**Chapter 4**

###  Pretest Determinations

#### Objectives

In this chapter you will learn how to determine the specific tolerances
to apply to a scale to be tested. You will study tolerance structure for
vehicle and axle-load scales and learn how to apply the tolerances to
specific scales. After studying this chapter, you should be able to:

1\. Complete a tolerance worksheet for a vehicle or axle-load scale.

2\. Determine the maximum test load to be applied during the test.

3\. Apply tolerances for tests to be conducted in the Test portion of
the field examination. These are:

-   decreasing-load test

-   increasing-load test

-   repeatability

-   shift test

-   substitution test

-   strain-load test

-   zero-load balance change

4\. Explain the meaning of the following terms:

> acceptance tolerance
>
> decreasing-load test
>
> discrimination (of an automatic-indicating scale)
>
> increasing-load test
>
> maintenance tolerance
>
> sensitivity requirement (SR)
>
> repeatability
>
> shift test
>
> strain-load test
>
> substitution test
>
> tolerance
>
> These terms are printed in **bold type** when first introduced.
> Complete definitions are listed alphabetically in Appendix A.

#### General Considerations

A major part of an official field examination is the test of the device
with field standard test weights to determine its performance in use.

A scale that would always weigh without error would be ideal. However,
since perfection cannot be attained, regulations are established that
set a limit on the amount of error that will be allowed. These
allowances for error are called **tolerances**. Tolerances fix the legal
limits of inaccuracy for a weighing or measuring device.

+-----------------------------------------------------------------------+
| **tolerance. -** A value fixing the limit of allowable error or       |
| departure from true performance or value.                             |
+-----------------------------------------------------------------------+
| # NIST Handbook 44, Appendix D, Definition of "Tolerance"             |
+-----------------------------------------------------------------------+

The tolerances that are established are based on several criteria, the
most important of which are:

-   The tolerances should not be so large that an allowable error would
    cause financial harm to either the buyer or the seller in a
    transaction.

-   The tolerances should not be so small that the cost of manufacturing
    and maintaining the scale is unreasonable. Note that these costs are
    usually passed on to the final purchasers of finished commodities.

Scale errors are expressed in terms of the difference between the
indicated (and/or recorded) weight and the actual weight of the load on
the scale. Scale errors are expressed in terms of registration: that is,
the actual weight is subtracted from the indicated weight to determine
the error value. Depending upon whether the difference is a positive or
negative value, errors are designated as either **overregistration** or
**underregistration**.

> [Overregistration]{.underline} means that the weight value indicated
> by the scale is more than the real weight. With a 5000-pound test
> weight on the scale platform, a display of 5020 lb is an example of
> overregistration. The error is +20 lb.
>
> [Underregistration]{.underline} means that the weight value indicated
> is less than the real weight. With a 5000-pound test weight on the
> scale platform, a display of only 4980 lb is an example of
> underregistration. The error is -20 lb.

According to Handbook 44, all tolerances for scales apply equally to
overregistration and to underregistration. For example, if the
applicable tolerance value for a given test is 20 lb and the test load
is 20,000 lb, any indication between 19,980 (actual weight minus the
tolerance value) and 20,020 (actual weight plus the tolerance value)
would be considered to be within the tolerance requirement for that
test.

The tolerances that are established in Handbook 44 are primarily for use
by weights and measures officials. As explained in Chapter 3, they are
not to be used by the owner of the device or a serviceperson either to
adjust a scale so that its registration benefits the owner or to
compensate for equipment that is in need of service or replacement.
Scales and other weighing and measuring devices are to be adjusted as
close to zero error as possible.

#### Applicable Requirements

Requirements are set forth in the Scales Code relating to the
application of tolerances to tests of scales. Three of these are
directly relevant to vehicle and axle-load scales.

+-----------------------------------------------------------------------+
| **T.N.2. Tolerance Application.**                                     |
|                                                                       |
| > **T.N.2.1. General.** ‑ The tolerance values are positive (+) and   |
| > negative (‑) with the weighing device adjusted to zero at no load.  |
| > When tare is in use, the tolerance values are applied from the tare |
| > zero reference; the tolerance values apply to certified test loads  |
| > only.­                                                               |
| >                                                                     |
| > **T.N.2.3.  Subsequent Verification Examinations.** ‑ For           |
| > subse­quent verification examinations, the tolerance values apply    |
| > regardless of the influence factors in effect at the time of the    |
| > conduct of the examina­tion. (Also see G‑N.2.)                       |
|                                                                       |
| **T.N.2.4.  Multi-Interval and Multiple Range (Variable               |
| Division‑Value) Scales.**  - ­For multi-interval and multiple range    |
| scales, the tolerance values are based on the value of the scale      |
| division of the range in use.                                         |
+-----------------------------------------------------------------------+
| #                                                                     |
|                                                                       |
| # NIST                                                                |
|  Handbook 44, Scales Code, Paragraphs T.N.2.1., T.N.2.3., and T.N.2.4 |
+-----------------------------------------------------------------------+

To meet the Handbook 44 requirement for accuracy, a scale must perform
within specified tolerances in the circumstances of its actual use.
During a field examination, tolerances are applied under the conditions
in which the scale is found, regardless of environmental conditions
(referred to as **influence factors** in Handbook 44), such as
temperature, barometric pressure, electromagnetic or radio frequency
interference, or power supply interruption.

During a test, tolerance values are applied to all indications and
recorded representations of a scale, including indicators, printers,
graduated balance indicators, and tare indicators.

As you will learn, Handbook 44 specifies several different tests of
vehicle and axle-load scales. Each of these tests involves comparing the
registration error under a specified set of conditions to the
**applicable tolerance**. The applicable tolerance for a given test of a
given scale is also specified in Handbook 44, but must be determined by
the inspector, based upon the following factors:

-   the amount of time that the scale has been in service since
    installation, a major overhaul, or a required repair,

-   the amount of the test load, and

-   the specific test being conducted.

The accuracy of many weighing and measuring devices, including scales,
can be expected to deteriorate gradually in service. In order to allow a
reasonable amount of use before devices have to be readjusted (and,
thereby, keep the cost of compliance with weights and measures
regulations reasonable), Handbook 44 specifies two sets of tolerances:
one set, called **acceptance tolerances**, applies to devices that are
"new" (as described below); the other set of tolerances, called
**maintenance tolerances** apply to devices that have been in service
for some time (see Handbook 44 paragraphs G‑T.1. and G‑T.2., discussed
below).

+-----------------------------------------------------------------------+
| **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.        |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, General Code, Paragraphs. G-T.1. and G-T.2.**     |
+-----------------------------------------------------------------------+

Note that acceptance tolerances apply not only to devices that are
literally "new," that is, those that have had 30 days or less of actual
service, but also to those that have been returned to service after a
major repair (in response to official rejection of the device) or
reconditioning, and so should be expected to perform "like new." The
values of acceptance tolerances are generally smaller than the values of
maintenance tolerances: for scales, acceptance tolerances are one-half
the value of the corresponding maintenance tolerances (see "**Tolerance
Values**," below).

You will generally determine the time in service from the device owner.
In some cases, you may wish to verify the service history by reviewing
transaction records for the preceding 30 days to determine whether or
not a scale has actually been in service during that time.

The Scales Code and the EPOs describe a number of different tests for
vehicle and axle-load scales, all but one of which involve tolerances.
Although every scale does not require all tests, and although most of
the tests use the same tolerances, specified test loads vary, and this
will affect tolerance values. You will, therefore, need to be able to
determine the applicable tolerance value for each test, based upon
specified parameters. These tests and their associated tolerances will
be described later in this chapter.

