USNWG on Belt-Conveyor Scales
Agenda
February 23-24, 2011
St. Louis, MO.
USNWG Belt-Conveyor Scales
Agenda
February 23-24, 2011
I. Carry-over items
a.
Recommendation: Add new paragraph S.1.6.1. to HB 44
Section 2.21. as shown:
S.1.6.1. Zero-load indicator. – The integrator shall display an indication that
defines a zero-balance condition when the unloaded condition of the belt
over a unit revolution or revolutions is within ± 0.12 % of the rated scale
capacity.
(Nonretroactive as of January 1, 2011)
(Added 201X)
During the 2008 USNWG BCS meeting a proposal was initiated in which a BCS
system would be required to include a means to indicate that the system is “ready” to
operate. This “ready” state would represent a stable zero condition before a
totalization operation would begin. This indication is similar to the “center of zero”
indication on many digital static-weigh scales.
The members of the USNWG initially supported the proposal as written in the S&T
Committee's recommendation. Some members of the WG questioned what would be
required of the operator when the device indicated an out-of-zero balance condition
prior to the weighing process, and they expressed their concerns about requiring the
operator to maintain the device within limits of an established zero reference. The
need for an associated user requirement to be developed was discussed that would
require that zero-balance condition be maintained during operation.
Al Page suggested that HB 44 BCS Code needs to require that these devices start at a
well defined zero at the beginning of the transaction. Steve Cook suggested that it
may be possible to meet this need by requiring a printed record of zero test results at
the beginning and ending of each transaction.
John Barton offered the draft proposal for a User Requirement seen below to be
considered by the group.
USNWG on Belt-Conveyor Scales
February 23-24, 2011
St. Louis, MO.
USNWG on Belt-Conveyor Scales
Agenda
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St. Louis, MO.
UR.3.5. Maintenance of No Load Zero – The zero-load balance condition
shall be maintained so that prior to any totalization of material, a zerobalance condition such as described in S.1.6.1. is indicated. Immediately
following any indication that a zero-load balance condition has exceeded the
limits described in paragraph S.1.6.1, the operator shall perform an
inspection of the belt-conveyor system to determine the cause of any
significant zero shift that has occurred. Any condition within the beltconveyor system that is determined to result in a shift of the zero load
indication shall be corrected prior to resuming the totalizing operation of
the belt-conveyor scale.
There was no consensus regarding the proposed addition of this item at the
conclusion of the USNWG 2009 meeting and the WG agreed that additional work on
the development of this item is needed. A sub group was formed and agreed to
review and develop the above draft and to continue work on this item. The sub
group which includes Peter Sirrico (Thayer Scale), Bill Ripka (Thermo Scientific),
James Hale (Southern Co.), Richard Harshman (NIST), Todd Dietrich (Kaskaskia
Scale), and Al Page (Montana Weights. and Measures – Ret.) agreed to further
develop this proposal by communicating through teleconference and email prior to
the USNWG meeting in February 2010.
The sub group is to consider what limits on the scale’s excursion from the zero
setting would require a rezeroing of the device. Phil Carpentier and Ian Burrell
expressed their wishes to be included in the continuing work of the sub group.
Discussions within the sub group took place through email and teleconference
following the 2010 USNWG meeting. At this time, prior to the 2011 USNWG
meeting, the sub group has not yet come to any consensus to the content in this draft
proposal and has not at this point developed language for consideration by the entire
WG. There are still key details of the development of any recommendation that are
to be decided upon, including whether or not this proposal should be abandoned
altogether.
Several points are still being debated including:
- What is the appropriate value of the allowable excursion from a zero condition
(+/- 0.12%)
- Should the allowable excursion from zero be a constant value or should there be a
value for the initial start of a “shift” and permit a larger value for use between
runs?
- What type of alarm or indication is needed?
o An audible or visible indication that is triggered upon each excursion from
the allowable zero deviation
o A record of each totalization where the time and duration of excursions
from the allowable tolerance are indicated
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Agenda
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- If a specification requirement is adopted, then is an associated User Requirement
a necessity?
- What action(s) is required of an operator when an out of zero conditions is
indicated?
- How are systems equipped with belt profiling software to be addressed?
Traditional zero verification is properly done over complete revolutions of the
belt.
- If a BCS system is equipped with belt profiling or mapping, is the need for
checking zero using complete revolutions of the belt obsolete?
USNWG member Paul Chase has provided an analysis of the item which is included
as an addendum to this agenda.
b.
