2015 NIST EPO No. 5

Prescription & Jeweler Scales-Mechanical (Rev 09/14) DRAFT Page 1 of 22

Examination Procedure Outline for

Prescription and Jeweler Scales Part 1 – Electronic

EPO No. 5 is divided into two parts. It is recommended that this outline be followed as minimum criteria for 

examining electronic digital indicating scales and balances used in prescription and jeweler applications. Included 

within Part 1, on pages 12 through 16, are procedures for verifying proper operation of a legal for trade counting 

feature on a Class I and Class II prescription scale. Part 2, which applies to mechanical equal arm scales and 

balances used in prescription and jeweler applications, begins on page 17.

Throughout the EPO, requirements that apply only to scales marked with an accuracy class designation are indicated 

with an asterisk (*). Nonretroactive requirements are followed by the applicable date in parentheses. 

Prescription scales are often located and used in areas where controlled substances are stored. Access to these areas, 

except by qualified personnel, is generally prohibited. Do not enter any restricted areas unless authorized to do so 

by personnel having proper authority to grant such access. 

SAFETY NOTES

When excerpting this Examination Procedure Outline for duplication, the NIST EPO Safety Annex (Safety 

Considerations and Glossary of Safety Key Phrases) should be duplicated and included with this outline.

Safety policies and regulations vary among jurisdictions. It is essential that inspectors or servicepersons be 

aware of all safety regulations and policies in effect at the inspection site and to practice their employer’s safety 

policies. The safety reminders included in this EPO contain general guidelines useful in alerting inspectors and 

servicepersons of the importance in taking adequate precautions to avoid personal injury. These guidelines can 

only be effective in improving safety when coupled with training in hazard recognition and control.

Prior to beginning any inspection, the inspector should read and be familiar with the NIST EPO Safety Annex -

“Safety Considerations and Glossary of Safety Key Phrases." The terms and key phrases in each safety reminder of 

this outline are found in the glossary of the EPO Safety Annex. The inspector is reminded of the importance of 

evaluating potential safety hazards prior to an inspection and taking adequate precautions to avoid personal injury or 

damage to the device. As a minimum, the following safety precautions should be noted and followed during the 

inspection. 

Clothing Personal Protection Equipment

e.g., Safety Shoes

Electrical Hazards Support – for Scale and Test Weights

First Aid Kit Transportation of Equipment

Lifting

Also: Chemicals and Hazardous Materials or Obstructions
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Equipment List:

Mass Standards Required1 Special Handling Equipment

Class I Scales: OIML E-2, ASTM 1, or standards of 

greater accuracy. Clean lint-free and dust-free gloves

Tweezers

Ash-free, acid-free filter paper

Class II Scales: OIML F-1, ASTM 3, or standards of 

greater accuracy.

Class III Scales: NIST Class F, or standards of greater 

accuracy.

Unmarked Scales: Use standards of the proper level of 

accuracy that comply with NIST Handbook 44 

Fundamental Consideration as detailed in footnote 1.

Note: The conversion tables for units of measurement commonly indicated on jewelers and prescription scales are listed in 

Appendix A to EPO No. 5. Verify the performance of the scale at multiple test points when different units of measurement are in 

commercial use for a particular application. 

Special care is required when handling mass standards to maintain the accuracy level needed to properly test most 

prescription and jewelers scales and scales marked Class I or Class II. 

1. Never touch standards with bare hands or otherwise contaminate standards by placing them on dirty surfaces.

2. Always wear suitable gloves or use tweezers when handling precision standards to prevent substances (oils, lint, 

moisture, etc.) from adhering to their metal surfaces. 

3. Special filter papers are available and should be used to prevent contact with dirty surfaces. 

4. Proper cleaning methods are to be used. 

5. Exercise care in storing and transporting standards to avoid physical damage.

H-44 General Code 

and Scales Code 

References

Inspection:

1. Accessibility for inspection, testing, and sealing...........................................................

2. Zero-load balance as found............................................................................................

Zero indication........................................................................................................

Zero-load adjustment ....................................................................................................

Zero-tracking (scales manufactured between 1/1/81 and 12/31/06).......................

Zero-tracking (scales manufactured on or after 1/1/07) .........................................

G-UR.2.3

UR.4.1.

G-S.5.2.2.(d), (1/1/86), 

S.1.1., S.1.1.1.(a), 

S.1.1.1.(b), (1/1/93)

S.2.1.1., S.2.1.2.

