OIML Comments and Suggestions 

12 February 2008

To Whom It May Concern

Re: Suggested Changes to OIML R50
The Secretariat TC9/SC2 by Morayo Awosola
Comments and Suggestions to further the Trade Use of Belt Weighing Technology

 

Dear Sirs,

We have pleasure in suggesting changes to OIML R50-1 since this document is the most significant belt weighing standard in the world. When the current version of OIML R50-1 was published, it brought about much posiitve change and progress in the industry. Now, in the light our own manufacturing and field experience, we would like to suggest enhancements which can further progress the technology and help better position belt weighing to assume the larger profile it deserves in national and international commerce.

  

Introduction

 

Control Systems Technology Pty Ltd is a manufacturer of loadcell based belt weighing equipment used extensively in Australia and exported to world markets. CST equipment is trade certified in Australia to OIML R50 standards as a Class 0.5 system, in Canada as a 0.1% system as well as a 0.5% system and in the United States of America under NTEP as a 0.25% system. CST has a strong interest in precision and trade certified belt weighing equipment.

 

The comments below apply to the OIML R 50-1 Working Draft which is to be commented by March 15 2008.

 

Later in this document, some of the suggestions are further described and discussed. The references to A1, A2 etc indicate the motivationj for the requested change.

 

Comments On Text Of OIML R 50-1 Working Draft

 

