SEPTEMBER 2007 

Applying Correct Tolerances 

By Rick Harshman 

Consider the scenario in which a commercial weighing or measuring device has been in 

service for more than 30 days and is being routinely tested within 30 days of having its 

performance adjusted. Assuming this is not a follow-up test of equipment under official 

rejection, what tolerances (maintenance or acceptance) apply? This question is one 

which seems to raise a lot of controversy within the weights and measures community. 

Although it may seem reasonable to think that acceptance tolerances should apply, this is 

not what NIST Handbook 44 (HB 44) necessarily requires. However, a case can be made 

for the application of either tolerance. The question is “why.” 

According to HB 44 Fundamental Considerations, two sets of tolerances (acceptance and 

maintenance) were established to apply to classes of equipment on which the magnitude 

of the errors of value or performance may be expected to change as a result of use. 

Acceptance tolerances are applied to new or newly reconditioned or adjusted equipment 

and are usually one-half the value of maintenance tolerances. Maintenance tolerances 

provide an additional range of inaccuracy within which equipment can be approved on 

subsequent tests, thereby permitting a limited amount of deterioration before the 

equipment will be officially rejected for inaccuracy and before reconditioning or 

adjustment will be required. A single set of tolerances is typically established for 

equipment that maintains its original accuracy regardless of use, such as glass milk 

bottles or graduates. 

The following tolerances are taken directly from NIST Handbook 44: 

NIST Handbook 44 G-T.1. Acceptance Tolerances and G-T.2. Maintenance 

Tolerances 

G-T.1. Acceptance Tolerances. - Acceptance tolerances shall apply to: 

 (a) equipment to be put into commercial use for the first time; 

 (b) equipment that has been placed in commercial service within the preceding 30 

days and is being officially tested for the first time; 

 (c) equipment that has been returned to commercial service following official 

rejection for failure to conform to performance requirements and is being 

officially tested for the first time within 30 days after corrective service; 

 (d) equipment that is being officially tested for the first time within 30 days after 

major reconditioning or overhaul; and 

 (e) equipment undergoing type evaluation. 

 (Amended 1989) 

G-T.2. Maintenance Tolerances. - Maintenance tolerances shall apply to equipment 

in actual use, except as provided in G-T.1. 
The various conditions under which acceptance and maintenance tolerances apply to 

commercial equipment are specified in HB 44 General Code paragraphs G-T.1. 

Acceptance Tolerances and G-T.2. Maintenance Tolerances. Paragraph G-T.1. specifies 

five different conditions under which acceptance tolerances apply. Note that only two of 

those conditions apply to equipment that has been in service for more than 30 days. The 

first condition—G-T.1.(c)—applies to equipment that has been returned to service 

following official rejection and is being tested within 30 days after corrective service. 

The second—G-T.1.(d)—applies to equipment that is being tested for the first time 

within 30 days after major reconditioning or overhaul. Paragraph G-T.2. specifies that 

maintenance tolerances shall apply to equipment in actual use, except as provided in 

G-T.1. 

Why wouldn’t HB 44 require the application of more stringent tolerances (i.e., 

acceptance tolerances) to commercial devices that have recently been adjusted or 

calibrated even when those devices had been in service for more than 30 days? A likely 

reason is that doing so would unfairly penalize that group of device owners who were 

diligent about maintaining the accuracy of their equipment. For example, device owners 

who were conscientious about having the accuracy of their devices properly maintained 

would suffer the unfortunate consequence of having officials apply acceptance tolerances 

to those devices (providing testing occurred soon after adjustments were completed). 

Device owners who were less diligent about maintaining the accuracy of their equipment 

would benefit because maintenance tolerance would continue to apply to that equipment. 

Thus, different tolerances could apply based upon how one elected to maintain their 

equipment. In summary, an unfair advantage would be created for those less diligent 

about maintaining the accuracy of their equipment and an improper message would be 

conveyed. 

Another specification comes into play when considering the General Code user 

requirement of HB 44. G-UR.4.3. Use of Adjustments reads: 

 

Does this mean that Handbook 44 requires the application of maintenance tolerances 

when conducting routine testing of devices that have recently been adjusted and have 

been in service for more than 30 days? The correct answer is “not necessarily.” 

Although G-T.1. and G-T.2. would require maintenance tolerance to be applied to such 

devices, General Code paragraph G-UR.4.3. Use of Adjustments, grants officials the 

G-UR.4.3. Use of Adjustments. – Weighing elements and measuring elements 

that are adjustable shall be adjusted only to correct those conditions that such 

elements are designed to control, and shall not be adjusted to compensate for 

defective or abnormal installation or accessories or for badly worn or otherwise 

defective parts of the assembly. Any faulty installation conditions shall be 

corrected, and any defective parts shall be renewed or suitably repaired, before 

adjustments are undertaken. Whenever equipment is adjusted, the adjustments 

shall be so made as to bring performance errors as close as practicable to zero 

value.
discretion of setting the defining limits of how close to zero error devices are to be 

adjusted. This paragraph specifies that whenever equipment is adjusted, the adjustments 

shall be so made as to bring performance errors as close as practicable to zero value. 

Thus, on one hand, HB 44 requires maintenance tolerances to be applied when routine 

tests are conducted on devices that have recently been adjusted and have been in service 

for more than 30 days. On the other hand, whenever these (or any other) devices are 

adjusted, officials are granted the authority to specify how close to zero value the 

performance errors are to be set, providing the degree of accuracy required is practicable. 

Thus, if an official deems it is reasonable for a particular device that has been in service 

for more than 30 days to be adjusted to within acceptance tolerances or better, the official 

may require, in accordance with HB 44, that when that device is adjusted, it be adjusted 

to within those limits. Paragraph G-UR.4.3. prevents a device owner from taking 

advantage of the tolerances by selectively adjusting a device to within a limit of his/her 

own choosing of the permissible tolerance. For example, if you find a device that has 

recently been adjusted and that is barely within maintenance tolerances, this might 

suggest the device owner is making a minimum effort to make the adjustments, and 

therefore, taking advantage of the tolerances. 

Note: Given the amount of liberty granted officials in setting the defining limits of such 

performance adjustments, some weights and measures administrators have developed 

guidelines outlining the tolerances to be applied when testing recently adjusted devices 

that have been in service for more than 30 days. These guidelines often differ depending 

upon the type of device being tested and the number of days that have elapsed since 

adjustment occurred. These guidelines may also include instructions detailing the actions 

to be taken by field officials when security seals have been removed from an adjusting 

mechanism or audit trail information has been altered without authorization. 

Thus, when trying to determine the appropriate tolerance to apply following an 

adjustment to the performance of a device, one must consider not only whether 

acceptance or maintenance tolerances apply, but also what adjustments are reasonable 

(i.e., practicable) for the particular device being tested. It is only after consideration of 

each of these points that the proper tolerances can be established and applied. 

For additional information regarding the correct application of HB 44 tolerances, please 

contact Rick Harshman at 301-975-8107 or by email at richard.harshman@nist.gov.