S.2.1.3.1.(a)

S.2.1.3.2.(b)

 1 In accordance with NIST Handbook 44, Fundamental Considerations Section 3 paragraph 3.2., the combined error 

and uncertainty of any standard used for testing must be less than one-third the applicable device tolerance. The use 

of the mass standards indicated for each of the scale accuracy classes listed will ensure conformance with this 

fundamental consideration. 
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H-44 General Code 

and Scales Code 

References

Inspection (Cont.):

3. Marking:

Identification (Refer to list of required markings provided in the table that follows)... G-S.1., S.6., S.6.3., 

Table S.6.3.a, 

Table S.6.3.b.

List of Required Markings for Jeweler and Prescription Scales

Manufacturer’s ID or Name Special Application (1/1/86)

Model Identifier Nominal Capacity 1

Acceptable Model Identifier Prefix 

(1/1/03)

Nominal Capacity and Value of Scale 

Division, “d” Displayed Together (1/1/83)

Serial Number (1/1/68)

Value of the Verification Scale Division, “e” 

if different from the value of the scale 

division “d” (1/1/86)

Serial Number Prefix (1/1/86) Temperature Limits Class III Scales 

(1/1/86)2 

Acceptable Abbreviations for “Serial” 

and “Number” (1/1/01)

Certificate of Conformance (CC) Accuracy Class (1/1/86)

Number or CC Addendum Number and 

Prefix (1/1/03)

1 The nominal capacity of a prescription scale that is not marked with an accuracy 

class designation can be assumed to be 1⁄2 apothecary ounce, unless otherwise 

marked. (Reference N.6.) 2 Applies only when the temperature range on the NTEP CC is narrower than and 

within -10 o

C to 40 oC (14 oF to 104 oF).

Note: Be aware that the required marking information included in this table was extracted from 

the 2013 edition of NIST Handbook 44 and is susceptible to change in future editions. 

Marking (continued):

Devices or main elements remanufactured after January 1, 2002. .........................

Interchange or reversal of parts. .............................................................................

Operational controls, indications, and features. .....................................................

Lettering .................................................................................................................

Visibility of identification ......................................................................................

4. General Considerations:

Selection of equipment...........................................................................................

Typical class for weighing applications. ................................................................

Accuracy class designation and parameters. ..........................................................

Nominal capacity of Prescription Scale..................................................................

Recommended Minimum Load.............................................................................. 

Maximum load. ......................................................................................................

Permanence. ...........................................................................................................

G-S.1.2

G-S.4.

G-S.6. (1/1/77)

G-S.7.

G-UR.2.1.1.

G-UR.1.1., G-UR.1.2., 

UR.1.

UR.1.1., Table 7a*

S.5.1.* (1/1/86), S.5.2.* 

(1/1/86)

N.6.

UR.3.1.

U.R.3.2.

G-S.3.
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Inspection (Cont.):

5. Installation. ....................................................................................................................

Indicating or recording element..............................................................................

Position of equipment (not applicable to prescription scales)................................

Initial zero-setting mechanism. ..............................................................................

Supports..................................................................................................................

G-UR.2.1.

G-UR.2.2.

G-UR.3.3.

S.2.1.5.(a), S.2.1.5.(b), 

(1/1/09)

UR.2.1.

SAFTEY REMINDER

− Check to ensure that scale supports provide a firm foundation for the scale under 

all loading conditions, including loading of the platform to device capacity!

Level indicating means and condition....................................................................

Customer Indications..............................................................................................

S.2.4., (1/1/86), UR.4.2.

S.1.8.4.

6. Design of indicating and recording elements: ...............................................................

Value of the scale division .....................................................................................

Recorded representations, General.........................................................................

Indicated and recorded representation of units (Appropriate abbreviations):

Equipment manufactured on or after January 1, 2008............................................

Equipment manufactured prior to January 1, 2008 ................................................

Multi-interval and multiple range scales................................................................

Value of the verification scale interval ..................................................................

Value of the tare division .......................................................................................

Tare mechanism .....................................................................................................

Combined Zero-Tare (“0/T”) key...........................................................................

Damping means......................................................................................................

Prepackaging scales................................................................................................

7. Design of the weighing element. ...................................................................................

8. Security and sealing:

Adjustable components ..........................................................................................

Adjustment mechanisms designed to be sealed......................................................

Provisions for sealing (does not apply to Class I) ..................................................

S.1.2. * (1/1/86), S.1.2.1. 

(1/1/89), G-S.5.3., 

G-S.5.3.1.

UR.1.3. (1/1/86), 

UR.1.3.1. (1/1/86)

G-S.5.6.

G-S.5.6.1. (a)

G-S.5.6.1. (b)

S.5.3.