Linearity in BCS performance:
Recent changes to NIST Handbook 44 Belt-Conveyor Scale Code requirements
N.2. Conditions of Tests, N.2.1. Initial Verification, and N.3.2. Material Tests have
prompted consideration of the ability of belt-conveyor scales to produce
weighing/totalization results that are linear throughout the range of the BCS
capacity.
The changes to the Belt-Conveyor Scale Code have reduced the required number of
test runs during an initial test. Test procedures now require a minimum of two test
runs, or a total of six test runs required if three flow rates are verified during the
initial verification. However in the case where only one flow rate is tested, a
minimum of four test runs are needed at that one flow rate.
The justification behind the change was that the majority of comments and
discussion heard during the proposal of these changes favored the exception since
the time and cost involved in performing all tests could not be justified when
performing the tests on systems normally operating at only a single flow rate.
Some members BCS USNWG have expressed concern that this exception now
permitted from minimum required number of tests will not provide sufficient testing
to verify a linear performance. This is a greater concern when testing devices that
have relatively smaller minimum test loads where the time during ramping up and
ramping down of the flow of material on the belt-conveyor occupies a larger
percentage of the totalization operation in this type of belt-conveyor scale system.
Additional discussions within the USNWG indicated that future work may be
needed to develop linearity testing for devices to compensate for the effect that
different flow rates while ramping up and ramping down may have with small
minimum totalized loads. Some of the issues related to linearization that require
deliberation are:
USNWG on Belt-Conveyor Scales
February 23-24, 2011
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USNWG on Belt-Conveyor Scales
Agenda
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St. Louis, MO.
- It has been reported that some regulatory jurisdictions do not permit a
linearization feature in a BCS device. What is the basis for the rejection of the
use of this feature by a field official?
- In what way is this feature identified so that its use is apparent to a regulatory
official?
- Is there a need to require an indication on a device that will indicate when
linearization is in use?
- How significant or to what degree will the ramping-up and ramping-down periods
of a belt affect the accuracy of totalization if the BCS does not hold its accuracy
in a linear fashion?
- Because linearity can be enhanced through software used in the system, it seems
there is a potential for certain significant physical and mechanical features
causing non-linear performance to be masked. What if any, are the specific
characteristics related to the BCS installation or mechanical components that may
deserve additional scrutiny?
- If this feature’s use is supported, how is its correct operation to be verified? Will
material testing at different flow rates be required on systems using a linearization
feature? The need for testing procedures to include in the Belt-Conveyor Scale
EPO and NCWM Pub 14 exists and the development of those procedures should
be undertaken by both the USNWG on BCS and the NTETC BCS Sector WG.
Bill Ripka has offered the following draft requirement that would explicitly permit
the use of a linearization feature.
N.x.x Linearization
If a Belt Conveyor Scale meets certification test requirements for initial stable
zero and is found to repeat at normal use flow rate within +/- 0.06% (1/4
tolerance), and if the system is within +/- 0.37% (1.5 times allowable tolerance)
at other required test flow rates, the calibration may be electronically linearized
to bring the full range of operational flow rates within acceptance tolerance.
Linearization may also be used to adjust belt conveyor scale systems, that meet
acceptance tolerance at all flow rates, to a more precise measurement condition
(i.e. reduce the weighing error to a value closer to 0.0%).
II. New business
a.
Development of a list of definitions and terminology
Within the HB 44 Belt-Conveyor Scale Code and other related industry documents,
terms and phrases are repeatedly used however these terms and phrases may have
different meanings or may not be used at all outside of the belt-conveyor scale
community. It has been suggested by some USNWG members that there is a need
USNWG on Belt-Conveyor Scales
February 23-24, 2011
St. Louis, MO.
USNWG on Belt-Conveyor Scales
Agenda
February 23-24, 2011
St. Louis, MO.
for a document listing these terms and phrases along with the definitions associated
with their use in the context of belt-conveyor scales.
Some terms to include may be:
- Ramp-up, ramp-down
- Belt profiling/mapping
- Flowrate
- Totalization
- Skirting
- etc.
b.
Belt Profiling or Mapping
The use of a belt-mapping or profiling to establish a zero reference for defined
lengths of the belt on belt-conveyor scale systems has entered into several interrelated topics considered by the USNWG. Current HB44 and OIML R50
requirements were developed for systems that average the weight of belt units by
continuously weighing the belt as it passes over the scale portion of the conveyor.
The belt-profiling feature establishes a zero reference for the belt-conveyor scale by
establishing a profile of the belt carcass where the weight of individual segmented
units of the belt are established and have distinct weight values associated with them.