Sect No,	Page No.	Comment	Reason
Page	5-17	Terminolgy. There may be impact on terminolgy as a result of other commnets, however, the terminology section is not specifically addressed here.	-
2.1	18	Accuracy Classes. Add Class 0.2. There is a seperate justification document for the introduction of a new more accurate class. This would require to be added across the whole document in all the relevant places if it were to be adopted.	A1
2.2,2.3,2.4, 2.5	19 20	Accuracy Classes, Add Class 0.2 Accuracy Classes, Add Class 0.2	A1
2.4 and 2.5	19,20	Clarify relevance of Minimum Totalised Load and Minimum Test Load. Specifically, make it clear that - the load obtained at maximum flow rate in one revolution of the belt (this is not applicable when all material tests load readings are obtained over a whole number of belt revolutions) Clarify that the Minium Totaliser Load and Minimum Test Load are governed by the same principle with regard to whole belt revolutions. There is a seperate discussion of totaliser update over whole belt revolutions Consider adding a clause; “Belt Scales may include a means of permitting all test load readings to be obtained over a whole number of belt revolutions. When such a facility is present and when Live Material tests are conducted for the purpose of pattern approval 'in-situ' tests or for subsequent re-verifcation, the MTL size need not exceed the load obtained at maximum flow rate in one revolution of the belt”. Also consider adding; “The means of permitting all test load readings to be obtained over a whole number of belt revolutions may consist ot one of the following methods. All or a sufficient part of totalisation may be withheld and only fully updated once per belt revolution. A zero image record of the belt may be recorded and synchronised with the belt so that there is no need to hold back any part of the totalisation. In the case of either (i) or (ii) above, a method of checking the agreement between the actual belt position and the belt image in the belt weigher electronics set shall be provided.”	A4
2.7	20	Would not other units be applicable in non SI units economies? Is this clause a statement to the effect that SI units are acceptable in Europe?	A2,A4
2.8.4.2	21	Add Class 0.2, 0.014%	A1
2.8.5.3	22	Add Class 0.2 at 0.02%	A1
2.8.5.4	22	Add Class 0.2 at 0.005%	A1
2.8.5.5	23	Add Class 0.2 at 0.0007%	A1
2.9.2	23	Add Class 0.2 and Consider tightening Zero Stability test. As currently written the error categories represent 100% of the mpe for belt weighing. To clarify, at 20% flow rate, a zero of 0.05% of FSD as written for a Class 0.5 instrument is an error of 0.25%. which is 100% of the allowable error. Clearly a belt weigher with this level of zero instability could not meet the requirements for a Class 0.5 instrument.	A1, A2
2.9.3	23	Add Class 0.2	A1
2.9.4	23	Add “this is not applicable when all material tests load readings are obtained over a whole number of belt revolutions” Read separate justification for this suggestion	A1
3.2.2	24	Should editing of legally relevant setup data be prevented unless the belt is stopped or unless the flow rate is zero? This provision seems un-necessarily restrictive.	A4
3.2.6	24	I assume this a reference to a totalising indicator. Should not the requirement be that there shall be an 'Out of Range' indication if any 'out of range flow' has occurred during the delivery? Further to this, the US Handbook 44 requires a graphical record of flow rate during a delivery. This would seem to be the only way to have a meaningful record of the validity of a delivery. There should also be some consideration and guidence given to what to do if the flow rate falls outside the legal range.	A4
3.2.7	24,25	The “or” conditions do not make it clear what is mandatory. This should be clarified or it will become a bone of contention. The clarification should be not only in what is mandatory but some more definition of precise meaning.	A4
3.3.1	25	Idler Sets around the weigh frame could easily be used to falsify results.The modification of alignment or spacing would have dramatic affects which may not be easily detectable by both the buer and/or the seller. Idler sets (Roller Track) should be specifically mentioned as requiring sealing. It might be useful for some methods of sealing to be suggested once this matter is discussed among interested parties. It seems to me that illegal adjustments to the roller track are far more easily achieved than illegal modiciations of software or data and deserve at least equal attention as electronics safeguards.	A3
3.3.2 b)	25	There may be an assumption that all belt weighing systems are based on general purpose computers. Adding an ID for who changes data is difficult to reliably implement, especially in less computerised – purpose specific weighing system controllers. I suggest that unless OIML wants to consider making biometric identification a requirement, that the need for ID against who makes changes be dropped as a requirement. We are opposed to the requirement for identification.	A3,A4
3.4.2.2	26	This seems to be a clause much more applicable to static scales. Belt weigher accumulations soon dwarf totaliser zero reading. As much as possible, irrelevant static scale influence should be removed from the standard.	A2,A4
3.4.3.1	26	Automatic Scale interval change. Surely relevant to static scales only and should be removed from standard.	A2,A4
3.4.5	27	This requirement is contrary to the needs of many users. I would substitute, “Totalisation indicating and printing devices shall clearly indicate when they are not engaged” and “Delivery systems shall be interlocked in such a way that no delivery can take place whenever the Totalisation indicating and printing device is not engaged”.	A1,A3
3.4.6 a)	27	Clarification required. Does this mean that no totaliser indication may be reset to zero unless the flow rate is zero or the belt is stopped? The implementation of this requirement would be very complex indeed. It would be no longer possible to use simple totaliser devices as legal for trade dislays unless they were non-resettable. This clause would require complex gateing of the reset of remote displays with the run condition of the conveyor.	A4
3.5. and 3.6	27& 28	Would suggest that other standards be referred to rather than having their content included.	A5
3.9	29	Given the power of a laboratory based OIML type approval, more requiements should be placed on weigh frames. Many in-adequate weigh frames are type approved, they often require 'testing until they work' in the field.	A2,A3
3.10.2	29	Conveyor Belt – consistent weight. Add “this is not applicable when all material tests load readings are obtained over a whole number of belt revolutions” Read separate justification for this suggestion.	A1,A3
3.10.4	30	Weigh Length. Provision should be made for sealing because it and idler alignment are too easily adjustable by relatively unsophistocated means.	A4
3.10.5	30	Belt Tension. Restriction on tensioning device and arc of contact should be withdrawn as they un-necessarily restrict technological developments.	A1
3.10.6	30	See also 3.6 Should not the requirement be that there shall be an 'Out of Range' indication if any out of range flow has occurred during the delivery? Further to this, the US Handbook 44 requires a graphical record of flow rate during a delivery. This would seem to be the only way to have a meaningful record of the validity of a delivery. There should also be some consideration and guidence given to what to do if the flow rate falls outside the legal range.	A4
3.10.7	30	The use of a belt scale weigh carriage operating in Static Mode as the Control Instrument is not understood. I can forsee the possibility of belt scales which will be accepted as giving live load performance on the basis of dead weight calibration, however this may not be what is intended, I would like to have more information about this idea.	A1,A4
4.1.3	33	Durability. There is a general need for the introduction of durability tests into R50, this is a major deficiency at this time. The US, Handbook 44 and NTEP incorporate a durability or permanence test which could be looked at for possible incorporation into type testing, however, there is also a need for durability testing in relation to subsequent and regular re-verification of trade certified belt weighers. At this time there is no evidence gathered for a trade certified belt weigher application that it has remained within accuracy limits over the 'in service' period. Due to the minimal mechanical testing of weigh frames in-situ in type testing, an 'in service' durability testing requiement would be prudent and would enhance the respect which industry has for this class of device. I propose that, at the re-verification interval, the accuracy of the system be checked with at least 3 live load tests, one at 20% , one at 100% and one at 50% before the systyem is re-calibrated to see if it is within the required 'in-service tolerance. If it passes, it would not require Re-Verification, unless the interval had reached 2 years. If performance is good, the re-verification period could be doubled up to a maximum of two years, if it is not, the re-verification interval is halved down to a minimum of three (3) months. If it cannot hold 3 months it cannot be used for trade purposes. In this regimen, there would also be the opportunity to continue at a reduced accuracy class. This principal is further described later. This suggestion if accepted would cause the most significant and far reaching change in the standard of world belt weighing world wide. I request that this idea be seriously considered. Should the world have to tolerate trade measurements equipment which could not easily stay within Class specification for at least 3 months?	A1,A2, A3,A4
5.3.1	40	Sub-sequent Re-Verification. See section above. We suggest an 'in-service' durability test. We see this as revolutionising the standard and respect of belt weighing. Suppliers of reputable equipment would not want to suggest that their eqipment could not meet our suggested requirement. Users would benefit not only from better equipment, but when purchasing better more stable equipment would be re-warded by less required testing.	A1,A2 A3,A4
6.1	40	General Tests. Where there is variable speed and a range of materials they may be mixed in pairs of tests and if accuracy is achieved, system will not be required to be tested over the full range of speeds or materials. The principle would be that a choice such as this which should make the whole test method more difficult is admissible, because if the equipment can pass this more difficult test it is suitbale for the range of duties which have been incorporated. This is to make a place for very good weighing systems which really can weigh different materials at different speeds accurately.	A1,A3
6.2.1	41	Do not understand 'Integral' Control Instrument. I understand it in relation to train in motion scales, but not belt scales. can forsee the possibility of belt scales which will be accepted as giving live load performance on the basis of dead weight calibration, however, is this what is intended by this entry?	A1,A4
A3.7.2	44	This seems to be static scale technology. Applicabaility not understood. If it is for the Control Instrument it should say so, however, might it not be better to refer to the relevant static scale standard.	A4
A5.3.2 & A5.3.3	48	Intergal Control instrument, See earlier comments about same matter	A1
A5.3.5	51	Zero Stability. Provide guidence on the use of Automatic Zero Tracking during tests and how often and at what times the system may be manually zeroed.	A4
	60- 71	EMI testing. Should this be included in this standard or would it be better to refer to the other relevant standards?	A2,A4
A9.2.1	74	Integral Control Instrument. Is this really applicable? See earlier comment.	A1