S.1.2.2.1., S.1.2.2.2.

S.2.3. (1/1/83)

S.2.3.

S.2.1.6.

S.2.5.

S.1.9.1., S.1.9.2.

S.4.

S.1.10.

G-UR.4.5.

G-S.8. (1/1/90), 

S.1.11.(a) (1/1/79), 

S.1.11. (b) (1/1/90), 

S.1.11.(c) (1/1/95)
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References

Inspection (Cont.):

9. Maintenance, use, and environmental factors:

Facilitation of fraud................................................................................................

Protection from environmental factors...................................................................

Method of operation ...............................................................................................

(scales having special designs)...............................................................................

Maintenance of equipment .....................................................................................

Abnormal performance...........................................................................................

Use of adjustments.................................................................................................

Scale modification..................................................................................................

10. Assistance ......................................................................................................................

Pretest Determinations:

1. Verification scale interval (e) .......................................................................................

For Class I and II scales equipped with a verification scale division (e) 

that does not equal the value of the displayed scale division (d), verify 

that the relationship between “e” and “d” conforms to the expression: 

d < e ≤ 10 d

For Class III scales that are marked with a manufacturer’s designated 

verification scale division (e), verify that the value of “e” is less than or 

equal to the value of “d.”

2. Tolerances:

Acceptance/maintenance ........................................................................................

Application.............................................................................................................

Tolerance values:

Determine the number of scale divisions (n) 2 using the following formula:

 

G-S.2.

UR.2.3.

G-UR.3.1.

UR.3.5.

G-UR.4.1.

G-UR.4.2.

G-UR.4.3.

UR.4.3.

G-UR.4.4.

S.1.2.2.1.*, S.1.2.2.2.*

G-T.1., G-T.2.

G-T.3., G-T.4., 

T.N.2.1.*, T.N.2.2.*, 

T.N.2.3.*

 2 In accordance with Handbook 44 Scales code Table 3, footnote 4, on a multiple range or multi-interval scale, the 

number of divisions for each range independently shall not exceed the maximum specified for the accuracy class. 

The number of scale divisions, n, for each weighing range is determined by dividing the scale capacity for each 

range by the verification scale division, e, for each range (i.e., do not add “n” for the ranges together). On a scale 

system with multiple load-receiving elements and multiple indications, each element considered shall not 

independently exceed the maximum specified for the accuracy class. If the system has a summing indicator, the nmax 

for the summed element shall not exceed the maximum specified for the accuracy class. (Added to Table 3 in 1997)
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and Scales Code 

References

Pretest Determinations (Cont.):

n 

Value of the verification scale division (e)

Scale capacity =

Scales marked with an accuracy class....................................................................

Scales not marked with an accuracy class..............................................................

3. Discrimination ...............................................................................................................

4. Minimum test weights and test loads.............................................................................

Test Notes:

1. Scales equipped with Automatic Zero-Tracking (AZT) Mechanism: 

manufactured between 1/1/81 and 1/1/07..............................................

manufactured on or after 1/1/07.............................................................

To verify correct operation of an AZT feature on a scale in which the value of the 

scale division (d) is equal to the value of the verification scale division (e), (i.e., the 

scale manufacturer has not declared a verification scale division) complete step a. 

through step f. below. 

a. Zero the scale with no load on the platter.

b. Apply all at once test weights equal to 0.7 d. 

c. Verify that the indicator is continuously displaying the value of 1 d. 

d. Zero the scale with the 0.7 d test weights remaining on the platter. 

e. Remove the 0.7 d test weights all at once.

f. The scale must display a continuous behind zero indication using one of the 

following means:

 display a value equal to minus 1 d;

 blank the display; or

 display error symbols that cannot be interpreted as a weight value.

 

If the scale is equipped with a verification scale division (e) that is not equal to the 

value of the scale division (d), the value of “d” rather than “e” is used to calculate the 

maximum permissible amount of weight that can be rezeroed during this test. 

However, in instances where multiplying the value of the scale division (d), whether 

or not the value of “d” and “e” are equal, by 0.7 results in a value that is smaller 

than the smallest available test standard, a field test of the AZT feature is not 

possible. 

T.N.3.1., Table 6, 

T.N.3.2., T.N.4, T.N.5

T.1.1., Table T.1.1.