The instrument synchronizes the application of a tare weight associated with distinct
segments of the belt with the movement of the belt segments over the scale portion
of the conveyor. If profiling is used there is a need to establish a procedure to
evaluate its effectiveness and ensure that it functions as intended an is maintained
during operation of the BCS. Evaluation should include verification of this feature’s
ability to track and compensate for changes in the belt carcass due to influences such
as changes in temperature and loading conditions. Procedure(s) need to be
developed to verify its operation during type evaluations. Points to be considered
under this item are:
- Are weights and measures jurisdictions prohibiting the use of belt-mapping or
profiling?
- Do regulatory officials verify the use of this type of feature? When in use, what
would make it obvious to a field inspector that the zero condition is being
maintained by profiling?
- If there is no indication that belt profiling is in use when it is in operation, does its
use advance the need for a requirement in HB44 such as the proposed S.1.3.1.
Zero Indication under item I.A. in this agenda?
- A significant variation in the consistency of a longer belt will cause an
instantaneous variation in the zero condition but introduce less of an adverse
effect to the totalized load then that of a shorter belt (smaller minimum totalized
loads) with significant variation in the consistency of the belt. On the other hand
USNWG on Belt-Conveyor Scales
February 23-24, 2011
St. Louis, MO.
USNWG on Belt-Conveyor Scales
Agenda
February 23-24, 2011
St. Louis, MO.
a longer belt will undergo larger changes in its characteristics (i.e., elasticity)
when subject to a temperature change. Should this feature be used on any belt or
will the size and construction of the belt dictate whether or not its use is justified?
- When a BCS is equipped with profiling there should be some method of
verification that this zero maintenance is performing as designed. What testing
procedures can be developed to verify its intended operation?
c.
Reference scales for use during material testing.
HB44 Section 2.21 (BCS Code) direct reference to scales used as control devices (or
reference scales) is limited to the timing of the test performed on the reference scale
used during a material test. However this HB44 section does contain a requirement
specifically directed to the accuracy of the material used in a material test as shown
below.
N.3.2.1.
Accuracy of Material. – The quantity of material used to
conduct a material test shall be weighed on a reference scale to an accuracy
within 0.1 %. Scales typically used for this purpose include Class III and III L
scales or a scale without a class designation as described in Handbook 44,
Section 2.20., Table T.1.1. Tolerances for Unmarked Scales.
(Added 1989) (Amended 1991, 1993, 1998 and 2000)
Appendix A in HB44 provides additional information for criteria regarding test
standards as shown below.
Tolerances for Standards. – Except for work of relatively
high precision, it is recommended that the accuracy of standards used in testing
commercial weighing and measuring equipment be established and maintained so
that the use of corrections is not necessary. When the standard is used without
correction, its combined error and uncertainty must be less than one-third of the
applicable device tolerance.
3.2.
Device testing is complicated to some degree when corrections to standards are
applied. When using a correction for a standard, the uncertainty associated with
the corrected value must be less than one-third of the applicable device tolerance.
The reason for this requirement is to give the device being tested as nearly as
practicable the full benefit of its own tolerance.
USNWG on Belt-Conveyor Scales
February 23-24, 2011
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USNWG on Belt-Conveyor Scales
Agenda
February 23-24, 2011
St. Louis, MO.
From these HB44 excerpts it is shown that reference scales must:
- have a verifiable precision that is within 1/3 the value of the tolerance applied to
the device under test, and;
- prior to being used as a control device, must have their accuracy verified to within
0.1% (i.e. acceptance tolerance).
Some inferences can be made to the nature of appropriate reference scale features
and configuration; however the need for specific requirements regarding features
such as minimum d, nominal capacity, and maximum number of divisions (nmax),
ability to perform single draft weighing are not currently part of U.S. requirements.
In addition to requirements addressing these specifications, is there a need for testing
procedures to ensure repeatability and stability (i.e. repeat test of reference scale
following the materials test)? These considerations may be warranted if a reference
scale is an integral part of a sales contract for belt-conveyor scales or other in-motion
weighing devices.
d.
Development of a training course for the test and inspection of
belt-conveyor scales.
The draft Field Reference Manual is a technical guide that has been developed and
targeted for use primarily by field regulatory officials. It is intended that this manual
will provide uniform procedures and practices to be used in the examination of beltconveyor scales. The manual contains detailed information that may also be of
interest to manufacturers and users of belt-conveyor scales.
The draft will undergo further changes in its organization and structure so that the
information will be better adapted to present to classes or groups. Reviews are
solicited to analyze and comment on the content. It is anticipated that NIST will be
prepared to conduct this training in the near future and recommendations for
locations and venues are also being sought.
e.