 

  

Discussion, proposed changes

  

Acceptance of More Difficult Tests as Valid Tests under OIML R50.

The normal situation when a control instrument is 3 x more accurate than the instrument under test is used in such a way that all errors in test results are attributed to the instrument under test.

Since all errors are always be attributed to the instrument under test. This makes it safe to vary test procedures in ways that may make accurate results more difficult to obtain since all error will be attributed to the instrument under test anyway. This principle determines that if the instrument still manages to pass, then the equipment deserves to be cerified.

Admissable changes to procedures would be any which might make the test more difficult such as;

Mixing different test materials in test pairs, ie Wheat and Coal or Coal and Iron Ore. Such a test would be seen as a more difficult test for a belt weigher, however we propose that it be accepted as part of the five pairs of tests required to verify a belt weigher.

Mixing different belt speeds in test pairs would also be admissible under this principle.

 

Provision for Class 0.2 belt scales.

We suggest that provision be made for Class 0.2 Belt Scales. We have some units which operate at this level of accuracy. We agree that it will be difficult to find a suitable control instrument to certify a Class 0.2 Belt Scale, however, we think this problem can be solved by using statistical methods. We are suggesting that a Class 0.2 instrument might be used to verify another Class 0.2 instrument and this can be verified scientifically with sufficient test samples.

A scientific and statistically correct method of verifying a belt scale with a control instrument of the same precision i.e. Class 0.2 with Class 0.2.

The statistical theory which governs the error between two normal distributions may be used to verify a Class 0.2 instrument against a Class 0.2 instrument within a 95% or 99% confidence interval.

This test can be scientifically validated using the methods in Uncertainty in Measurement ISO Guide by Robin E. Bentley.

  

The significance of using a system whose totaliser updates only once per belt revolution.

 In the case of a belt weighing system whose totaliser updates only once per belt revolution the normal weighing errors which can result from the variation in the weight of a belt along its length can be eliminated.