T.N.7.2.*

N.3., Table 4

S.2.1.3.1.(a)

S.2.1.3.2.(b)
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Test Notes (Cont.):

Be aware that step a. through step c. verify the correct operation of an AZT feature 

on the positive side of zero while step d. through step f. verify the correct operation 

of an AZT feature behind zero. A continuous display of 1 d (in step c.) and a 

continuous display of a behind zero indication (i.e., in accordance with any of the 

means noted in step f.) confirms the correct operation of an AZT feature on both 

sides of zero. However, if the displayed indication is zero following the completion 

of step b. or step e., the AZT feature may not be functioning properly. In this case, 

verify the feature is not functioning properly by repeating the AZT test before 

rejecting the device. 

2. If the scale is equipped with a semiautomatic zero-setting mechanism (i.e., 

pushbutton zero), a pushbutton tare feature, or a ticket printer, verify the correct 

operation of motion detection........................................................................................

3. If the scale is equipped with a ticket printer, print a ticket at several test loads. Verify 

that recorded values agree with corresponding indicated values and that all values are 

appropriately and correctly identified on the ticket. If scale is equipped with

computing capability, verify correct money value agreement.......................................

Also verify that any options for obtaining a recorded representation are appropriate. 

The customer may be given the option of not receiving the recorded representation. 

If the system is equipped with the capability, the customer may also be given the 

option of receiving the recorded representation electronically in lieu of or in addition 

to a hard copy. ...............................................................................................................

S.2.1.2.(b), S.2.5.1.(b) 

G-S.5.1, G-S.5.2. 

(except G-S.5.2.1.), 

G-S.5.3., G-S.5.3.1., 

G-S.5.4., G-S.5.5., 

G-S.5.6.

G-S.5.6.

4. Check proper design of tare auto-clear, if scale is so equipped.....................................

5. Discrimination. ..............................................................................................................

S.2.3. (1/1/83)

N.1.5. (1/1/86), N.1.5.1., 

T.N.7.2.*

Note: A test for discrimination should be performed when environmental conditions permit 

and when the test standards available for testing are adequate in amount and are of suitable 

denominations. Steps a. through d., of the Procedures for Testing Discrimination at or 

Near Zero Load and Procedures for Testing Discrimination Near Maximum Capacity

indicated below, specify the total amount and the minimum denominations of the test 

standards needed to properly perform a discrimination test. 
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H-44 General Code 

and Scales Code 

References

Test Notes (Cont.):

Procedures for Testing Discrimination

at or Near Zero Load for a Scale, where d = e

The procedures for testing discrimination near zero load on scales equipped with a 

scale division value (d) that is equal to the value of the verification division (e), 

(i.e., d = e) are indicated in step a. through step f. below and illustrated in Figure 1.

a. With the scale on zero, place decimal weights on the scale equal to the value 

of 1 d.

b. Zero the scale and place a load equal to the value of 5 d on the platter.

c. Remove the decimal weights in 0.1 d increments until the indication flickers 

between the values representing 4 d and 5 d. If the indication does not flicker 

but indicates a steady 4 d value, add 0.1 d. If the scale indicates 5 d, it is at the 

breakpoint in the zone of uncertainty. (remove the 0.1 d if it was added to the 

4 d value to verify the breakpoint)

d. Add a test load equal to 1.4 d to the scale.

e. The indication should read a stable 6 d (i.e., the addition of 1.4 d should change 

the displayed indication 2 d)

f. If the scale passes the discrimination test at a load near zero, a discrimination 

test should also be performed near the maximum capacity of the scale.

Example: Discrimination Test near Zero Load for a Scale, where d = e

Scale capacity: 610 x 0.01 g
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References

Test Notes (Cont.):

Procedures for Testing Discrimination

near Maximum Capacity for a Scale, where d = e

The procedures for testing discrimination near maximum load on scales equipped 

with a division value (d) that is equal to the value of the verification division (e), are 

indicated in step a. through step e. below and illustrated in figure 2.

a. With the scale on zero and no load on the load-receiving element, apply test 

weights equal to 1.4 d and zero the device.

b. Add a test load sufficient in amount to cause the scale to indicate a stable value 

near maximum capacity.

c. With the scale indication stable at a near capacity test load, add test standards in 

0.1 d increments until the indication continually flickers back and forth between 

the indication that was stable and the next higher increment. Then add 0.1 d to 

cause the indication to become stable at the higher of the two flickering 

increments.

d. Remove the 1.4 d test weights.

e. To pass the discrimination test near maximum capacity, the removal of the 1.4 d 

test weights should cause the indication to change (decrease) by 2 d.