#### Tolerance Values

The Scales Code specifies different tolerances for many scales,
depending upon whether or not they are marked with an accuracy class.
The tolerances for unmarked scales are summarized in Table T.1.1. for
each type of scale. However, according to this table, the tolerances for
marked and unmarked vehicle and axle-load scales are the same, that is,
the tolerances specified for Class III L scales. The table is,
therefore, not reproduced.

**Maintenance Tolerance Values**

Maintenance tolerances are specified in paragraph T.N.3.1. and Table 6.

+-----------------------------------------------------------------------+
| **T.N.3.1.  Maintenance Tolerance Values.** -- The maintenance        |
| tolerance values are as specified in Table 6.                         |
|                                                                       |
| +------+----------+------+------+--------+-------+-------------+      |
| | **T  |          |      |      |        |       |             |      |
| | able |          |      |      |        |       |             |      |
| | 6.** |          |      |      |        |       |             |      |
| |      |          |      |      |        |       |             |      |
| | *    |          |      |      |        |       |             |      |
| | *Mai |          |      |      |        |       |             |      |
| | nten |          |      |      |        |       |             |      |
| | ance |          |      |      |        |       |             |      |
| | Tole |          |      |      |        |       |             |      |
| | ranc |          |      |      |        |       |             |      |
| | es** |          |      |      |        |       |             |      |
| |      |          |      |      |        |       |             |      |
| | **   |          |      |      |        |       |             |      |
| | (All |          |      |      |        |       |             |      |
| | va   |          |      |      |        |       |             |      |
| | lues |          |      |      |        |       |             |      |
| | in   |          |      |      |        |       |             |      |
| | this |          |      |      |        |       |             |      |
| | t    |          |      |      |        |       |             |      |
| | able |          |      |      |        |       |             |      |
| | are  |          |      |      |        |       |             |      |
| | in   |          |      |      |        |       |             |      |
| | s    |          |      |      |        |       |             |      |
| | cale |          |      |      |        |       |             |      |
| | divi |          |      |      |        |       |             |      |
| | sion |          |      |      |        |       |             |      |
| | s)** |          |      |      |        |       |             |      |
| +------+----------+------+------+--------+-------+-------------+      |
| | T    |          |      |      |        |       |             |      |
| | oler |          |      |      |        |       |             |      |
| | ance |          |      |      |        |       |             |      |
| | in   |          |      |      |        |       |             |      |
| | s    |          |      |      |        |       |             |      |
| | cale |          |      |      |        |       |             |      |
| | d    |          |      |      |        |       |             |      |
| | ivis |          |      |      |        |       |             |      |
| | ions |          |      |      |        |       |             |      |
| +------+----------+------+------+--------+-------+-------------+      |
| |      | 1        | 2    |      | 3      |       | 5           |      |
| +------+----------+------+------+--------+-------+-------------+      |
| | C    | Test     |      |      |        |       |             |      |
| | lass | Load     |      |      |        |       |             |      |
| +------+----------+------+------+--------+-------+-------------+      |
| | I    | 0 ‑ 50   | 50   | 200  | 20­0    |       |             |      |
| |      | 000      | 001  | 000  | 001 +  |       |             |      |
| |      |          | ‑    |      |        |       |             |      |
| +------+----------+------+------+--------+-------+-------------+      |
| | II   | 0 ‑  5   | 5    | 20   | 20 001 |       |             |      |
| |      | 000      | 001  | 000  | +      |       |             |      |
| |      |          | ‑    |      |        |       |             |      |
| +------+----------+------+------+--------+-------+-------------+      |
| | III  | 0 -      | 501  | 2    | 2 001  | 4 000 | 4 001 +     |      |
| |      | 500      | ‑    | 000  | ‑      |       |             |      |
| +------+----------+------+------+--------+-------+-------------+      |
| | IIII | 0 -      | 51 ‑ | 200  | 201 ‑  | 400   | 401 +       |      |
| |      | 50       |      |      |        |       |             |      |
| +------+----------+------+------+--------+-------+-------------+      |
| | III  | 0 ‑      | 501  | 1    | (Add   |       |             |      |
| | L    |  500     | ‑    | 000­  | 1d for |       |             |      |
| |      |          |      |      | each   |       |             |      |
| |      |          |      |      | addi   |       |             |      |
| |      |          |      |      | tional |       |             |      |
| |      |          |      |      | 500 d  |       |             |      |
| |      |          |      |      | or     |       |             |      |
| |      |          |      |      | fr     |       |             |      |
| |      |          |      |      | ac­tion |       |             |      |
| |      |          |      |      | th     |       |             |      |
| |      |          |      |      | ere­of) |       |             |      |
| +------+----------+------+------+--------+-------+-------------+      |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code Paragraph T.N.3.1. and Table 6.**     |
+-----------------------------------------------------------------------+

Since vehicle and axle-load scales are Class III L devices, we are
concerned with the last line in Table 6. As can be seen, for this class
the tolerance value, shown on the top line, increases by 1 d for every
additional 500d of test load. Since the maximum number of scale
divisions for this class of scales is 10,000, such a scale, tested at
full capacity, would have a tolerance of 20d.

Because the test of a scale involves a number of different test loads,
including all the tolerance "break points" --- the maximum load in each
range (500d, 1,000d, 1,500d, and so on) --- up to the amount of
available test weights or the capacity of the scale, whichever is less,
you will find that it saves time to determine the tolerances for each of
the 500d ranges, up to the maximum load, as shown in the example in
Figure 4-1 for a vehicle scale with 20-lb scale divisions.

+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| #                                                                     |
|                                                                       |
| # Figure 4-                                                           |
| 1. Maintenance Tolerance Table for Vehicle Scale with 20 lb Divisions |
+-----------------------------------------------------------------------+

In some cases, however, a test load will not correspond to a tolerance
break point --- for example, if the test procedure calls for a test load
that is equivalent to 600d. In that event, you will have to calculate
the tolerance. To calculate the maintenance tolerance for a vehicle or
axle-load scale, use the following steps.

-   Determine the test load expressed in scale divisions (divide the
    test load by the scale division, converting the units of the test
    load to those of the scale division if necessary). For example, if 6
    tons of test weights are to be applied to a scale with 20 lb
    divisions, the test load would be 12,000 lb ÷ 20 lb = 600d.

-   Divide the test load by 500; if the result has a fractional
    remainder, round up to the [next]{.underline} whole number. To
    continue with the same example, if 600d of test load will be
    applied, 600d ÷ 500 = 1.2, which rounds up to 2. This is the
    maintenance tolerance value in scale divisions, in this example, 2d.

Although this may seem complicated, you will find that in practice it is
not; all you are doing is determining the number of whole 500 units in
the test load.

**Acceptance Tolerance Values**

The acceptance tolerance for vehicle and axle-load scales is simply
one-half the applicable maintenance tolerance.

  -----------------------------------------------------------------------
  **T.N.3.2. Acceptance Tolerance Values.** - The acceptance tolerance
  values shall be one-half the maintenance tolerance values.

  **NIST Handbook 44, Scales Code, Paragraph T.N.3.2.**
  -----------------------------------------------------------------------

For example, the acceptance tolerance for a scale with 20 lb scale
divisions and a test load of 600d would be ½ x 2d = 1d (or 20 lb).

**Decreasing-Load Test Tolerances**

The Decreasing-Load Test is performed on all automatic-indicating
scales. For vehicle and axle-load scales, the test load for the
Decreasing-Load Test is one-half the maximum load applied during the
Increasing-Load Test. The tolerances to be applied for the
Decreasing-Load Test are those for Class III L scales specified in
paragraph T.N.3.1. and Table 6 and paragraph T.N.3.2. of the Scales
Code.