There are two ways to achieve this objective, (i)Holding back the totaliser accumualtion and releasing it only as each belt revolution completes or (ii)Holding in memory an image of the weight of the conveyor belt and using this as a dynamic zero adjustment.

Holding back Totaliser Accumulation.

At zero, variations in belt weight should cause cause both negative and positive flow rates as the belt revolves. A correctly zeroed system uses a zero constant which represents the average weight of the conveyor belt. Totaliser results released at the same point on the belt each time (after whole revolutions) include no error from belt weight variation, the average of the belt weight and the average zero constant used have the same arithmetic affect.

To achieve this result, it is only necessary that enough weight is held back so that there is a reservoir available from which to count down weight when below average weight sections of belt pass over the weigh frame. There are two ways to do this, (1)hold back enough weight to provide a reservoir for negative flow rates at zero, or (2)hold back all totaliser update until the same point on the belt passes a given point every revolution.

In either case, the totaliser is only 'up to date' once each belt revolution.

The CST system has a user settable amount which is held back, it can be set to include all the material in a belt revolution, or it can be set to hold back a sufficient amount, for example 1 tonne. There is also a belt position counter on the screen which counts 00 to 99 and then to 00 again, the totaliser is updated when the counter rolls over from 99 to 00. OIML may require more elaborate measures to ensure that only 'up to date' totaliser figures are used in transactions.

Zero Image Method

When a belt Zero Image is employed, multiple records of the weight of the belt are kept in memory and are available synchronised with belt movement so that the weight of the belt over the weigh frame at any time is able to be zeroed off with the correctly stored weight of that piece of belt. The result is that there is little influence from belt weigh variation as the belt circulates, it is no longer necessary to update the totaliser once per belt revolution, in fact the totaliser can be updated at any time without influence from belt weight variation.

CST proposes that this method of zero compensation be accepted as fulflling the requirement that the totaliser update only once per belt revolution. This is a modern method reflecting what is available with new computer based technology and this type of feature should be accepted in trade use equipment to improve the serviceability of belt weighers.

Variation of indication at Zero Test. (MVZ)

The maximum variation at Zero Test is applicable if the Minimum Totalised Load (MTL or TL) is conducted over less than three belt revolutuions. The MVZ test is conducted over the duration of the test, ie the duration of the passage of the MTL or TL and limits the maximum variation of the indication of the totaliser to no more than 70% of the Maximum Permitted Error of the test. So, for a Class 0.5 instrument which is allowed an error of 0.25%, no more than 70% of this ie 0.18% is permitted to come from zero variation.

This test is designed to protect the purchaser or seller from errors in the measured delivery which result from belt weight variation by controlling this source of error to no more than 70% of the MPE.

In the case of systems which utilise a method of eliminating zero error in such tests by either (i) Updating the Totaliser (fully) only once per belt revolution or (ii) Keeping multiple zeros synchronised with the conveyor belt, the Maximum Variation at Zero Test is not necessary and we suggest is not applicable.

CST would proposed to make the case that the maximum variation at zero test is not applicable if 'Totaliser Update once per belt revolution' feature is available and in use. A MVZ test may be applied during pattern approval tests to check that system under test has an adequate method of implementing 'totaliser update only once per belt revolution', however, it should not be required for subsequent re-verification.

Another reason to drop this test is that it is not clearly understood how to implement this test on the various weighing products in the market.

  

Permanence provisions which could enhance the value of OIML R50, borrowing here from NTEP regulations.

CST propose that the addition of Permanence provisions would be a logical extension of the OIML R50 regulations. It should be noted that Permanence tests are a part of the US NTEP system. In the case of the field tests for the NTEP candidate, the test unit must pass the live load tests (3 tests at 0.25%) and then must pass the same tests again 6 months later without adjustment.

CST hold the view that OIML compliant equipment can be applied improperly in the field and there is little that can be done under the current regulations to ensure that a belt weigher remains within the accuracy requirements of the standard. We agree that the initial five pairs of tests are to some extent arduous, however, current regulations leave some capacity for 'testing until it works' so that a marginal system can be verified. It is likley that such systems which have been verified today may not pass in one week or one months time and it seems prudent to change the provisions of OIML R50 to encourage the sale of safely compliant equipment rather than barely compliant equipment.