Example: Discrimination Test Near Maximum Capacity for a Scale, where d = e

Scale capacity: 610 x 0.01 g
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and Scales Code 

References

Test Notes (Cont.):

The procedures for testing discrimination near zero load and near maximum scale 

capacity when the value of the minimum scale division (d) is not equal to the value of 

the minimum verification scale division (e), are as follows:

a. Zero the scale with no load on the platter.

b. Apply a test load equal to 2 e to the platter and observe the displayed indication.

c. Apply an additional test load of 2 d to the platter and observe the displayed 

indication.

d. The addition of 2 d in step c. must increase the displayed indication by 2 d. 

e. For the discrimination test at or near maximum capacity, apply a test load near 

scale capacity, then remove 2 d and observe the change in the displayed indication. 

f. The displayed indication must change by 2 d when a test load of 2 d is removed.

g. The results of these tests confirm that a scale is able to detect small changes in 

weight being added or removed from the platter over the scale’s entire weighing 

range.

Tests:

1. Level condition and Zero-load balance (verify before beginning the increasing-load 

test).

2. Increasing-load test (test loads approximately centered on the platter).......................

For scales marked with an accuracy class, test at several points in each tolerance 

range including at or near the highest test load that can be applied to the scale 

without the tolerance increasing to the next tolerance range. For scales not marked 

with an accuracy class, test at several points to capacity, including test loads at or 

near one-quarter, one-half, and three-quarters of scale capacity and at full capacity. 

3. Shift test (one-third nominal capacity test load3

)........................................................

The shift test can be conducted during the increasing load test once one-third 

capacity test load is achieved.

N.1.1.

N.1.3.7.(a), T.N.4.4.*

 3 NIST Handbook 44 Scales Code paragraph N.1.3.7.(a) defines one-third nominal capacity test load as test weights 

in amounts of at least 30% of scale capacity, but not to exceed 35% of scale capacity.
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Tests (Cont.):

Shift Test Pattern

(One-third capacity test load)

The numbered boxes identify the positions for placing the shift test load.

4. RFI/EMI (if a problem is suspected) Radio Frequency Interference Electromagnetic 

Interference. ...............................................................................................................

5. Check repeatability of, and agreement between indications throughout the test. .........

6. Over-capacity test..........................................................................................................

7. Decreasing-load test ......................................................................................................

For scales marked with an accuracy class and having a total number of scale divisions 

(d) or verification scale divisions (e), if “e” has been declared, greater than or equal to 

100 000 for Class I scales or 10 000 for Class II scales, test with loads equal or near 

the highest value of each tolerance band. For example, on a Class II scale, test at 

loads equal or near 20 000 d and 5000 d if a verification scale division has not been 

declared and at or near 20 000 e and 5000 e if a verification scale division has been 

declared. For all other scales, test at one-half maximum test load applied during the 

increasing-load test.

8. Return to zero – verify the scale returns to a zero indication after the entire test 

load has been removed .................................................................................................

G-N.2., G-UR.1.2., 

G-UR.3.2., G-UR.4.2., 

N.1.6., T.N.9*

G.S.5.2.2.(a), G-S.5.4., 

T.N.5.

S.1.7 (a), S.1.7.(b) 

(1/1/93)

N.1.2., N.1.2.1*, 

N.1.2.2.

N.1.9., G-UR.4.2. 

4

2

3

Figure 3. Rectangular Platter

Shift Test Pattern.

1

2 4

3

Figure 4. Circular Platter

Shift Test Pattern.

1
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It is recommended that the additional requirements in this outline be followed for a Class I or 

Class II prescription scale equipped with a legal for trade counting feature. The counting 

feature is based on commodity weight; therefore, accuracy of the weighing function must be 

verified before conducting a test of the counting feature.

Additional Tests for Prescription and Jeweler Scales with a 

Legal for Trade Counting Feature.

H-44 General Code 

and Scales Code 

References

Inspection:

1. Marking: 

minimum individual piece weight and minimum sample piece count ...................

special application. .................................................................................................

operational controls, indications, features..............................................................

2. Zero indication of count. ...............................................................................................

A scale with a legal for trade counting feature shall be capable of indicating 

zero in the count mode of operation. 

3. Tare mechanism ............................................................................................................

Value of the tare division .......................................................................................

An operational counting feature is only legal for prescription filling 

applications on a compliant 

Class I or Class II device.

A prescription scale with a legal for trade counting feature must be marked:

“Counting Feature for Prescription Filling Only”

Class I or Class II prescription scales equipped with an operational counting 

feature that does not comply with all applicable NIST Handbook 44 

requirements for the count feature must be marked on both the customer and 

operator’s side:

“The Counting Feature is Not Legal for Trade.”

S.6.6.