OIML R50 Update
The international standard OIML R50 Continuous totalizing automatic weighing
instruments (belt weighers) is now in the form of its 3rd Committee Draft and is
undergoing further revision. Review and comment on this draft revision took place
in July 2010. The U.S. responded with numerous comments regarding the technical
content which are under consideration by the secretariat and presumably will be part
of the discussion at an upcoming meeting of the Technical Committee (TC9/SC2) in
April 2011.
Two topics that have received a great deal of scrutiny and have been the topic of
considerable debate are:
USNWG on Belt-Conveyor Scales
February 23-24, 2011
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USNWG on Belt-Conveyor Scales
Agenda
February 23-24, 2011
St. Louis, MO.
- A newly proposed 0.2 accuracy class; and
- Required permanence testing of belt-conveyor scales
As mentioned in item II. C. (Reference Scales) above, the introduction of a 0.2
accuracy class may influence the further development of U.S. standards.
The proposal of in-situ permanence testing has met with opposition from several
member states within the Technical Committee. The notion is not being readily
accepted and its introduction into R50 seems to be in doubt.
Feedback is solicited from USNWG members so that a formal U.S. position can be
decided upon prior to the TC9/SC2 meeting in April.
III. Annex A
Analysis provided by USNWG member Paul Chase of item I.A. above regarding
proposal to add new paragraph S.1.6.1. to NIST HB44.
CHASE TECHNOLOGY INC
Crosby, MN 56441
Phone 218-545-2356
mjc@crosbyironton.net
7 December 2010
“Center of Zero”
Belt conveyor scales
The USNWG for belt-conveyor scales has discussed the concept of a zero
indicator similar to the center-of-zero provisions of the scale code. Following
are some thoughts on such a provision for belt-conveyor scales.
Reference to static scales
Scales such as truck scales and platform scales have a center-of-zero
provision as defined in HB44[2.20]
S.1.1.1.
Digital Indicating Elements.
(a) A digital zero indication shall represent a balance condition that is within
± ½ the value of the scale division.
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USNWG on Belt-Conveyor Scales
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St. Louis, MO.
(b) A digital indicating device shall either automatically maintain a “center-ofzero” condition to ± ¼ scale division or less, or have an auxiliary or
supplemental “center-of-zero” indicator that defines a zero-balance
condition to ± ¼ of a scale division or less. A “center-of-zero” indication
may operate when zero is indicated for gross and/or net mode(s).
[Nonretroactive as of January 1, 1993]
(Amended 1992 and 2008)
Center-of-zero for these scales provides that the scale will always have a
precise and accurate zero condition.
Belt-conveyor scales
Zero balance for a belt-conveyor scale is somewhat more complicated. The
tare weight of the belt varies along the length of the belt during a revolution
of the belt. As a result any zero measurement that includes less than a
complete revolution of the belt is really incomplete.
S.3.1 requires that a zero operation be conducted over a complete revolution
of the belt.
S.3.1.
Design of Zero-Setting Mechanism. – Automatic and semiautomatic
zero-setting mechanisms shall be so constructed that the resetting operation is carried
out only after a whole number of belt revolutions and the completion of the setting or the
whole operation is indicated. An audio or visual signal shall be given when the
automatic and semiautomatic zero-setting mechanisms reach the limit of adjustment of
the zero-setting mechanism.*
(Amended 1999 and 2002)
N.3.1.1 further requires that the zero operation be conducted over a
minimum of three belt revolutions for electronic integrators.
N.3.1.1. Determination of Zero. – A “zero-load test” is a determination of the
error in zero, expressed as an internal reference, a percentage of the full-scale
capacity, or a change in a totalized load over a whole number of complete belt
revolutions. For belt-conveyor scales with electronic integrators, the test must be
performed over a period of at least 3 minutes and with a whole number of complete
belt revolutions. For belt-conveyor scales with mechanical integrators, the test shall
be performed with no less than three complete revolutions or 10 minutes of
operation, whichever is greater.
(Added 2002)
N.3.1.2 defines zero balance as acceptable when 3 successive tests are all
within 0.06% of scale capacity
N.3.1.2. Test of Zero Stability. – The conveyor system shall be operated to warm
up the belt and the belt scale shall be zero adjusted as required. A series of zeroload tests shall be carried out immediately before conducting the simulated load or
USNWG on Belt-Conveyor Scales
February 23-24, 2011
St. Louis, MO.
USNWG on Belt-Conveyor Scales
Agenda
February 23-24, 2011
St. Louis, MO.
materials test until the three consecutive zero-load tests each indicate an error which
does not exceed  0.06 % of the totalized load at full scale capacity for the duration
of the test. No adjustments can be made during the three consecutive zero-load test
readings.