**Agreement of Indications**

When a scale system is equipped with multiple indicating and recording
elements, all indicated and recorded representations of the same value
must agree as closely as possible. As mentioned earlier, exact agreement
between analog indications and digital indications or recorded
representations within a system is not always possible, but the
variation is strictly limited. Even two independent analog devices
within the same system may disagree by a small amount. Because it is
always desirable to limit such variation as much as possible, an
additional tolerance applies to the agreement between indicators or
recorded representations. Handbook 44 addresses agreement of indication
requirements for three different scale designs.

The first is a scale with two or more indicating and/or recording
elements that are intended to be used independently of one another. An
example is a scale with a digital indicator and printer that also has a
"standby" dial and printer to be used in case of a power failure or
other failure of the digital indicator.

In this case, the tolerances are applied independently to each separate
element or combination of elements. In other words, there is no
tolerance for agreement between the indications.

+-----------------------------------------------------------------------+
| **T.N.4.1. Multiple Indicating/Recording Elements.** - In the case of |
| a scale or weighing system equipped with more than one indicating     |
| element or indicating element and recording element combination,      |
| where the indicators or indicator/recorder combination are intended   |
| to be used independently of one another, tolerances shall be applied  |
| independently to each indicator or indicator/recorder combination.    |
|                                                                       |
| (Amended 1986)                                                        |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraph T.N.4.1.**                 |
+-----------------------------------------------------------------------+

The second design is a scale that is equipped with various means of
indicating the weight of the same load, which are intended to be used in
combination. An example is a weighbeam with counterpoise weights, a dial
indicator with tare bars, or a weighbeam with an automatic-indicating
attachment such as a weight-o-graph.

+-----------------------------------------------------------------------+
| **T.N.4.2. Single Indicating/Recording Element.** - In the case of a  |
| scale or weighing system with a single indicating element or an       |
| indicating/recording element combination, and equipped with component |
| parts such as unit weights, weighbeam and weights, or multiple        |
| weighbeams that can be used in combination to indicate a weight, the  |
| difference in the weight value indications of any load shall not be   |
| greater than the absolute value of the applicable tolerance for that  |
| load, and shall be within tolerance limits.                           |
|                                                                       |
| (Amended 1986)                                                        |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraph T.N.4.2.**                 |
+-----------------------------------------------------------------------+

In this case, all indications must be individually within the applicable
tolerance [and]{.underline} the indications or recorded representations
produced by the different means must not differ by more than the
absolute value of the applicable tolerance. For example, consider the
following:

Type of indicator: Dial with tare bars

Value of scale division: 20 lb

Test load: 20,000 lb

Maintenance tolerance value: 2d or 40 lb

Acceptable tolerance range: 19,960 lb to 20,040 lb

Dial indication: 20,020 lb

In this case, the tare bar could indicate no more than 20,040 pounds
(20,020 + 40 = 20,060; however, 20,060 is not within the acceptable
tolerance range) and no less than 19,980 pounds (20,020 - 40 = 19,980
lb) in order for the scale to meet the agreement requirement in T.N.4.2.
and be within the applicable tolerance. If the dial had indicated 20,000
pounds, the tare bar reading still could have been no more than 20,040
pounds, but the minimum reading could have been 19,960 pounds.

Finally, the third design is a scale equipped with two analog indicators
as part of the same element, which are used to indicate the same load.
Usually, one indicator is provided for the customer and one is provided
for the device operator. An example would be a vehicle scale with twin
dials, one on the front and one on the back of the indicator.

+-----------------------------------------------------------------------+
| **T.N.4.3. Single Indicating Element/Multiple Indications.** - In the |
| case of an analog indicating element equipped with two or more        |
| indicating means within the same element, the difference in the       |
| weight indications for any load other than zero shall not be greater  |
| than one-half the value of the scale division (d) and be within       |
| tolerance limits.                                                     |
|                                                                       |
| (Amended 1986)                                                        |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraph T.N.4.3.**                 |
+-----------------------------------------------------------------------+

In this case, the readings of both indicators must be within tolerance
and must agree within one-half the value of the scale division. If you
are testing a scale with a scale division value of 20 pounds, the
maximum difference permitted between the two dial indications is 10
pounds. For example, with a test load of 20,000 pounds on the scale and
a maintenance tolerance of 40 pounds, if the indicator of one dial was
on the 20,020-pound graduation and the indicator of the other dial was
half-way between the 20,020-pound and the 20,040-pound graduations, the
indications would both be within tolerance and they would agree within
the 10-pound limit; however, if the indicator of the second dial was
closer to the 20,040 graduation or on that graduation, the indication
would still be within tolerance but would not meet the agreement
requirement specified in paragraph T.N.4.3.

**Tolerances on Shift Tests**

In the shift test, two different loads not exceeding the Concentrated
Load Capacity or section capacity are applied at various points on the
load-receiving element, usually over each section and sometimes also at
the midpoints between the sections. At each point, the indicated and
recorded weights are observed. Each of the readings must be within the
applicable tolerance.

+-----------------------------------------------------------------------+
| **T.N.4.4. Shift or Section Tests.** - The range of the results       |
| obtained during the conduct of a shift test or a section test shall   |
| not exceed the absolute value of the maintenance tolerance applicable |
| and each test result shall be within applicable tolerances.           |
|                                                                       |
| (Added 1986)                                                          |
|                                                                       |
| **absolute value. --** The absolute value of a number is the          |
| magnitude of that number without considering the positive or negative |
| sign.                                                                 |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraph T.N.4.4. and Appendix D,   |
| Definitions**                                                         |
+-----------------------------------------------------------------------+

In addition, the range of weight values obtained from the separate
weighings may not exceed the absolute value of the applicable
[maintenance]{.underline} tolerance. As described in NIST Handbook 44,
Appendix D, Definitions, the absolute value of a number is that number
without any plus ("+") or minus ("-") signs. For example, the absolute
value of a maintenance tolerance value of +/- 60 lb is 60 lb (without
any signs). To determine the range of a group of section test results,
subtract the smallest from the largest indicated section weight; the
result is the range, which must not exceed the absolute value of the
maintenance tolerance.

For example, consider a four-section scale with a CLC of 50,000 lb and a
20-lb scale division. A total of 30,000 lb of certified test weights is
available for the test. The scale has been in service for more than 30
days; therefore, maintenance tolerances apply. If the sections are
tested with the entire available weight, the tolerance will be (30,000
lb ÷ 20 lb) ÷ 500 = 3d = 60 lb. Suppose the following weights are
registered:

Section 1 30,040 lb

Section 2 29,980 lb

Section 3 30,000 lb

Section 4 29,960 lb

Since readings between 29,940 lb and 30,060 lb meet the maintenance
tolerance for this test, each individual section is so far in
compliance. However, the [range]{.underline} of values is 30,040 -
29,960 = 80 lb. Since this exceeds the tolerance of 3d = 60 lb, the
scale fails to meet that portion of the requirement.

**Repeatability**

A fundamental criterion for accuracy of a weighing or measuring device
is its **repeatability**, that is, its ability to reproduce the same
indicated measurement of the same test load under reasonably similar
conditions. Again, the absolute value of the applicable maintenance
tolerance is applied as a tolerance to the difference between successive
weighings of the same load under similar conditions.

  -----------------------------------------------------------------------
  **T.N.5. Repeatability.** - The results obtained from several weighings
  of the same load under reasonably static test conditions shall agree
  within the absolute value of the maintenance tolerance for that load,
  and shall be within applicable tolerances.