Table S.6.3.a., 

Table S.6.3.b. (note 13)

G-S.6.

S.1.1.

S.2.3.

S.2.3. (1/1/83)
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Additional Tests for Prescription and Jeweler Scales with a 

Legal for Trade Counting Feature.

H-44 General Code 

and Scales Code 

References

Pretest Determinations:

1. Tolerances for count......................................................................................................

2. Determine the scale’s maximum counting capacity (i.e., the maximum number of 

pieces the scale is capable of indicating) by inserting the appropriate corresponding 

values marked on the device into the following formula: 

minimum individual piece weight

nominal capacity

For example: Given a scale marked as follows: 

Capacity: 610 x 0.01 g 

Minimum individual piece weight: 0.03 g

Minimum sample piece count: 10 pieces

Using the formula, the maximum count value in pieces is calculated as follows:

20 333.3333 pieces

0.03 g/piece

610 g =

Then truncate the result to the nearest whole number of 20 333 pieces. 

3. Select at least three count values to verify the count accuracy of a legal for trade 

counting feature similar to that illustrated below:

 

A count value between 90 and 100 pieces inclusive;

A count value between 180 and 200 pieces inclusive; and 

A count value equal or near the scale’s maximum counting capacity as 

determined in 2. above. 

Note: When choosing the count value near a scale’s maximum counting capacity, select a whole 

number that is slightly lower than the maximum possible count. For example, one possible 

choice at the maximum end of the counting capacity for the scale described above is 

20 300. The resulting value of 20 300 is slightly less than the actual truncated value of 

20 333 and is less likely to introduce rounding errors when converted to a corresponding 

weight value. The result is also more practical when selecting denominations of test 

standards for use in testing the count accuracy than if 20 333 were used.4

T.N.3.10., 

Table T.N.3.10

 4 When the 20 300 piece count is converted to weight by multiplying it by the scale’s marked “minimum individual 

piece weight” of 0.03 g, the resulting weight value equals 609 g. If 20 333 is multiplied by this same factor, the 

resulting weight value equals 609.99 g. It would be considered acceptable to perform the test at 20 333 pieces using 

a test load of 609.99 g, providing test standards were available in small enough denominations for converting this 

count value to its exact equivalent in weight; however, it is easier and more practical to use test standards in 

denominations of whole grams. 
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Additional Tests for Prescription and Jeweler Scales with a 

Legal for Trade Counting Feature.

H-44 General Code 

and Scales Code 

References

Pretest Determinations (Cont.):

4. Convert the selected count values to weight by multiplying each count value by the 

minimum individual piece weight5 marked on the device. 

Given the same device for this example, convert selected count values of 100, 190, 

and 20 300 pieces to weight using the formula: piece count x minimum individual 

piece weight = amount of test weight to be applied.

 

100 x 0.03 g = 3 g

190 x 0.03 g = 5.7 g

 20 300 x 0.03 g = 609 g

Test Notes:

1. Accuracy of test standards ............................................................................................

The combined error and uncertainty in any test standard used for testing must 

be less than one-third the applicable tolerance. Refer to the list of acceptable 

standards for use in testing precision scales on page 5E-2 of this EPO.

2. Special handling precautions for test standards:

Special care is required when handling mass standards to maintain the 

accuracy level needed to properly test most prescription and jewelers scales 

and scales marked Class I or Class II. Never touch standards with bare hands 

or otherwise contaminate standards by placing them on dirty surfaces. 

Always wear suitable gloves or use tweezers when handling precision 

standards to prevent substances (oils, lint, moisture, etc.) from adhering to 

their metal surfaces. Special filter papers are available and should be used to 

prevent contact with dirty surfaces. Proper cleaning methods are to be used. 

Exercise care in storing and transporting standards to avoid physical damage.

3. If scale is equipped with a ticket printer, print a ticket at several test loads. Verify 

that recorded values agree with corresponding indicated values and that all values are 

appropriately and correctly identified on the ticket. .....................................................

Fundamental 

Considerations

G-S.5.1., G-S.5.2. 

(except G-S.5.2.1.), 

G-S.5.3., G-S.5.3.1., 

G-S.5.4., G-S.5.5., 

G-S.5.6.

 5 Although the weight of an individual piece in any given prescription to be counted will often be greater than a 

scale’s marked minimum individual piece weight, the sample used for the purpose of verifying count accuracy is 

established from minimum values (i.e., minimum individual piece weight and minimum sample piece count) marked 

on the device.
2015 NIST EPO No. 5

Part 1 - Electronic

Prescription & Jeweler Scales-Electronic (Rev 09/14) Page 15 of 22

Additional Tests for Prescription and Jeweler Scales with a 

Legal for Trade Counting Feature.

