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Old 18th Sep 2021, 9:46 am   #1
John_BS
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Default Plea for new 'cautionary' thread!

My Unigor 4 multimeter movement was not perfectly balanced, such that if the meter was used vertically the needle would move a couple of percent from zero. I knew it was a taut-band movement, but forgot that it's also only 10uA fsd.

Blithely opened up expectng to find the normal counter-weights set at 90 degrees, only to find the arrangement show attached.

Needless to say the first attempt to adjust the balance resulted in the error increasing by at least twenty-fold. After numerous "tweaks" I cut my losses and now the meter is only a couple of times worse than when I started.

So, in order to balance the "Success Stories" thread, we need a "Disasters" or "Cautionary tales" where such hapless behaviour is put on parade for others to learn from, scratch their head and chuckle into their beards.

John
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Old 18th Sep 2021, 3:07 pm   #2
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Default Re: Plea for new 'cautionary' thread!

I'm not familiar with the taut-band style movement, but can the
counter Weights not be adjusted by placing a very small dab of Shellac
Polish in the right places, just as Avo Meter Movements are balanced?




Ian
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Old 18th Sep 2021, 3:18 pm   #3
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Default Re: Plea for new 'cautionary' thread!

Ian: yes, you're right. But the meters I've played with previously had three weights: one at 180 degrees to the needle, and the other pair at +/- 90 degrees. So you set the needle horizontal and balance with the 180 degree, then vertical etc. But with this meter the two weights interact: their sum balances the needle when horizontal, and their difference when it's vertical. So any change to either produces offsets in both directions.

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Old 18th Sep 2021, 3:29 pm   #4
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Default Re: Plea for new 'cautionary' thread!

How about a Dab of Shellac right in between where the
Two Counter Weights meet?
Which hopefully, would be like adjusting the 180 Degree Weight that isn't there!

Why did they do it like that I wonder?

I would have Thought a single Balance Weight at 180 would have been enough.




Ian
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Old 18th Sep 2021, 4:51 pm   #5
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Default Re: Plea for new 'cautionary' thread!

Just thinking aloud....
If you set the meter upright but at 45 degrees such that one of the counterweights is vertically down it will have minimum effect.
Rock the meter from horizontal to the above position and balance using the other counter weight.
Repeat but with the counterweight you have just balanced vertically down.

Peter
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Old 18th Sep 2021, 5:59 pm   #6
John_BS
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Default Re: Plea for new 'cautionary' thread!

I see, so you're saying something like the attached? I think that deals with getting the two counter-balances equal in moment (left-right), but when the meter is rotated such that the pointer is horizontal, you won't necessarily have the correct "sum" mass to balance the needle.

John
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Old 18th Sep 2021, 6:10 pm   #7
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Default Re: Plea for new 'cautionary' thread!

Looked at as shown attached, you can see that the necessary condition is that the two moments M2 & M3 need to be very similar, and only differ if the needle is not balanced L-R. However, their sum must balance the relatively large moment of the needle M1.
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Old 18th Sep 2021, 6:15 pm   #8
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Default Re: Plea for new 'cautionary' thread!

Shellac? I just nick some of SHMBO's nail varnish. If this isn't available most shops supplying the dame will have an old bottle or two of an out of fashion colour which they are more than willing to give away.
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Old 18th Sep 2021, 6:31 pm   #9
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Default Re: Plea for new 'cautionary' thread!

That's precisely what I ended up using: I have my own stash of garish colours.....
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Old 18th Sep 2021, 7:04 pm   #10
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Default Re: Plea for new 'cautionary' thread!

Quote:
Originally Posted by John_BS View Post
I see, so you're saying something like the attached? I think that deals with getting the two counter-balances equal in moment (left-right), but when the meter is rotated such that the pointer is horizontal, you won't necessarily have the correct "sum" mass to balance the needle.

John
Thats what I had in mind.
As I said though, just thinking aloud.

Peter
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Old 18th Sep 2021, 7:09 pm   #11
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Question Re: Plea for new 'cautionary' thread!

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In order to balance the "Success Stories" thread, we need a "Disasters" or "Cautionary tales" where such hapless behaviour is put on parade for others to learn from, scratch their head and chuckle into their beards.
John
So far, that part of your OP has not been addressed. Is there a likelihood of such an additional section being created?

Al. / Sept. 18th.
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Old 18th Sep 2021, 7:15 pm   #12
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Default Re: Plea for new 'cautionary' thread!

The other problem with Unigor taut-band movements is that the die-cast aluminium frame can distort, making the movement useless. I have four Unigors, only two of which are useable, but those two are among my favourite analogue meters.
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Old 18th Sep 2021, 7:18 pm   #13
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Default Re: Plea for new 'cautionary' thread!

Quote:
those two are among my favourite analogue meters.