  **NIST Handbook 44, Scales Code, Paragraph T.N.5.**
  -----------------------------------------------------------------------

For example, consider a 100,000 x 20 lb scale with a 20,000 lb test load
applied. If a weight value of 19,960 lb is displayed under one
application of the 20,000 lb load, and a repeat application of the same
load displays 20,040 lb, the individual readings are within the
maintenance tolerance of +/- 40 lb; however the scale is not within
tolerance for repeatability. The repeatability tolerance, which is the
absolute value of the maintenance tolerance, is 40 lb; the difference in
two repeat readings in the example is 80 lb (20,040 lb ‑ 19,960 lb =
80 lb).

Note that T.N.5. specifies the absolute value of the
[maintenance]{.underline} tolerance, whether or not the system is being
tested for acceptance. Here, a situation may arise where successive
weighings of the same test load meet the repeatability requirement, even
though they fail to meet the tolerance requirement. For example, on the
scale just described, if successive weighings give indications of 20,000
lb and 20,040, the second weighing exceeds the acceptance tolerance for
the test (20 lb), even though the difference between the weights is
within the tolerance for repeatability (40 lb).

**Sensitivity Requirement (SR)**

Nonautomatic indicating elements, such as weighbeams, are required to
meet a minimum requirement for the degree of responsiveness to a change
in load. According to Handbook 44, the **sensitivity requirement**
(**SR**) is "the minimum change in position of rest of the indicating
element or elements of the scale in response to the increase or
decrease, by a specified amount, of the test load on the load-receiving
element of the scale" (for example, the minimum movement of a dial
pointer when a specified amount of weight is applied to the scale).

Although not precisely a tolerance, the SR has traditionally been
included in the tolerance section of the Scales Code. The test loads for
the sensitivity test are given in terms of the scale division and,
consequently, must be converted to pounds prior to the test. The test
loads for the SR test for marked and unmarked scales are identical, but
are specified in different paragraphs of the Scales Code. The test loads
for [unmarked]{.underline} vehicle and axle-load scales is set forth in
T.2.7.; the test load for [marked]{.underline} scales in T.N.6.1.

+-----------------------------------------------------------------------+
| **T.2.7.** \[Sensitivity test load for unmarked\] **Vehicle,          |
| Axle-Load, Livestock, and Animal Scales.**                            |
|                                                                       |
| **T.2.7.1. Equipped with Balance Indicators.** 1d                     |
|                                                                       |
| **T.2.7.2. Not Equipped with Balance Indicators.** 2d or 0.2 percent  |
| of the scale capacity,                                                |
|                                                                       |
| whichever is less.                                                    |
|                                                                       |
| \* \* \*                                                              |
|                                                                       |
| **T.N.6.1.** \[Sensitivity\] **Test Load**                            |
|                                                                       |
| \(a\) The test load for sensitivity for nonautomatic-indicating       |
| vehicle, axle-load, livestock,. and animal scales shall be 1d for     |
| scales equipped with balance indicators, and 2d or 0.2 percent of the |
| scale capacity, whichever is less, for scales not equipped with       |
| balance indicators.                                                   |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraphs T.2.7. and T.N.6.1. (a)** |
+-----------------------------------------------------------------------+

The specified changes in equilibrium corresponding to the specified test
loads for unmarked and marked scales are similar, and for all practical
purpose identical, except for one detail. The paragraphs from the Scales
Code (T.3. and T.N.6.2.) are printed side by side on the next page.
Figure 4-2 summarizes the requirements.

# 

+-----------------------------------------------------------------------+
| +-----------------+-----------------+-----------------+-----------+   |
| |                 | **Test          | **Equilibrium   | **Eq      |   |
| |                 | Load---Marked   | Change Unmarked | uilibrium |   |
| |                 | and Unmarked**  | Scales**        | Change\   |   |
| |                 |                 |                 | Marked    |   |
| |                 |                 |                 | Scales**  |   |
| +-----------------+-----------------+-----------------+-----------+   |
| | **With Balance  | 1d              | Greater of 0.25 | Same as   |   |
| | Indicator**     |                 | inch change in  | unmarked\ |   |
| |                 |                 | position\       | [e        |   |
| |                 |                 | or 1 graduation | xcept]{.u |   |
| |                 |                 | or\             | nderline} |   |
| |                 |                 | width of target | **        |   |
| |                 |                 | area            | *0.20***\ |   |
| |                 |                 |                 | inch      |   |
| |                 |                 |                 | change in |   |
| |                 |                 |                 | position  |   |
| +-----------------+-----------------+-----------------+-----------+   |
| |                 |                 |                 |           |   |
| +-----------------+-----------------+-----------------+-----------+   |
| | **With Trig     | Lesser of 2d or | Tip of beam     | Same as   |   |
| | Loop but no     | 0.2%\           | moves\          | unmarked  |   |
| | Balance         | of scale        | from center of  |           |   |
| | Indicator**     | capacity        | trig\           |           |   |
| |                 |                 | loop to the top |           |   |
| |                 | (whichever, is  | or\             |           |   |
| |                 | less)           | bottom          |           |   |
| +-----------------+-----------------+-----------------+-----------+   |
| |                 |                 |                 |           |   |
| +-----------------+-----------------+-----------------+-----------+   |
| | **With neither  | Lesser of 2d or | Position of     | Same as   |   |
| | Trig Loop nor   | 0.2%\           | weighbeam or    | unmarked  |   |
| | Balance         | of scale        | lever system    |           |   |
| | Indicator**     | capacity        | moves from      |           |   |
| |                 |                 | midway between  |           |   |
| |                 | (whichever, is  | stops to either |           |   |
| |                 | less)           | limit of motion |           |   |
| +-----------------+-----------------+-----------------+-----------+   |
+-----------------------------------------------------------------------+
| # Figure 4-2. Sensitivity Requirements                                |
+-----------------------------------------------------------------------+

+-----------------------------------+-----------------------------------+
| **\[For Unmarked Scales\]**       | **\[For Marked Scales\]**         |
|                                   |                                   |
| **T.3. Sensitivity Requirement,   | **T.N.6.2. Minimum Change of      |
| Equilibrium Change Required. -**  | Indications. -** The addition or  |
| The minimum change in equilibrium | removal of the test load for      |
| with test loads equal to the      | sensitivity shall cause a minimum |
| values specified in T.2. shall be | permanent change as follows:      |
| as follows:                       |                                   |
|                                   | \(a\) For a Scale with a Trig     |
| \(a\) Scale With a Trig Loop but  | Loop but Without a Balance        |
| Without a Balance Indicator. -    |                                   |
| The position of rest of the       | Indicator. - The position of the  |
| weighbeam shall change from the   | weighbeam shall                   |
| center of the trig loop to the    |                                   |
| top or bottom, as the case may    | change from the center to the     |
| be.                               | outer limit of the trig loop.     |
|                                   |                                   |
| \* \* \*                          | \(b\) For a Scale With a Balance  |
|                                   | Indicator. - The position of the  |
| \(c\) Scale With a Single Balance | indicator shall change one        |
| Indicator and Having a Nominal    | division on the graduated scale,  |
| Capacity of 500 Pounds or         | the width of the central target   |
| Greater. - The position of rest   | area, or the applicable value as  |
| of the indicator shall change     | shown below, whichever is         |
| 0.25 (1/4) inch or one division   | greater:                          |
| on the graduated scale or the     |                                   |
| width of the central target area, | > Scale of Class III, III L, or   |
| whichever is greater. However,    | > IIII with a maximum capacity of |
| the indicator on a batching scale | > more than 70 pounds (30 kg):    |
| shall change 0.125 (1/8) inch or  | > 0.20 inch (5mm);                |
| one division on the graduated     |                                   |
| scale, whichever is greater.      | \(c\) For a Scale Without a Trig  |
|                                   | Loop or a Balance Indicator. -    |
| \* \* \*                          | The position of rest of the       |
|                                   | weighbeam or lever system shall   |
| \(e\) Scale With Neither a Trig   | change from the horizontal or     |
| Loop Nor a Balance Indicator. -   | midway between limiting stops to  |
| The position of rest of the       | either limit of motion.           |
| weighbeam or lever system shall   |                                   |
| change from the horizontal, or    |                                   |
| midway between limiting stops, to |                                   |
| either limit of motion.           |                                   |
+-----------------------------------+-----------------------------------+
| **NIST Handbook 44, Scales Code,  |                                   |
| Paragraphs T.3. and T.N.6.2.**    |                                   |
+-----------------------------------+-----------------------------------+