H-44 General Code 

and Scales Code 

References

Tests:

Counting feature test ............................................................................................................

The counting feature test requires the scale meet all three test criteria listed in 1. 

through 3. below. It is first necessary to determine the “minimum sample size in weight” 

using the formula: 

Minimum sample size in weight = minimum individual piece weight x minimum 

sample piece count

The values for “minimum individual piece weight” and “minimum sample piece count,” 

must be appropriately marked and identified on a prescription scale equipped with a legal 

for trade counting feature. 

1. Minimum sample piece count........................................................................................

Verify that the scale’s counting feature will not accept a sample containing less than 

the “minimum sample piece count” by completing the following steps:

a. Verify that the scale is in the counting mode of operation. 

b. With the scale operating in the counting mode, apply a sufficient load to the 

platter to cause the displayed indication to equal the “minimum sample size in 

weight.” 

c. Attempt to input a sample piece count less than the “minimum sample piece 

count” marked on the device.

d. The counting feature must reject the entry.

2. Minimum individual piece weight of the sample ..........................................................

Verify that a scale’s counting feature will not accept the entry of an individual piece 

weight that is less than the “minimum individual piece weight” by completing the 

following steps:

a. Verify that the scale is in the counting mode of operation.

b. With the scale operating in the counting mode, apply a sufficient load to cause 

the displayed indication to be less than the “minimum sample size in weight.”

c. Attempt to enter the “minimum sample piece count” marked on the device.

d. The counting feature must reject the entry.

3. Count accuracy verification...........................................................................................

Complete the following steps in order to verify the accuracy of a legal for trade 

counting feature: 

a. Place the scale in the counting mode of operation. 

b. Zero the scale with no load on the platter.

N.1.10.

S.1.2.3.(b), S.2.5.3., 

N.1.10.

S.1.2.3.(a), S.2.5.3., 

N.1.10.

N.1.10, T.N.3.10.
2015 NIST EPO No. 5

Part 1 - Electronic

Page 16 of 22 Prescription & Jeweler Scales- Mechanical (Rev 09/14) DRAFT

Additional Tests for Prescription and Jeweler Scales with a 

Legal for Trade Counting Feature.

H-44 General Code 

and Scales Code 

References

Tests (Cont.):

c. Apply a load sufficient in amount to cause the displayed weight indication to 

equal the value of the “minimum sample size in weight” as marked on the 

device.

d. Enter the value of the “minimum sample piece count” as marked on the device 

using the keyboard or other input means.

e. Remove the sample load that was applied in step c.; scale must indicate zero 

pieces.

f. Apply test standards equivalent in value to each of the test points selected, 

including the test point near highest possible count.

g. Observe the displayed count indication after each test point load is applied and 

verify that no count errors exceed the applicable tolerances indicated in Table 

T.N.3.10.

h. Remove the entire test load; scale must indicate zero pieces.

i. Return the scale to the weigh mode, and verify a zero balance indication. 

 

4. Display of total quantity ................................................................................................

5. Minimum count .............................................................................................................

6. Correct stored piece weight ...........................................................................................

S.2.5.3.

UR.3.11

UR.3.12.
2015 NIST EPO No. 5

Prescription & Jeweler Scales-Mechanical (Rev 09/14) DRAFT Page 17 of 22

Examination Procedure Outline for

Jeweler and Prescription Scales Part 2 – Mechanical Equal Arm

It is recommended that this outline be followed for mechanical equal arm scales and balances used in prescription 

and jeweler applications. Requirements that apply only to scales marked with an accuracy class designation are 

indicated with an asterisk (*). Nonretroactive requirements are followed by the applicable date in parentheses.

Prescription scales are often located and used in areas where controlled substances are stored. Access to these areas, 

except by qualified personnel, is generally prohibited. Do not enter any restricted areas unless authorized to do so 

by personnel having proper authority to grant such access. 

Safety Notes – See EPO 5 Part 1

Equipment List – See EPO 5 Part 1

H-44 General Code 

and Scales Code 

References

Inspection:

1. Accessibility for inspection, testing, and sealing...........................................................

2. Zero-load balance as found............................................................................................

Zero indication........................................................................................................

Zero-load adjustment ..............................................................................................

Manual zero-setting mechanism (not applicable to Class I and Class II scales 

equipped with a balance ball).................................................................................

3. Marking:

Identification (Refer to list of required markings provided in EPO Part 1.) .................

Devices or main elements remanufactured after January 1, 2002 ..........................

