Republished with permission of Jim Truex
National Conference on Weights and Measures
2017 Issue 1
Jim Truex, NTEP Administrator
When reading an NTEP Certificate of Conformance (CC) we see the load cell used in a scale is identified as non-NTEP in some case. Many have inquired as to why NTEP allows the use of load cells that are not traceable to their own NTEP CC or “non-NTEP” cells.1
While NIST Handbook 44 does not specifically state that non-NTEP load cells are permitted for use in weighing instruments; Handbook 44 does contain several hints that not all load cells used in commercial scales are required to have an NTEP certificate.
- If we look at NIST Handbook 44, Scales Code, Marking Requirements, Table S.6.3.a., the heading of the fourth column reads “Load Cell with CC (11).” This column heading implies that if these marking requirements are for load cells with a Certificate of Conformance, there must be load cells without a Certificate of Conformance or non-NTEP load cells.
- Also in the Scales Code, paragraph S.6.3., reads in part: “Scales, main elements of scales when not contained in a single enclosure for the entire scale, load cells for which Certificates of Conformance (CC) have been issued under the National Type Evaluation Program (NTEP, and other equipment necessary to a weighing system.” The statement, “load cells for which Certificates of Conformance (CC) have been issued” is also implying there must be load cells without a Certificate of Conformance or non-NTEP load cells.
- Additionally, if we look at Scales Code paragraph S.5.4. we find the following stated: “This requirement does not apply to complete weighing/load-receiving elements or scales, which satisfy all the following criteria:”
- the complete weighing/load-receiving element or scale has been evaluated for compliance with T.N.8.1. Temperature under NTEP (meaning the complete weighing device, including load cell(s) has been tested in an environmental chamber);
- the complete weighing/load-receiving element or scale has received an NTEP Certificate of Conformance;”
Again, no clear statement, but the fact that the formula shown in S.5.4. does not apply if the specified conditions are met tells us that the use of load cells without a Certificate of Conformance or non-NTEP load cells is acceptable when the complete weighing/load-receiving element or complete scale has undergone and satisfied influence factor testing.
So, NIST Handbook 44 tells us that not all load cells have their NTEP certificate but does not tell us why. However, it does tell us that the complete scale (containing the load cell) or weighing element (containing the load cell) must be evaluated for compliance with T.N.8.
To understand more we must go to NCWM Publication 14, Weighing Devices, NTEP Technical Policy, Section B.1., which contains the following statement:
“The main elements and components (indicating elements and load cells) of scales with a capacity greater than 2000 lb. must be tested separately for compliance with the influence factor requirements.”2
Is there any penalty for using non-NTEP cells? Yes, in fact there is a potential penalty for the certificate holder. NCWM Publication 14, Technical Policy allows for the “Substitution of Load Cells in Scales” using a set of specific requirements to determine if the cells are metrologically equivalent. The first requirement in evaluating metrologically equivalent is that both load cells must have been evaluated separately and have a separate load cell Certificate of Conformance. Obviously, since a non-NTEP load cell was used in the cases we are addressing, it does not have an NTEP certificate and cannot be substituted. In these cases, the certificate holder must resubmit the device to NTEP for full performance testing to allow the use of another load cell. Reason being that NTEP does not have values for non-NTEP load cells, such as nmax or vmin to determine equivalency.
Now, we can use this information to piece together the facts pertaining to the use of non-NTEP cells used in complete scales and weighing/load-receiving elements.
- NIST Handbook 44 recognizes the use of load cells that do not have their own NTEP Certificate of Conformance.
- NCWM Publication 14 recognizes the use of non-NTEP load cells in scales 2000 lb. capacity and less.
- The scale or weighing element containing a non-NTEP load cell or load cells must be tested by NTEP for compliance to influence factor (T.N.8) requirements.
- If a non-NTEP load cell is used it is identified on the NTEP certificate and the load cell cannot be substituted, otherwise the device must be resubmitted to NTEP for a complete performance test using the different cell. When this scenario happens, an amended certificate identifying the non-NTEP cells used during the evaluations will be issued.
Hopefully this article has helped eliminate some of the confusion and answer questions that may have been on your mind.
1. This article was published about two years ago, but we have answered many recent inquiries from the weights and measures community confused about the use the term “non-NTEP” on some NTEP certificates. It appears to be especially confusing when some of the scale models use load cells traceable to NTEP certificate but others, on the same certificate, use non-NTEP certified cells that must be identified on the certificate because of the load cell substitution requirements specified in NCWM Publication 14, Technical Policy. All the scale models listed on an NTEP certificate are covered by the certificate or they would not be listed on the certificate. The “non-NTEP” designation applies to the load cells used in that particular model, version, capacity, etc.
2. Note: If we read further we will find some exceptions but most of those would result in a provisional NTEP certificate not a full NTEP certificate.