**Discrimination**

The discrimination test for automatic-indicating elements is analogous
to the sensitivity test for nonautomatic-indicating elements. However,
for the discrimination test, the addition or subtraction of a specified
test load must produce a minimum change in indication. There are
separate specifications for analog and digital indicators (see paragraph
T.N.7. below). Again, the specified values are expressed in terms of
scale divisions, and so must be converted to pounds. The determination
of the zone of uncertainty will be discussed further in Chapter 6.

+-----------------------------------------------------------------------+
| **T.N.7. Discrimination**                                             |
|                                                                       |
| **T.N.7.1. Analog Automatic Indicating (i.e., Weighing Device with    |
| Dial, Drum, Fan, Etc.).** -A test load equivalent to 1.4d shall cause |
| a change in the indication of at least 1.0d. (See N.1.5.)             |
|                                                                       |
| **T.N.7.2. Digital Automatic Indicating.** - A test load equivalent   |
| to 1.4d shall cause a change in the indicated or recorded value of at |
| least 2.0d. This requires the zone of uncertainty to be not greater   |
| than 0.3 times the value of the scale division. (See N.1.5.1.)        |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraphs T.N.7., T.N.7.1., and     |
| T.N.7.2.**                                                            |
+-----------------------------------------------------------------------+

**Maximum and Minimum Test Loads**

The tolerances described in this chapter are determined according to the
test load that is applied. In the case of large-capacity scales like
vehicle and axle-load scales, test loads are often limited by the
available certified test weights. As explained in Chapter 2, Handbook 44
specifies the minimum test weights and test loads required for testing
different capacity scales.

+-----------------------------------------------------------------------+
| **N.3. Minimum Test Weights and Test Loads^2^.** ‑ The minimum test   |
| weights and test loads for in‑service tests (except railway track     |
| scales) are shown in Table 4. \[See Table 4 for footnote ^2^.\]       |
|                                                                       |
| (Added 1984) (Amended 1988)                                           |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraph N.3.**                     |
+-----------------------------------------------------------------------+

Table 4 is reproduced on the next page along with the accompanying
paragraph N.3. Since most vehicle scales have capacities of more than
40,000 lb, the minimum test weights, according to Table 4, are 12.5
percent of the scale capacity or 10,000 lb, whichever is greater. For a
vehicle scale with a capacity of 200,000 lb, the minimum test weight
would therefore be 0.125 × 200,000 = 25,000 lb.

In addition, a larger minimum **test load** of 25 percent of the scale
capacity is required. The test load consists of certified weights plus
any other non-certified weights or loads that are used in testing (for
example, a vehicle used in the strain-load load test of a vehicle
scale). Thus, a minimum test load for a scale with 200,000 capacity
would be 0.25 × 200,000 = 50,000, at least half of which would be
certified test weights.

Examples used in this and subsequent chapters will assume that 30,000 lb
of certified test weights are available and that additional
non-certified weights are available to obtain a total test load of
60,000 lb.

While having minimum test weights and loads is essential to conducting
effective tests of vehicle and axle-load scales in the field, it is also
essential that test loads not be applied in a manner that places a
damaging strain on the scale. Specific guidelines for applying test
loads are given in Chapter 6. However, an essential limitation should be
observed in all test situations:

-   The test load applied to a scale section, or to the span of a scale
    deck between sections, should never exceed the Concentrated Load
    Capacity that is required to be marked by the manufacturer on scales
    manufactured after January 1, 1989, or the section capacity
    specified by the manufacturer of a scale manufactured prior to that
    date.

