Analysis of NTEP Proposal to the Weighing Sector, August 2014

Analysis of NTEP Proposal to the Weighing Sector, August 2014

By Henry Oppermann

Weights and Measures Consulting

Proposal discussed by the NTEP Weighing Sector:

S.5.4. Relationship of Load Cell Verification Interval Value to the Scale Division. – The relationship of the value for the load cell verification scale interval, vmin, to the scale division,d, for a specific scale installation using National Type Evaluation Program (NTEP) load cells shall comply with the following formulae where N is the number of load cells in the scale (such as hopper or vehicle scale weighing/load-receiving elements):

Note: When the scale installation contains two or more W/LREs where the output of each W/LRE produces its own independent weight display and is thus capable of operating as an independent NTEP certificated scale in a commercial application, the value of “N” should be the number of load cells in each individual W/LRE.

[The remainder of the paragraph is not changed.]

Justification Provided for the Proposal:

Two reasons were given to justify the proposed change.

(1) The change was offered to address two different interpretations of how the vmin calculation should be applied tovehicle scales with three platforms to provide weight indications for each platform for the axle weights and the total weight of the vehicle. These scales are often used for law enforcement of highway weight laws and as commercial scales in truck stops to allow truck drivers to check their gross weight and axle weights. The intent of the proposed change is to provide an exemption for these scales so that the vmin calculation is performed for each weighing element, rather than applied to the entire scale.

(2) The second justification given is that since vmin is a measure of the change at zero load as the temperature changes, scales that are equipped with an automatic zero-tracking mechanism will automatically correct for changes in the zero reference when the scale returns to zero. Consequently, it is argued that the vmin value is not really important and more flexibility should be allowed regarding the change in zero reference due to changes in temperature. Furthermore, since each weighing element in the three-platform vehicle scale has its own A/D converter and separate zero tracking, then vmin should be calculated based upon the number of load cells in each weighing element that has its own A/D converter and its own weight indication.

Analysis of the Proposal

Justification 1:

What problem are we trying to solve?The first justification for the proposal is that there are different interpretations of how the vmin calculation should be applied to a vehicle scale weighing system that consists of three independent weighing elements, each with its own A/D converter and weight indication. The fact is that the issue of interpretation was settled by a vote of the NCWM in 1990. The NTEP Weighing Sector discussed thisissue at two meetings in 1989, but the Weighing Sector could not come to agreement on an interpretation. The S&T Committee had provided its interpretation in January 1989, but many on the Weighing Sector were not willing to accept the S&T Committee. The S&T Committee interpretation was that “…a multiple-platform scale system is a single system and shall be treated as a single scale.”[1] Since the Weighing Sector did not agree to accept the interpretation of the S&T Committee, the S&T Committee put its interpretation up for a vote at the 1990 NCWM. In July 1990, the NCWM voted to adopt the recommendation of the S&T Committee. The last sentence in item 320-11 of the S&T report states, “The significance of this interpretation is that not only must each independent weighing device meet the requirements of Handbook 44, but the entire weighing system must meet all requirements that would apply if the device were a single scale.”[2] See the Appendix for the documentation of these discussions and the vote of the NCWM in 1990.

It was pointed out at the Weighing Sector meeting in August 2014 that the paragraph S.5.4., which states the relationship of the load cell verification interval value to the scale division, was not added to the Scales Code until 1993, Hence, it was argued that the vmin calculation was not considered when the interpretation was adopted in 1990 that the entire weighing system must comply with the requirements of Handbook 44, not just the individual weighing elements.

This is not the case, because the value of vmin was an important aspect of load cell performance throughout the time that NTEP was establishing the program to test load cells for influence factors. A more stringent requirement for the relationship of vmin to the scale division existed in the NTEP load cell policy in the 1988 checklist. The NTEP load cell testing procedures were described in the 1987 Report of the NCWM. In 1988, the NTEP Board of Governors adopted a change to the relationship of the load cell minimum verification scale division to the value of the verification scale division for the scale. Hence, the relationship was established in NTEP technical policy long before the requirement was added to Handbook 44.

Weights and measures officials were expected to verify the appropriateness of load cells for specific applications using the values for vmin. This is stated in the 1989 Report of the National Conference on page 149, where the location of the marking information should be on scales and load cells. The S&T report states:

“2. since weights and measures officials must do some disassembly to verify that the vmon rating of a load cell is appropriate for the scale application, then the identification information could be located in a protected area, but still conveniently available to the official when verifying the load cell information.”

