Figure 8: NIST HB44, Table N.3.2: N

Figure 8: NIST HB44, Table N.3.2: Number of sample weights per test (referred to in OIML documents as “test weighings”)
Special endurance tests for ACWs in the USA
Whereas for reasons of occupational safety OIML R 51
performs all influence factor tests in accordance with
Chapter 6.4.5 only up to a maximum load of 20 kg in
automatic operation (and permits static tests for loads
in excess of 20 kg in non-automatic operation), all tests
in the United States are performed with the entire
weighing range in automatic operation.
This comparative strictness is due to other factors as
well. In addition to HB44, ACWs in the United States
also have to fulfill the requirements of NCWM
Publication 14 (Pub 14) (see Figure 8). This publication
includes a permanence test, an uninterrupted 100-hour
endurance test in automatic operation without a single
stop of the conveyor system. After this test is concluded,
the accuracy of the ACW in automatic operation is
checked again. During these 100 hours, the conveying
speed is at least 80 % of the maximum speed. The
packages moved across the conveyor belt and weighed
dynamically have to weigh at least 75 % of the maximum
capacity (Max) of the ACW. Furthermore, no large gap
may be present between two successive packages,
forcing the ACW to have weight applied to it as
continuously as possible. In the case of fast ACWs,
several hundred thousand packages are weighed during
this test. Inferior conveyor systems which wear out
quickly will not survive this endurance test.