+-----------------------------------------------------------------------+
| **N.3. Minimum Test Weights and Test Loads^2^.** ‑ The minimum test   |
| weights and test loads for in‑service tests (except railway track     |
| scales) are shown in Table 4. \[See Table 4 for footnote ^2^.\]       |
|                                                                       |
| (Added 1984) (Amended 1988)                                           |
|                                                                       |
| +-------------+------------------+--------+----------------------+    |
| | **Table     |                  |        |                      |    |
| | 4.**        |                  |        |                      |    |
| |             |                  |        |                      |    |
| | **Minimum   |                  |        |                      |    |
| | Test        |                  |        |                      |    |
| | Weights and |                  |        |                      |    |
| | Test        |                  |        |                      |    |
| | Loads^1^**  |                  |        |                      |    |
| +-------------+------------------+--------+----------------------+    |
| | Device      | Minimums (in     |        | (where practicable)  |    |
| | capacity    | terms of device  |        |                      |    |
| |             | capacity)        |        |                      |    |
| +-------------+------------------+--------+----------------------+    |
| |             | Test weights     | Test   |                      |    |
| |             | (greater of)     | lo     |                      |    |
| |             |                  | ads^2^ |                      |    |
| +-------------+------------------+--------+----------------------+    |
| | 0 to 150 kg | 100 %            |        |                      |    |
| |             |                  |        |                      |    |
| | (0 to       |                  |        |                      |    |
| | 300 lb)     |                  |        |                      |    |
| +-------------+------------------+--------+----------------------+    |
| | 151 to      | 25 % or 150 kg   | 75 %   | Test weights to dial |    |
| | 1 500 kg    | (300 lb)         |        | face ca­paci­ty,       |    |
| |             |                  |        | 1 000 d, or test     |    |
| | (301 to     |                  |        | load to used         |    |
| | 3 000 lb)   |                  |        | ca­pacity, if great­er |    |
| |             |                  |        | than mini­mums        |    |
| |             |                  |        | speci­fied            |    |
| |             |                  |        |                      |    |
| |             |                  |        | During initial       |    |
| |             |                  |        | verification, a      |    |
| |             |                  |        | scale should be      |    |
| |             |                  |        | tested to capacity.  |    |
| +-------------+------------------+--------+----------------------+    |
| | 1 501 to    | 12.5 % or 500 kg | 50 %   |                      |    |
| | 20 000 kg   | (1 000 lb)       |        |                      |    |
| |             |                  |        |                      |    |
| | (3 001 to   |                  |        |                      |    |
| | 40 000 lb)  |                  |        |                      |    |
| +-------------+------------------+--------+----------------------+    |
| | 20 001 kg+  | 12.5 % or        | 2      |                      |    |
| |             | 5 000 kg (10     | 5 %^3^ |                      |    |
| | (40 001     | 000 lb)          |        |                      |    |
| | lb+)        |                  |        |                      |    |
| +-------------+------------------+--------+----------------------+    |
| | ^1^ If the  |                  |        |                      |    |
| | amount of   |                  |        |                      |    |
| | test weight |                  |        |                      |    |
| | in Table 4  |                  |        |                      |    |
| | combined    |                  |        |                      |    |
| | with the    |                  |        |                      |    |
| | load on the |                  |        |                      |    |
| | scale would |                  |        |                      |    |
| | result in   |                  |        |                      |    |
| | an unsafe   |                  |        |                      |    |
| | condition,  |                  |        |                      |    |
| | then the    |                  |        |                      |    |
| | appropriate |                  |        |                      |    |
| | load will   |                  |        |                      |    |
| | be          |                  |        |                      |    |
| | determined  |                  |        |                      |    |
| | by the      |                  |        |                      |    |
| | official    |                  |        |                      |    |
| | with        |                  |        |                      |    |
| | statutory   |                  |        |                      |    |
| | authority.  |                  |        |                      |    |
| |             |                  |        |                      |    |
| | ^2^The term |                  |        |                      |    |
| | \"test      |                  |        |                      |    |
| | load\"      |                  |        |                      |    |
| | means the   |                  |        |                      |    |
| | sum of the  |                  |        |                      |    |
| | combination |                  |        |                      |    |
| | of field    |                  |        |                      |    |
| | standard    |                  |        |                      |    |
| | test        |                  |        |                      |    |
| | weights and |                  |        |                      |    |
| | any other   |                  |        |                      |    |
| | applied     |                  |        |                      |    |
| | load used   |                  |        |                      |    |
| | in the      |                  |        |                      |    |
| | conduct of  |                  |        |                      |    |
| | a test      |                  |        |                      |    |
| | using       |                  |        |                      |    |
| | s           |                  |        |                      |    |
| | ubstitution |                  |        |                      |    |
| | test        |                  |        |                      |    |
| | methods.    |                  |        |                      |    |
| | Not more    |                  |        |                      |    |
| | than three  |                  |        |                      |    |
| | su          |                  |        |                      |    |
| | bstitutions |                  |        |                      |    |
| | shall be    |                  |        |                      |    |
| | used during |                  |        |                      |    |
| | s           |                  |        |                      |    |
| | ubstitution |                  |        |                      |    |
| | testing,    |                  |        |                      |    |
| | after which |                  |        |                      |    |
| | the         |                  |        |                      |    |
| | tolerances  |                  |        |                      |    |
| | for         |                  |        |                      |    |
| | strain-load |                  |        |                      |    |
| | tests shall |                  |        |                      |    |
| | be applied  |                  |        |                      |    |
| | to each set |                  |        |                      |    |
| | of test     |                  |        |                      |    |
| | loads.      |                  |        |                      |    |
| |             |                  |        |                      |    |
| | ^3^ The     |                  |        |                      |    |
| | scale shall |                  |        |                      |    |
| | be tested   |                  |        |                      |    |
| | from zero   |                  |        |                      |    |
| | to at least |                  |        |                      |    |
| | 12.5 % of   |                  |        |                      |    |
| | scale       |                  |        |                      |    |
| | capacity    |                  |        |                      |    |
| | using known |                  |        |                      |    |
| | test        |                  |        |                      |    |
| | weights and |                  |        |                      |    |
| | then to at  |                  |        |                      |    |
| | least 25 %  |                  |        |                      |    |
| | of scale    |                  |        |                      |    |
| | capacity    |                  |        |                      |    |
| | using       |                  |        |                      |    |
| | either a    |                  |        |                      |    |
| | s           |                  |        |                      |    |
| | ubstitution |                  |        |                      |    |
| | or          |                  |        |                      |    |
| | strain-load |                  |        |                      |    |
| | test that   |                  |        |                      |    |
| | utilizes    |                  |        |                      |    |
| | known test  |                  |        |                      |    |
| | weights of  |                  |        |                      |    |
| | at least    |                  |        |                      |    |
| | 12.5 % of   |                  |        |                      |    |
| | scale       |                  |        |                      |    |
| | capacity.   |                  |        |                      |    |
| | Whenever    |                  |        |                      |    |
| | practical,  |                  |        |                      |    |
| | a           |                  |        |                      |    |
| | strain-load |                  |        |                      |    |
| | test should |                  |        |                      |    |
| | be          |                  |        |                      |    |
| | conducted   |                  |        |                      |    |
| | to the used |                  |        |                      |    |
| | capacity of |                  |        |                      |    |
| | the scale.  |                  |        |                      |    |
| | When a      |                  |        |                      |    |
| | strain-load |                  |        |                      |    |
| | test is     |                  |        |                      |    |
| | conducted,  |                  |        |                      |    |
| | the         |                  |        |                      |    |
| | tolerances  |                  |        |                      |    |
| | apply only  |                  |        |                      |    |
| | to the      |                  |        |                      |    |
| | known test  |                  |        |                      |    |
| | weights or  |                  |        |                      |    |
| | s           |                  |        |                      |    |
| | ubstitution |                  |        |                      |    |
| | test load.  |                  |        |                      |    |
| |             |                  |        |                      |    |
| | (Amended    |                  |        |                      |    |
| | 1988, 1989  |                  |        |                      |    |
| | and 1994)   |                  |        |                      |    |
| +-------------+------------------+--------+----------------------+    |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraph N.3. and Table T.4.**      |
+-----------------------------------------------------------------------+

Since the nominal capacity of a scale marked with an accuracy class must
be less than the sum of the capacities of the scale sections
(specifically, less than or equal to the CLC times the number of
sections minus 0.5, as explained in Chapter 3), a test load equal to the
full capacity may be distributed over the entire deck, provided that the
maximum load on any one section or between sections is not exceeded.

## Substitution Test

In a substitution test, unknown material or objects are substituted for
known test weights as described in NIST Handbook 44 Scales Code
paragraph N.1.11. and in Appendix D, Definitions. The tolerances are
applied to the scale based on the substitution test load as noted in
Scales Code paragraph T.N.3.11. This process is used as part of the
increasing load test when sufficient weights are unavailable to reach
the required test load specified in Table 4. The substitution process
shall be repeated no more than three times.

In the substitution process, known test weights are placed on the scale;
a scale reading is taken using error weights to determine the exact
scale indication; the reading is noted; and the known test weights are
removed. Next, unknown material or objects are added to the scale until
the scale indication reads the same as when the known test weights were
on the scale; error weights are again used to ensure the reading is
duplicated. This process represents one substitution of the known test
weights.

Once the substitution process has been completed, the test can be
continued using the known test weights. When all of the known test
weights have once again been applied, the scale reading is taken using
error weights as described above and the known test weights are removed.
Additional unknown material or objects are substituted for the known
test weights using the same process as described above.

+-----------------------------------------------------------------------+
| **N.1.11. Substitution Test. -** In the substitution test process,    |
| material or objects are substituted for known test weights, or a      |
| combination of known test weights and previously quantified material  |
| or objects, using the scale under test as a comparator. Additional    |
| test weights or other known test loads may be added to the known test |
| load to evaluate higher weight ranges on the scale.                   |
|                                                                       |
| **T.N.3.11. Tolerances for Substitution Test. -** Tolerances are      |
| applied to the scale based on the substitution test load.             |
|                                                                       |
| **substitution test. --** A scale testing process used to quantity    |
| the weight of material or objects for use as a known test load.       |
|                                                                       |
| **substitution test load. --** The sum of the combination of field    |
| standard test weights and any other applied load used in the conduct  |
| of a test using substitution test methods.                            |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraphs N.1.11. and T.N.3.11. and |
| Appendix D, Definition**                                              |
+-----------------------------------------------------------------------+

**Strain-Load Test**

In a strain-load test, an unknown quantity of material or objects are
used to establish a reference load point to which test weights or a
combination of test weights and substitution test loads are added. This
process is used when available test weights are less than maximum
capacity of a vehicle or axle-load scale and another means must be found
to test the upper portion of the scale's capacity. This process is
referenced in NIST Handbook 44 Scales Code paragraph N.1.12. and in
Appendix D, Definitions. The tolerances are applied to the scale based
on the known test weights or substitution test load as noted in Scales
Code paragraph T.N.3.12.