(Added 2002) (Amended 2004 and 2009)
A static scale is in a no-load condition under normal circumstances. The
scale therefore has adequate opportunity to achieve an accurate zero. In
most applications a belt-conveyor scale operates under load for an extended
time period ranging from hours to perhaps days. During these operating
periods there is often not enough belt-empty time to complete a three
revolution zero test. There may be some short periods of no-load that would
allow an approximation of a zero test.
The requirements for a belt-conveyor scale zero test are currently included in
HB44[2.21] and any “center-of-zero” provision would need to be compatible
with the current regulation. It may be reasonable to consider some sort of
warning or alarm when the zero appears to be significantly deficient. To use
this approach some value at which to provide a warning or alarm must be
established. Any zero test performed over a portion of a belt revolution does
not have the precision or reliability of a full zero balance test so a wider
range of acceptable variation is appropriate.
One method to use as a basis for the amount of allowable zero error is to
base it on HB44[2.21] N.3.1.3. As currently written N.3.1.3 allows an
excursion of 3 divisions during a revolution of the belt. It is difficult to relate 3
divisions to some short term variation of zero, but a starting point for
discussion is to provide a warning if a short term empty-belt test accumulated
more than 3 divisions.
There has been discussion of a system that stores a “map” of the belt weight
at increments along the belt. If this system were in use it would be possible
to define a zero warning alarm as a percent of full scale belt load over some
period such as 20 seconds or 30 seconds.
There is rationale in HB44[2.21]T.1.1 to define an acceptable zero-balance
condition during testing.
T.1.1.
Tolerance Values – Test of Zero Stability. – Immediately after material
has been weighed over the belt-conveyor scale during the conduct of any material test
run, the zero-load test shall be repeated. The change in the accumulated or subtracted
weight during the zero-load test shall not exceed 0.12 % of the totalized load at full scale
capacity for the duration of that test. If the range of zero adjustments during a complete
(official) verification test exceeds 0.18 % of the totalized load at full scale capacity for
the duration of the zero-load test, the official with statutory authority may establish an
interval for zero-load testing during normal operation.
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(Added 2004) (Amended 2009)
This requirement relates to testing, but does not address normal use of the
belt-conveyor scale. S.3.1 makes reference to “Except for systems that record
the zero load reference at the beginning and end of a delivery…” and acknowledges
that some scales perform a zero test before and after a delivery. It is not
stated as a requirement.
A possible approach for a zero-balance warning
Following is a first-pass attempt to combine the information included above
to develop a method of zero-balance warning for belt-conveyor scales. The
intent is to list several items for discussion with users, manufacturers, and
regulatory personnel.
1. Define the criteria used to establish that the belt is empty and an autozero procedure can begin. This is currently undefined in HB44. In the
past some manufacturers have used a delay time after the load has
decreased below 0.x% of capacity as a start time for an auto-zero
procedure. Because this procedure provides a warning, not an actual
change in zero, the start time should be as short as possible.
2. Establish some minimum time to “look” at zero before publishing a
warning.
3. Establish some zero-balance limit above which the warning will be
issued. Referring to T.1.1 perhaps 0.18% of full scale is appropriate.
4. Add a User Requirement to define what the user should do when such an
alarm (or alarms) occurs.
Summary:
The above list of items would seem to overly define and restrict manufacture
of belt-conveyor scale electronics. Whatever definition would be used for
items 1-4 above, there is most probably some other definition that would
provide an adequate warning of zero unbalance. I recommend that the
USNWG abandon the “center-of-zero” requirement.
I see nothing in HB44 currently that would prevent a manufacturer from
providing a warning similar to that discussed above. It appears difficult to
define the concept adequately to make it a requirement.
As is apparent in the earlier discussion belt-conveyor scales are uniquely
vulnerable to zero balance problems. Because of the difficulty in defining a
zero-balance warning system, it may be easier to alert the user to the need
for closer attention to zero balance.
The language of T.1.1 applies only during testing. Perhaps a new user
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February 23-24, 2011
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USNWG on Belt-Conveyor Scales
Agenda
February 23-24, 2011
St. Louis, MO.
requirement similar to T.1.1 could define a shortened zero test (one
revolution) to be conducted at the end of a weighing operation or after 24
hours, whichever came first. If that value exceeded 0.18% some additional
testing and/or maintenance could be required.
Best regards,
Paul W Chase
USNWG on Belt-Conveyor Scales
February 23-24, 2011
St. Louis, MO.