By Phil Carpentier, Todd Dietrich, Bill Ripka and Dale Poblenz
The National Institute of Standards and Technology (NIST) Handbook 44 (HB44) is a set of specifications, tolerances and other technical requirements for weighing and measuring devices. State Weights and Measures Officials use this document to test and approve commercial weighing devices; hopper scales, truck scales, track scales, belt-conveyor scales, etc. Changes were recently implemented to NIST HB44 for the belt-conveyor scale section 2.21.
In 2004, a major specification change to HB44 was adopted requiring a three point material test: one at the normal use capacity, another at 35% capacity, and a third at a point between the normal use capacity and 35%. The change eliminated the ability to place a belt-conveyor scale into commercial use via testing at a single flow rate.
This revised specification was incorporated following tests on existing commercial installations to see if a shorter material test was possible. Note: The tests were performed on installations designed to operate at a single flow rate thus tested and placed into commercial operation based on a single flow rate. The basis for the change was a minor non linearity found on some installations when operated at other than the normal flow rate.
The specification under question was printed as follows:
N.2. Conditions of Tests—A belt-conveyor scale shall be tested after it is installed on the conveyor system with which it is to be used and under such environmental conditions as may normally be expected. Each test shall be conducted with test loads no less than the minimum test load. (Amended 1986 and 2004)
N.2.1. Initial Verification—A belt-conveyor scale system shall be tested at the normal use flow rate, 35% of the maximum rated capacity, and an intermediate flow rate between these two points. The system may also be tested at any other rate of flow that may be used at the installation. (Added 2004)
The new requirement caused some users an unnecessary hardship and expense. Their systems had to be designed to material test at the three flow rates even though they would only operate at a single flow rate. To further complicate this new specification, some states required the performance of three material tests at each flow rate. Because the systems had to be modified for this once a year test, the tests often took several hours and, in some cases, more than one day.
Zeros do change over time and temperature and some states wouldn’t allow zero adjustment for the duration of the series of material tests. Some of the installations that took several hours or more than a day to complete failed the initial material test. Belt-conveyor scales are linear but, if the installation and material handling systems are not designed for a range of flow rates, the test results can be nonlinear.
For the past several years, NIST piggy backed a Belt-Conveyor Scale Working Group (BCSWG) meeting following the February National Weighing and Sampling Association (NW&SA) technical meeting in St. Louis. Many of the belt-conveyor scale service companies, manufacturers, consultants and users attend the NW&SA technical meetings.
During the 2008 BCSWG meeting, concerns were expressed with this issue and group members pointed out the International Standards for Belt-Conveyor Scale (OIML R-50) allows a single point material test on the initial test and also requires adjusting zero prior to each material test performed. The Weights & Measures Division (W&MD) of NIST is working with the international community trying to make the U.S. specifications and tolerances align with the international standards where possible.
The technical advisors from W&MD noted the concerns and brought them forward to the Specifications & Tolerances Committee (S&TC) of the National Conference on Weights and Measures (NCWM) via an amendment proposal, requesting an allowance for single point testing on systems verified to operate within 10% of a single flow rate at least 80% of operational time, and to add a requirement to check and adjust zero between each material test load. The proposal also clearly defined the number of tests to be conducted at each flow rate.
The HB-44 change proposal was forwarded to the four regional Weights and Measures Associations which geographically represent the country (Western, Central, Southern and Northeastern) for discussion and development. Anyone with an interest in a proposal is welcome to add, comment or make suggestions regarding the issue during open testimony at the regional conferences. The S&TCs for the regional associations discuss and summarize the testimony received for submittal back to the NCWM S&TC, recommending the issue move forward as a voting item, remain as a development