The strain-load test is conducted by removing all loads from the scale
and zeroing the scale. Next, an unknown load is placed on the scale and
error weights are used to establish a beginning reference load point.
Known test weights are then applied to the scale and error weights used
to determine the resulting scale reading. This resulting scale reading
is compared with the beginning reference load point, and the difference
between the two is determined. The error is determined by comparing this
difference with the known test load that was applied to the scale. The
process for conducting a strain-load test is discussed in more detail in
Chapter 6.

+-----------------------------------------------------------------------+
| **N.1.12. Strain-Load Test. -** In the strain-load test procedure, an |
| unknown quantity of material or objects are used to establish a       |
| reference load or tare to which test weights or substitution test     |
| loads are added.                                                      |
|                                                                       |
| **T.N.3.12. Tolerances for Strain-Load Test. -** The tolerances apply |
| only to the test weights or substitution test load.                   |
|                                                                       |
| **Strain-load test. --** The test of a scale beginning with the scale |
| under load and applying known test weights to determine accuracy over |
| a portion of the weighing range. The scale errors for a strain-load   |
| test are the errors observed for the known test loads only. The       |
| tolerances to be applied are based on the known test load used for    |
| each error that is determined.                                        |
+-----------------------------------------------------------------------+
| **NIST Handbook 44, Scales Code, Paragraphs N.1.12. and TN.3.12. and  |
| Appendix D, Definition**                                              |
+-----------------------------------------------------------------------+

#### 

#### 

#### Constructing A Tolerance Worksheet

It is good practice to make all tolerance determinations before
beginning any test. This will permit you to plan the test to make the
most efficient use of available test weights if they are less than the
capacity of the scale and will help insure that nothing important is
overlooked. One effective approach is to use tolerance worksheets, which
not only record applicable tolerance values but also break the test down
into its essential components. Three sample worksheets have been
developed for use in this module. Blank worksheets are reproduced in
Appendix C. You should find that these worksheets, along with the EPOs,
make the pretest determinations portion of the examination as efficient
as possible.

One worksheet is provided for each of the following types of vehicle and
axle-load scales:

-   full capacity weighbeams

-   mechanical dials

-   electronic scales

Separate worksheets have been developed for these different types of
scales even though most tolerances are similar because of differences in
test application and procedure. You will learn about these topics in the
next chapters. At present, you should concentrate on learning how to
fill out the worksheets. Examples of completed worksheets are given
below, and a partially completed example is provided on which to
practice the tolerance determinations. You will be given additional
opportunities to practice and review worksheets with your instructor.

Before turning to the examples, you should be aware that the test method
known as "tolerance testing" is employed exclusively in this manual, and
it is recommended for use in field examinations. In tolerance testing
you determine by direct reading whether or not the displayed weight
value at each test load is within tolerance for that test load. This
will not define the [exact]{.underline} amount of scale error in each
case, but does determine whether or not the scale performance is within
acceptable limits.

Some jurisdictions use "error testing." In this method, small test
weights are used to determine the exact amount of error at a particular
test load. Although this method provides more accurate indications of
the error than tolerance testing, most jurisdictions use tolerance
testing because:

-   it takes less time,

-   official actions are based on devices being in or out of tolerance,
    not on the [exact]{.underline} amount of error, and

-   it is less affected by environmental factors such as wind because
    larger test loads are used.

Consequently, tolerance testing procedures are used in this module.

In tolerance testing, the range of weight values that are within
tolerance for each test load is called the "acceptable range of scale
indication." Weight values are determined by direct reading from the
indicator. [All]{.underline} weight values must fall within the
"acceptable range of scale indication." If any of the test values fall
outside that range, the scale fails to meet the tolerance requirements
of Handbook 44.

For example, when a 10,000-lb test load is placed on the scale, and the
maintenance tolerance is 20 lb, the acceptable range of scale indication
is:

10,000 lb - 20 lb = 9,980 lb

10,000 lb + 20 lb = 10,020 lb.

Any test value in the range from 9,980 lb to 10,020 lb is acceptable. A
value above or below that range means the scale is out of tolerance and
cannot be approved.

It is good practice to calculate the "acceptable range of scale
indication" as a part of the pretest determination of tolerances. The
sample worksheets provide space for entering those values.

## Sample Tolerance Worksheets

The worksheets are organized in two sections, as can be seen in Figure
4-3. In the upper section of the worksheet, along the left-hand side,
are listed the various capacities of the scale, the value and number of
scale divisions (d and n), and the total available certified test
weights. In addition, space is provided to record whether the scale is
marked III L and whether acceptance or maintenance tolerances are
applicable. You should fill in data in the spaces provided.

The right-hand portion of the top section presents a summary of the
weights that correspond to the 500d increments that are used to
determine tolerances in Table 6. The first value, 500d, is calculated by
multiplying the value of the scale division by 500. Subsequent values
are multiples of 500d, and can be easily filled in. To avoid confusion,
values should not be filled in above the amount of available test
weights. This summary will make it easier to determine specific
tolerances as you continue filling in the worksheet.

The lower portion of the worksheet summarizes test loads, tolerances,
and acceptable ranges of scale indications for each of the tests
prescribed in the EPOs. You start by filling in the test loads column.
As you can see in Figure 4-3, many of the test loads correspond to whole
or fractional capacities (one-half and full fractional bar capacity,
section capacity, etc.). Where reference is made to the **tolerance
break point**, these are the largest values in the test load ranges in
Table 6. Tests are made at tolerance break points because at that end of
the range the tolerance is smallest in proportion to the test load (0.2
percent). Keep in mind the minimum test weights and test loads specified
in Table 4 of the Scales Code as you complete the worksheet.

The notes in the table state that a minimum test of a Class III L scale
shall consist of one test from zero to at least 25 percent of the scale
capacity and then one strain-load test to at least the **used capacity**
of the device. Used capacity refers to the largest load normally weighed
on the scale. This can be determined by asking the scale operator or
owner about the typical use of the scale, considering the application of
the scale, and observing the vehicles being weighed. It will not be
necessary to include the used capacity on the worksheet; instead, under
the Strain-Load Test, you will enter the amount of known (certified)
test weights available to conduct the test.

When the test loads column is filled in, the remainder of the worksheet
determinations are mechanical:

-   Fill in the second column, **Tol. In "d"** by consulting Table 6 for
    the test loads listed in the first column, [except]{.underline} for
    the **Zero-Load Balance Change**. This value will always be 1d for
    maintenance tests and 0.5d for acceptance tests (according to
    N.1.9.).

-   Fill in the third column, **Tol. In Pounds**, by multiplying the
    value in the second column by the scale division and recording the
    result.

-   Fill in the fourth column, **Acceptable Range of Scale Indication,
    Minimum Pounds**, by subtracting the value in the third column
    (tolerance in pounds) from the value in the first column (test load)
    and recording the result.

-   Fill in the fifth column, **Acceptable Range of Scale Indication,
    Maximum Pounds**, by adding the value in the third column to the
    value in the first column.

