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#1 GMO

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Posted 07 June 2016 - 08:25 AM

I'm looking at putting in dynamic checks only into my weight control system because static checks on checkweighers are meaningless, however, I don't know what tolerance to allow.

 

So I could allow the zone of indecision (+/- 3 sigma) but that seems excessive.  Any more ideas or justification for them please?



#2 Ehab Nassar

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Posted 07 June 2016 - 12:56 PM

Hi GMO ,

 

I depends on the line capability , usually I'm looking for the control limits ( upper and lower ) with the specification limits  ( upper and lower )  , if the control limits within the specification limits , that the process is capable ,then I will study the line Sigma level, and work on it , by time I ll try to improve the performance and so the sigma level.

 

So if your data from the line indicated that you are at level 2 , if you adjust it on level 3 you will lose and so on , accordingly you have to know what is your current Sigma level

 

Br,

Ehab



#3 GMO

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Posted 09 June 2016 - 05:38 AM

Hi GMO ,

 

I depends on the line capability , usually I'm looking for the control limits ( upper and lower ) with the specification limits  ( upper and lower )  , if the control limits within the specification limits , that the process is capable ,then I will study the line Sigma level, and work on it , by time I ll try to improve the performance and so the sigma level.

 

So if your data from the line indicated that you are at level 2 , if you adjust it on level 3 you will lose and so on , accordingly you have to know what is your current Sigma level

 

Br,

Ehab

 

That makes no sense to me at all I'm afraid.  Sigma means standard deviation which we know.  What are you referring to with Level 2 and 3?



#4 Charles.C

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Posted 09 June 2016 - 07:30 AM

Hi GMO,

 

I guess yr query overlaps the t1/t2 criteria.

 

Personally i have always used crude, homemade, monitoring procedures to keep within the final EC weight requirements.

 

The options discussed near end of attachment (pg 23 etc) may be of some relevance to yr query. Not really my area of expertise though.

 

Attached File  Guidance_-_The_Weights_and_Measures__Packaged_Goods__Regulations_2006_v.4_December_2015.pdf   644.89KB   93 downloads


Kind Regards,

 

Charles.C


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#5 GMO

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Posted 09 June 2016 - 08:43 AM

Hi GMO,

 

I guess yr query overlaps the t1/t2 criteria.

 

Personally i have always used crude, homemade, monitoring procedures to keep within the final EC weight requirements.

 

The options discussed near end of attachment (pg 23 etc) may be of some relevance to yr query. Not really my area of expertise though.

 

attachicon.gifGuidance_-_The_Weights_and_Measures__Packaged_Goods__Regulations_2006_v.4_December_2015.pdf

 

Thanks but I've checked that document; no help.  It doesn't really overlap the T1 / T2.  The question is, how do you check on a routine basis that calibration hasn't drifted on your checkweigher?  Checking a T2 pack rejects doesn't check that I'm afraid.  Checking a static weight is meaningless as checkweighers don't check statically and are subject to vibrations, belt issues etc which could massively impact on readings.  Obvious thing is to run over an "acceptable" pack you've weighed offline and check it's reading within a reasonable tolerance.  BUT... what is the tolerance?  Strictly speaking it could be +/- 3 sigma and that would be irrespective of your process variability because the value for sigma is according to your process but even in the most tightly controlled process +/- 3 sigma can be huge!



#6 Ehab Nassar

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Posted 09 June 2016 - 09:07 AM

That makes no sense to me at all I'm afraid.  Sigma means standard deviation which we know.  What are you referring to with Level 2 and 3?

the below link may be explain it for you , then it will make sense for you 

https://www.isixsigm...-one-six-sigma/

 

The sigma level determined through the std dev. it has a levels from 1 to 6 , but the it is indicating the DPMO of the process.

So if the process performance at 2  Sigma the DPMO is 308,537  whoever it is 66,807 at 3 sigma

to determine at which level we should set our process we need to know what is the required specification , process capability (cpk) and the performance of the process at which sigma level . So after that you can determine to which sigma you can go.

 

hope this is make it sounds good , and make sense !,

BR,

Ehab

 

  

 

 

 

 

 


#7 Charles.C

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Posted 09 June 2016 - 01:13 PM

Hi GMO,

 

Perhaps I should have added that I've never used a checkweigher. So i have never been forced to encounter the ZOI.

 

ZOI is discussed at considerable depth in an earlier (2001) Welmec document attached in the older thread (post 7) linked below. I am unsure if the Packer rules are still identical (still 3 anyway). Maybe you have seen this document also.

 

4 types of checkweigher are (maybe were) apparently in use. From a very quick glance the choice appears relevant to the interpretation of ZOI and its estimation.

 

Some of the maths requires fairly heavy thinking so I didn't read the document to any great depth.

 

I notice you were already mulling over this issue in 2010. :smile:

 

Sorry if equally unhelpful as 2015 document.

 

http://www.ifsqn.com...er-calibration/


Kind Regards,

 

Charles.C


#8 GMO

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Posted 09 June 2016 - 03:37 PM

the below link may be explain it for you , then it will make sense for you 

https://www.isixsigm...-one-six-sigma/

 

The sigma level determined through the std dev. it has a levels from 1 to 6 , but the it is indicating the DPMO of the process.

So if the process performance at 2  Sigma the DPMO is 308,537  whoever it is 66,807 at 3 sigma

to determine at which level we should set our process we need to know what is the required specification , process capability (cpk) and the performance of the process at which sigma level . So after that you can determine to which sigma you can go.

 

hope this is make it sounds good , and make sense !,

BR,

Ehab

 

  

 

No this is not what I'm talking about.  The dpmo is just a fancy way of talking about a normal distribution but in any case that still doesn't help me.  So, say, for example, a pack weighs 200g and the standard deviation of a given pack when passed over for a ZOI check (sigma) is an entirely acceptable 0.5g.  The standard deviation is not controlled by anything in your process but simply the variability of your checkweigher.  This might be due to all kinds of things which are controllable (decent set up) but mostly uncontrollable.

 

To have a decent assurance that the weight is real within the bounds of your variability introduced via the uncertainty in your checkweigher; you should really say a weight is correct passing over a checkweigher if it is within +/- 3 sigma which is approx. 95% of the data.  That, however, includes weights from 198.5 to 201.5g.  So if it is weighing 1.5g heavy, my check would work but an average weight on my line of 200.5g would actually be illegal because the real average weight was 199.0g.

 

What I'm saying, perhaps inarticulately, is that it's nothing to do with your process variability, that is not my question, what it is, is that due to the natural variability of checkweighers, you cannot do an accurate calibration check because you do not know what is due to checkweigher variability and what is to do with being out of calibration.  The only way you can do it is to pass the same pack over the checkweigher again and again (i.e. do a ZOI check) and check the average corresponds with the weight you expect but that is a massive time commitment.  Retailer codes of practice demand that you do a calibration check daily but how people generally do it is to do a static weight check which, as I've already explained, is a complete waste of time...

 






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