CST propose that at the time of re-verification, usually 12 months, that limited live load tests be conducted to ensure that the belt weigher is still accurate. These test may consist of only 3 tests at approximately 20%, 50% and 100% of flow. If the equipment passes, then CST suggest that the re-verification time could be doubled to a maximum of 2 years however, if it fails, the re-verification time be halved to a minimum of 3 months and systems which cannot hold calibration for 3 months not be stamped.

CST suggests that this addition to the standard would close the testing loophole which exists due to the non-type testing of each live installation. Equipment whose design lends itself only to the most favourable belt weighing applications would not then seem to compare favourably with more substantial and more stable equipment which is more suitable to the task.

Author. L. Ian Burrell
Managing Director
Control Systems Technology Pty Ltd
Australia

 

Control Systems Technology Pty Ltd. holds the following belt weigher approvals:
USA NTEP 0.25% Belt Weigher Certificate 02-135
Canada MC 0.1% Belt Scale Approval AM-5612 2007
MC 0.5% Belt Scale Approval AM-5442 2007 (first approved 2002)
Australia 6/14D/13 Class 0.5 Belt Weigher 2003 (first approved 1996)
OIML Certificate of Conformity 2003 Class 0.5 Belt Scale R50/1997-AU-03.01
UK MID application in process

  

Motivations For Suggested Changes To OIML R 50-1

The following are the reasons why we believe that the changes to OIML R50 which we have suggested should be adopted.

Future Proofing OIML R50, permiting innovation

Some aspects of the existing OIML R50 standard, though prudent, are required only in the context of older technology. Suggestions around totaliser updating in whole belt revolutions and zero imaging for instance are much more viable with current technology. The improved technology makes it easier to apply trade certified belt weighers and so they are more likley to be applied in industry. We suggest that everything that is in the standard which is based purely on the limitations of current technology be removed from the standard. We suggest that Class 0.2 systems be allowed also as a way forward for the improvement of belt weighing technology.

OIML R50's very significant world influence

Many world economies do not have a strong legal framework for the trade use of belt weighing, however, the OIML R50 document is very widely available in these economies and is sometimes mis-used. As an example, in Chile 'trade certification' of belt weighers is carried out purely on the basis of static calibration, this is clearly not in accordance with the standard however, OIML R50 is used as the authority for the certification. Hence there is a world wide responsibility to make OIML R50 clear and un-ambiguos. For instance the references to the belt scale being used as the Control Instrument could be mis-construed. Also, the zero stability tests which I have pointed out represent 100% of the allowable error are a problem because, in the absense of live load testing, systems may be accepted whose zero stability alone is not good enough to permit certification.

Making Belt Weighers more Acceptable for Trade Use.

In many situations, belt weighers can do a better job than the available technologies, so any reasonable step which would enhance the acceptability of belt weighing should be taken. For example, ships draft survey and weighing in motion systems for trains can easily be eclipsed by a good belt weigher installation, if it can be certified and easily maintained. We would argue that if the technology can meet the task and it should not be restricted by a standard such as OIML R50 which may be couched in a context of older technology. One measure which really assists the application of belt weighers is the possibility of using a test load which is less than one belt revolution. CST has used this in a number of instances here in Australia.

Need for Clarification

Some of the comments and suggestions are based around the need for clarification of what is really meant by the clauses in R50. As a designer, manufacturer and maintainer of belt weighing equipment I look at each clause and try to work out what it means for us as a manufacturer and for the end user. If it is not clear it is likely to become a bone of contention for the industry, hence the need for clarification of R50 at this time of review.

References to other Standards

Some countries have recommended a revision with regard to EMC and software in order to keep pace with the developments of other OIML recommendations (R76-1) and corresponding developments in Europe (MID and WELMEC-Guides). We notice that parts of these standards have been included in the Working Draft. We think however that some of the issues in these standards are quite complex and that the incomplete importing of these standards is frought with problems for the future. We would prefer to just see references to and clarifications as to the applicability of these standards rather than the inclusion of some of their provisions by direct insertion. For example, some of the comments about software do not seem to seperate between special purpose electronics sets and general purpose computers used as a part of a belt weigher. For instance, the idea that the person who makes a change to a scale parameter be identified in the audit trail seems to imply a general purpose computer while our company does not have confidence in general purpose computers for belt weighing. The WELMEC Software Guide MID 2004/22/EC does distinguish between these two approaches, designating them as Type-P and Type U and it could be referred to in OIML R50-1 when it is helpful to do so.