The marking requirement for the value of vmin on weighing elements was adopted in 1987 and nonretroactive as of January 1, 1988.[3] Consequently, the relationship of the value of vmin to the value of the scale division was well established when the NCWM adopted in 1990 the S&T interpretation that “…not only must each independent weighing device meet the requirements of Handbook 44, but the entire weighing system must meet all requirements that would apply if the device were a single scale.”

The S&T Committee recommended that the relationship of vmin to the scale division be added to the Scales Code in 1993 “so it is readily available to enforcement officials.”[4]

Justification 2:

It was said at the August 2014 meeting of the Weighing Sector that the interpretation adopted by the NCWM in 1990 does not preclude submitting a proposal to change Handbook 44 to include an exemption for vehicle scales with three independent weighing elements, each with its own weight display and a total weight for the loads on all three platforms. However, the nature of the proposed exemption and the manner in which it is being developed are troubling. First of all, the proposed exemption is very narrow. Essentially, the exemption applies to vehicle scales with separate weighing elements that are used to obtain axle weights. If the rationale provided in Justification 2 is considered substantive, then why isn’t the proposal to eliminate the requirement for the relationship of vmin to the scale division for all scales that have automatic zero-tracking mechanisms? Why isn’t there an effort being made to change OIML Recommendations 60 and 76 to drop the relationship requirement or to request the same exemption internationally? Why not propose to exempt load cells with digital output also, since each one has its own A/D converter?

One troubling aspect of the proposed change is that the narrowness of the exemption appears to promote an exemption primarily to benefit one company for one model of load cell of one particular capacity (that doesn’t meet the relationship requirement) for one specific scale application. Why is the exemption requested when there are many NTEP load cells (with the capacity used in these scales) that satisfy the relationship of vmin to the scale division of the vehicle scale? Why did NTEP submit the proposed change to Handbook 44, rather than the company that would appear to benefit from the change?

A review of active NTEP Certificates of Conformance (CCs) for load cells (as of September 6, 2014) shows that 65 of 70 load cells (about 93%) with a capacity of 75,000lb are available and that satisfy the relationship of vmin to the scale division of the vehicle scale. Please see the chart at the right for a histogram of vmin values for 75,000lb capacity load cells covered by active NTEP CCs.

If one were to use larger load cells (as listed on active NTEP CCs with capacities in the range of 88,000 to 110,000lb, when the 75,000lb capacity is not available) in the scale, then there are still 24 out of 35 load cells (about 69%) available that satisfy the relationship of vmin to the scale division of the vehicle scale. Please see the chart below and to the right for a histogram of vmin values for load cells with these larger capacities that covered by active NTEP CCs.

Based on this analysis of the proposed change, there is insufficient justification to provide an exemption to the requirement regarding the relationship of vmin to the scale division of the vehicle scale, especially for only one type of vehicle scale. Even if the exemption were written to apply to all vehicle scales, there are many load cells that satisfy the relationship of vmin to the scale division of the vehicle scale so that there is no need for any exemption from this long-established requirement.

Furthermore, why should weights and measures officials deal with anarrow exemption to a requirement in Handbook 44 to accommodate this proposed change?

Commercial Applications

The suggested change discussed by the Weighing Sector references when a “…scale installation contains two or more W/LREs where the output of each W/LRE produces its own independent weight display and is thus capable of operating as an independent NTEP certificated scale in a commercial application…” The axle-load weight indications for each weighing/load-receiving element are not legal-for-trade weights values. Compliance with Handbook 44 should not be based upon not legal-for-trade weight indications. Furthermore, each separate weighing/load-receiving element cannot be used separately as an independent vehicle scale, since split-weighing is prohibited in commercial applications.

Conclusion

I am opposed to the proposed change to grant a very narrow exemption to the requirement that specified the relationship of vmin to the scale division of the vehicle scale. The need for the exemption has not been adequately justified. There is no need for the enforcement official to deal with an exemption that is so narrowly stated, when there are many load cells available that can be used and that satisfy the existing requirement in Handbook 44.