Ditto, but don't try to "improve" them!
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Old 18th Sep 2021, 9:49 pm   #14
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Default Re: Plea for new 'cautionary' thread!

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That's precisely what I ended up using: I have my own stash of garish colours.....
Thats it !!! you are using the wrong colour.



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Old 19th Sep 2021, 6:22 am   #15
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Default Re: Plea for new 'cautionary' thread!

Quote:
Originally Posted by John_BS View Post
I see, so you're saying something like the attached? I think that deals with getting the two counter-balances equal in moment (left-right), but when the meter is rotated such that the pointer is horizontal, you won't necessarily have the correct "sum" mass to balance the needle.

John
You got me thinking so I decided to try and calculate it out.

Let us assume
1) That the mass of the pointer is evenly distributed along the length of the pointer, this means that for our calculations the mass is effectively concentrated halfway along the pointer.
2) the mass of the short arm holding the counterweight can be ignored and all the mass of each counterweight is concentrated in the counterweight itself.
3) the two counterweights are identical, there is no lateral imbalance
4) the two counterweight arms are at 90 degrees to each other.

For balance the moments on the left hand side must equal those on the right hand side.

We will consider two cases
a) where one of the counterweights is vertically down. From assumption 4 above then the pointer is at 45 degrees and the other counterweight is horizontal.

b) where the pointer is horizontal and the counter weight are are at 45 dgrees above and below horizontal

Let the mass of the pointer be MP, the effective distance of the pointer mass from the pivot be LP, the mass of one counter weight be MC and the distance of the counterweight from the pivot be LC

In case a)

Since the pointer is 45 degrees the mass of the pointer is operating at an effective (horizontal) distance of LP / (root 2)
One counterweight is vertically down so is not affecting balance.
The other counterweight is horizontal.

Balance is achieved when MP x LP / (root 2) = MC x LC
rearranging the above then the equation for balance in case a) is

MP x LP = (root 2) x MC x LC

In case b)
The pointer is horizontal and both counterweights are acting at an angle of 45 degrees, one above horizontal and one below.
Since the counterweights are 45 degrees the mass of the counterweight is operating at an effective (horizontal) distance of LC / (root 2)

Balance is now achieved when MP x LP = 2 x MC x LC x (root 2)

Now 2 divided by (root 2) is (root 2)

Substituting into the equation for balance we get

MP x LP = (root 2) x MC x LC

The same as case a) QED

It looks like my suggested way to balance the meter should work.

Peter
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Old 19th Sep 2021, 8:53 am   #16
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Default Re: Plea for new 'cautionary' thread!

Hi Peter and John, I spent many years as an instrument mechanic in the nuclear industry calibrating metered equipment. If it were just balancing the pointer it would be easy to just manufacter it the correct size, no need for balance weights. The purpose of the weights therefore are to correct for everything else physically connected, like the coil, former, Spring arrangement, taut band etc. These you cannot calculate for so therefore, balancing weights are provided and trial and error is the preferred technique.

Dave
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Old 19th Sep 2021, 6:50 pm   #17
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Default Re: Plea for new 'cautionary' thread!

Dave: that's pretty much how I had to work in the end.

Peter: I can see that your tecnique works if the weights are at rt angles, but I don't recall thinking /observing that they were. Sadly I didn't take a photo.

John
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Old 19th Sep 2021, 7:27 pm   #18
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Default Re: Plea for new 'cautionary' thread!

I am not familiar with Unigor, only 50uA AVO PMMC movements...and i don't think i have a single one which maintains a stable 'zero' when moving the meter from horizontal to vertical. I consider it good if they don't shift more than 3% of FSD! (I say this without knowing whether Unigor say their meter can be used in either orientation)

This alternative arrangement of balance weights would totally throw me.

The cock-up thread could perhaps be integrated with the success stories thread with alteration of title since both are equally helpful. Like the time i bought an early AVO and drilled out what appeared to be sheared off terminal threads..only to find out that the earlier ones had narrower threads and i had overbored it too much for new ones to fit....and Richard had offered me original screw in terminals FOC which i could no longer use. Fortunately it was a meter which looked like it had taken part in a cheese rolling competition down a steep hill.

Dave
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Old 19th Sep 2021, 9:07 pm   #19
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Default Re: Plea for new 'cautionary' thread!

Quote:
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Peter: I can see that your tecnique works if the weights are at rt angles, but I don't recall thinking /observing that they were. Sadly I didn't take a photo.

John
It actually works at other angles, the math is easier at 90 degree as you can use pythagoras hence the root 2. At other angles you have to use trig but it still works.
The smaller the angle between the counter weights the less lateral balancing effect there is, the greater the angle the larger mass you need to balance the pointer. The calibration technique is the same.

Peter
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