You will learn how to use the completed worksheet as an aid in
conducting the tests in Chapter 6. For the present, review each of the
following examples to make sure that you understand how the tolerances
are determined. Begin by reading the notes which precede each example,
which are intended to clarify some of the items relating to test
objectives and procedures (these topics will be reviewed later in the
module). When you have reviewed each of the three examples, proceed to
Example 4 and complete the blank worksheet that follows the example
(except that, you do not have to complete the "Acceptable Range of Scale
Indication" columns). Make notes of any questions you have, so that you
can go over them with your instructor.

**Example 1**

**Tolerance Worksheet for a Scale with a Full-Capacity Weighbeam**

[Scale Data]{.underline}

> Scale with full capacity beam
>
> Scale capacity: 100,000 × 10 lb (fractional bar 990 lb × 10 lb)
>
> Section capacity: 50,000 lb
>
> [Not]{.underline} marked with accuracy class
>
> Since the scale has been in service over 30 days since installation or
> repairs, maintenance tolerances apply.

[Available certified test weights]{.underline}: 30,000 lb

[Notes]{.underline}

**Increasing-Load Test and Shift Test Tolerances**. The Increasing-Load
Test determines whether or not the scale performs accurately as larger
loads are applied to the scale. Use tolerances from Table 6 for the
Increasing-Load Test. List test loads at the tolerance break points to
the limits of available test weights or CLC or section capacity. The
shift test determines the performance of the scale under off-center
loading and provides an opportunity to check the agreement of scale
indications. Table 6 values apply to each reading taken during the shift
test; however, the range of results obtained during the test must not
exceed the absolute value of the maintenance tolerance applicable. The
shift test should be conducted with test loads equal to one-half and the
full CLC or section capacity if an adequate amount of certified test
weights (equivalent to the CLC or section capacity) is available; if
not, the test should be conducted with one-half and the full amount of
certified test weights.

**Strain-Load Test Tolerances**. With vehicle and axle-load scales,
certified test weights to the capacity of the scale are seldom
available. In the strain-load test, a heavy load --- often a vehicle ---
is applied and standard test weights are added. Tolerances are applied
only to the "added weight value" of the standard weights applied. A full
explanation of procedures for the Strain-Load Test will be presented in
Chapter 6.

**Zero-Load Balance Change**. When all test load is removed from the
scale deck, the scale should return to zero within the tolerance for
zero load.

  -----------------------------------------------------------------------
  ![](media/image1.png){width="6.49375in" height="7.794444444444444in"}

  **Figure 4-3. Sample Tolerance Worksheet, Scale With a\
  Full Capacity Weighbeam**
  -----------------------------------------------------------------------

**Example 2**

**Tolerance Worksheet for Mechanical Dial Scale**

[Scale Data]{.underline}

> Vehicle scale equipped with dial and printer
>
> Dial capacity: 20,000 lb
>
> Equipped with four 20,000-lb unit weights
>
> Section capacity: 50,000 lb
>
> Scale capacity: 100,000 lb
>
> Scale division: 20 lb
>
> [Not]{.underline} marked with accuracy class
>
> Since the scale has just gone back into service last week after being
>
> repaired, acceptance tolerances apply.

[Available certified test weights]{.underline}: 30,000 lb

[Notes]{.underline}

**Increasing-Load Test**. Test at least at each quarter of the dial
face. Also test every unit weight possible, at least one test load per
unit weight.

**Decreasing-Load Test**. This test is performed only for
automatic-indicating scales. It tests the ability of the scale to
perform as strain is gradually removed. Comparison of performance on
this test with performance on the Increasing-Load Test may indicate the
condition of the equipment; the greater the difference, the stronger the
indication that system components are worn or fittings are loose.

+-----------------------------------------------------------------------+
|                                                                       |
+-----------------------------------------------------------------------+
| ![](media/image2.png){width="5.995138888888889in"                     |
| height="7.354166666666667in"}                                         |
+-----------------------------------------------------------------------+
| #                                                                     |
|                                                                       |
| #                                                                     |
|                                                                       |
| # Figure 4-4. Sample Tolerance Worksheet, Mechanical Dial Scale       |
+-----------------------------------------------------------------------+

**Example 3**

**Scale Equipped with Electronic Digital Indicator**

[Scale Data]{.underline}

> Electronic digital vehicle scale
>
> CLC: 50,000 lb
>
> Scale capacity: 100,000 lb
>
> Scale division: 20 lb
>
> The scale is marked accuracy class III L
>
> It has been in service more than 6 months; therefore, maintenance
>
> tolerances will apply.

[Available certified test load]{.underline}: 30,000 lb

Tests are similar to those for scales with mechanical dial indicators;
except that, test loads for the Increasing-Load Test are set at
tolerance break points.

  -----------------------------------------------------------------------

  ![](media/image3.png){width="5.998611111111111in"
  height="7.789583333333334in"}

  **Figure 4-5. Sample Tolerance Worksheet, Scale With\
  Electronic Digital Indicator**
  -----------------------------------------------------------------------

**Example 4**

**Electronic Vehicle Scale**

[Scale Data]{.underline}

> Electronic digital vehicle scale
>
> Section Capacity: 70,000 lb
>
> Scale Capacity: 100,000 lb
>
> Scale Division: 20 lb
>
> The scale is not marked with an accuracy class designation.
>
> The scale is being returned to service after major overhaul;
> therefore, acceptance tolerances will apply.

[Available certified test load]{.underline}: 30,000 lb

Based upon this data, complete the tolerance worksheet on the next page.

TOLERANCE WORKSHEET

ELECTRONIC --- VEHICLE OR AXLE-LOAD SCALE

\- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-

CLC or SECTION CAPACITY: lb 500 d - lb

SCALE CAPACITY: lb 1000 d - lb

VALUE OF SCALE DIVISION: lb 1500 d - lb

NUMBER OF SCALE DIVISIONS: 2000 d - lb

AVAIL. CERT. TEST WEIGHTS: lb 2500 d - lb

MARKED III L yes no 3000 d - lb

3500 d - lb

TOLERANCE TO BE APPLIED: ACCEPTANCE MAINTENANCE

\- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-

ACCEPTABLE RANGE

TEST TOL. TOL. OF SCALE INDICATION

TEST LOAD LOAD IN IN MINIMUM MAXIMUM

DESCRIPTION POUNDS "d" POUNDS POUNDS POUNDS

\- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-

INCREASING-LOAD AND SHIFT TEST

at each tol. break point:

SHIFT TEST

at 1/2 CLC, sect. cap., or test load:

range of results must be within:

CONTINUE INCREASING-LOAD AND SHIFT TEST

at each tol. break point:

SHIFT TEST

at CLC, sect. cap., or test load cap.:

range of results must be within:

DECREASING-LOAD TEST

at one-half avail. load:

STRAIN-LOAD TEST

certified test load only:

ZERO-LOAD BALANCE CHANGE:

\- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-

## [Summary]{.underline}

In an ideal situation, a vehicle or axle-load scale would perform
without error at every weight value throughout its range. In practice,
however, the precision of construction and adjustment of equipment will
vary. Repeated use of the equipment will cause wear at critical points,
thus changing the accuracy of weight values delivered. Consequently,
official tolerance values are set that are small enough to protect all
parties, yet large enough to insure that purchase and maintenance of
equipment will not be unreasonably costly.

Before you can test a scale for compliance with allowable tolerance
values, you must determine the proper tolerances to apply. Applicable
tolerances are based upon the amount of time in service since
installation, a major repair, or corrective action, the size of test
loads, and test parameters.

It is good practice to construct a tolerance worksheet that shows
applicable tolerances for different test loads and acceptable ranges of
scale indications.