was successful)
Then I tried to increase the sensitivity by aligning screw 3.

First surprise: aligning sensitivity shifted also the zero point, too.
Second surprise: originally I supposed that turning screw 3 a bit counterclockwise shifts the center of gravity of the whole moving arm (seesaw) higher so its endpoints will be a little bit wider (up to 10 mg instead of 9). But surprisingly, it happened the opposite: turning screw 3 a bit counterclockwise, it reached 8 mg only.

Thereafter I reset the original positions of both screws as don't want to make the original status worse so the 9/10 of sensitivity still remained.

So any advice/proposal are kindly welcome )

Thanks in advance,

regards
Peter

Interesting scale, but to me, it looks like a possible reloading mistake waiting to happen.

Danny

 

[BuffaloBill]

Very cool!

 

[Kracken McGraw]

In the world of calibration with respect to mass balances and scales, one of the most important performance characteristics is called the uncertainty.

Some of these types of instruments worked remarkably well, and others were a calibration nightmare.

This scale comes from a place and time behind what we called The Iron Curtain so it is unlikely folks from the west would have any specific familiarity with it.

I have no experience with this particular model but inherited and ran many of these kinds of scales in my early career.

Before you spend too much time and resource on your scale, consider if your goal will match up with the uncertainty of this scale.

It is a cool reminder of the times before transducers and integrated circuits.

How does it give you the weight reading?

 

[RegionRat]

How does it give you the weight reading?

Typically, you combined the poise values and the reading in that window to achieve a balance.

It was really just like any other simple lab balance, but with far more protection for those poise weights and with a really sensitive indicator.

Other models used a noble metal fine chain arc to establish the last digits. We had several types and models that were used to measure very small while being able to support relatively heavy samples.

When you consider our typical hobbyist reloading scales, like the Sartorius or the FX120 types, it was amazing that these mechanical lab balances supported more sample weight all while going down to a fraction of a milligram.




Here is an old Sartorius photo I found on the internet that shows the specs on a few of their older models, and another one of a Sartorius Selecta with the cover off. Those large cylinders were air dashpots to dampen the oscillations without oil or magnets.

 

[peter63]

Hi Guys!

First of all, I would like to thank your attention and comments.

As I wrote, gramms and centigramms can be added/set by the four dials. These dials obviously move the (conter)weights mechanically. These fix counterweights are precise and OK.
Milligramms can be read on the illuminated windows-scale as #.# mg. This scale is moved by the moving arm (seesaw) of the balance. The lower sensitivity thus practically means that the left/right endposition of the seeaws is less than it should be.

This principle was generally used in analytical balance worldwide: Sartorius(D), Mettler-Toledo(US) and Ainsworth(US) also used it until the invention of digital balances.

How does it give the weight reading? Obviously with the sum of dials and optical scale. I attached two photos on is: the weight is 0.70 gramm + 16 milligramm = 0.7016 gramm.

 

[peter63]

Exactly!
Selecta balance also has these three alignmint screws.