Appendix

MEETING SUMMARY

Technical Committee on National Type Evaluation

Weighing Industry Sector

June 13-14, 1989

VI. Multiple weighing elements interfaced with a single indicating element

a)Number of scale divisions allowed for the summed weight indication

b)Proper operation of the displays for individual weighing elements and the summed display

c)Clarify the S.4.3. requirement for indicating which weighing elements are included in the summed display

The number of vehicle scales consisting of three or more individual weighing elements used simultaneously to obtain a gross weight for commercial transactions are increasing. The individual weighing elements are used to obtain axle-load weights to determine compliance with highway laws and, because the entire vehicle is weighed as a single draft, the summed weight can be used for commercial transactions.

This approach to weighing has raised a question of how the number of scale divisions permitted in class III or IIIL scales should be applied. The Technical Committee was strongly divided on this issue. A number of weights and measures officials and industry representatives supported treating each weighing element as a separate scale and the summing of the indications from each weighing element is the same as summing the indications from separate scales.

Others believed that the use of multiple platforms in an application determines how it is classified. The argument was that the multiple platforms are being used in the same application as a class IIIL scale. If the platforms were not separate, the 10,000 division limit for class III L scales would apply. Consequently, a system consisting of multiple weighing elements must be considered as a single system and is limited to 10,000 divisions.

It was decided that this issue was not a type evaluation issue. The issue was referred to the Specifications and Tolerances Committee.

MEETING SUMMARY

Technical Committee on National Type Evaluation

Weighing Industry Sector

October 31 - November 1, 1989

IV.Multiple weighing devices interfaced with a single indicating element

This approach to weighing has raised a question of how the number of scale divisions permitted in class III or IIIL scales should be applied. For example, a vehicle scale may have three platforms with the following capacities: 100,000 x 20lb, 100,000 x 20lb, and 60,000 x 20lb. The sum of these scale capacities is 260,000lb for a total of 13,000 divisions. Should the maximum number of divisions for class III scales apply to the summed indication? An argument could be made that since the total weight is only the sum of digital indications, then the 10,000 division limitation should not apply. However, a request had been received to permit four 20,000-lb x 5-lb capacity scales be used in combination to provide a scale with a capacity of 80,000 x 5lb to essentially circumvent the class III L limit on the number of scale divisions. Consequently, the 10,000 division limit should apply to the summed indication because the summed indication is being used as the basis for commercial trade.

The S&T Committee briefly discussed this item at the 1989 NCWM. The Committee concluded that a multiple-platform scale system is a single system and shall be treated as a single scale. The S&T Committee requested that the Technical Committee continue to discuss this item to see if a consensus could be reached on the requirements that should apply.

There was a reluctance from several members of the Technical Committee to accept the position of the S&T Committee without further review. Consequently, the Technical Committee did not reach a consensus on this issue. The discussion is documented below to provide input to the S&T Committee to review the issue.

Discussion: Henry Oppermann reported the position of the S&T Committee. The S&T position is that three permanently installed axle-load scales used to obtain the gross weight of a truck from three axle-load scales is to be treated as a single system and that class IIIL requirements apply to the indicator that is totalizing the weights from the other weighing devices. This means that all of the sections in the three weighing devices must agree within the limit set by T.N.4.4.

Jim Truex distributed the Ohio memorandum stating the Ohio policy regarding these devices. The Ohio position is consistent with the criteria proposed for consideration by the Technical Committee.

Ross Andersen: The criterion "simultaneously displayed" is not sufficient to define which devices are to be treated as a single weighing system. For example, multiple sets of hopper scales, e.g., 1,000x 1lb and 10,000x10lb, are often used to batch asphalt and the individual weight indications could be added together to get an accurate weight. I agree that a multiple-weighing-device vehicle scale should be treated as a single system, but two or more independent hopper scales used to batch asphalt should not be considered a single system.

Joe Giannina: Why should the division values on multiple weighing devices be the same? If a system consists of independent weighing devices, then each device should be tested separately.

Henry Oppermann: The division values for all of the weighing devices should be the same, otherwise anyone testing the complete system would not know what tolerance to apply to the system if each weighing device has a different scale division.

Tom Stabler: Under the General Code, all of these weighing devices must have the same scale division value.

Terry James: It is the normal practice of weights and measures enforcement officials to test multiple-weighing-device systems as if each weighing device is an independent scale. Permitting each weighing device to have 10,000 divisions does not violate Handbook 44. Using a multiple-weighing-device scale and adding up the values to obtain a gross weight is no different than adding the weights on a calculator.