Interchange or reversal of parts. .............................................................................

Operational controls, indications, and features. .....................................................

Lettering .................................................................................................................

Visibility of identification ......................................................................................

G-UR.2.3

UR.4.1.

S.1.1.

S.2.1.1.

S.2.1.2.

G-S.1., S.6., S.6.3., 

Table S.6.3.a, Table 

S.6.3.b.

G-S.1.2

G-S.4.

G-S.6. (1/1/77)

G-S.7.

G-UR.2.1.1.
2015 NIST EPO No. 5

Part 2 – Mechanical Equal Arm

Page 18 of 22 Prescription & Jeweler Scales- Mechanical (Rev 09/14) DRAFT

H-44 General Code 

and Scales Code 

References

Inspection (Cont.):

Marking (continued):

4. General Considerations:

Selection of equipment...........................................................................................

Typical class for weighing applications. ................................................................

Accuracy class designation and parameters. ..........................................................

Recommended minimum load................................................................................

Maximum load. ......................................................................................................

Permanence. ...........................................................................................................

5. Installation. ....................................................................................................................

Position of equipment (not applicable to prescription scales)................................

Supports..................................................................................................................

Level indicating means and condition....................................................................

Level condition.......................................................................................................

G-UR.1.1., G-UR.1.2., 

UR.1.

UR.1.1., Table 7a*

S.5.1.* (1/1/86), S.5.2.* 

(1/1/86)

U.R.3.1.*, Table 8*

U.R.3.2.

G-S.3.

G-UR.2.1.

G-UR.3.3.

UR.2.1.

S.2.4. (1/1/86)

UR.4.2.

SAFTEY REMINDER

− Check to ensure that scale supports provide a firm foundation for the scale under all 

loading conditions, including loading of the platform to device capacity!

6. Design of indicating and recording elements:

Appropriateness......................................................................................................

Value of the scale division .....................................................................................

Indicated and recorded representation of units (Appropriate abbreviations):

Equipment manufactured on or after January 1, 2008.....................................

Equipment manufactured prior to January 1, 2008 .........................................

Value of the verification scale interval ..................................................................

Value of the tare division .......................................................................................

Tare mechanism .....................................................................................................

Graduations and indications...................................................................................

Weighbeams and poises .........................................................................................

Adjustable components ..........................................................................................

Balance indicator....................................................................................................

Damping and arresting mechanism ........................................................................

7. Design of the weighing element. ...................................................................................

8. Security and sealing:

Adjustable components ..........................................................................................

Adjustment mechanisms designed to be sealed......................................................

G-S.5.1., G-S.5.2., 

(except G-S.5.2.2.)

S.1.2. * (1/1/86), 

G-S.5.3., G-S.5.3.1.

G-S.5.6.1. (a)

G-S.5.6.1. (b)

S.1.2.2.1., S.1.2.2.2.

S.2.3. (1/1/83)

S.2.3.

S.1.3., S.1.4., G-S.5.7.

S.1.5., S.1.6.

S.1.10.

S.2.2., S.2.2.1., S.2.2.2. 

(1/1/89)

S.2.5., S.2.5.2.

S.4.

S.1.10.

G-UR.4.5.
2015 NIST EPO No. 5

Part 2 – Mechanical Equal Arm

Prescription & Jeweler Scales-Electronic (Rev 09/14) Page 19 of 22

H-44 General Code 

and Scales Code 

References

 

Inspection (Cont.):

9. Maintenance, use, and environmental factors:

Facilitation of fraud................................................................................................

Protection from environmental factors...................................................................

Method of operation ...............................................................................................

(scales having special designs)...............................................................................

Maintenance of equipment .....................................................................................

Abnormal performance...........................................................................................

Use of adjustments.................................................................................................

Scale modification..................................................................................................

10. Assistance ......................................................................................................................

G-S.2.

UR.2.3.

G-UR.3.1.

UR.3.5.

G-UR.4.1.

G-UR.4.2.

G-UR.4.3.

UR.4.3.

G-UR.4.4.

Pretest Determinations:

1. Tolerances:

Acceptance/maintenance ........................................................................................

Application.............................................................................................................

Intermediate values ................................................................................................

Tolerance values:

Determine the number of scale divisions (n) using the following formula: 

n = 

Value of the verification scale division (e)

Scale capacity

Scales that are marked with an accuracy class designation

Principles.......................................................................................................

Tolerance application ....................................................................................

Maintenance tolerance values........................................................................

Acceptance tolerance values..........................................................................

Ratio tests......................................................................................................

Single indicating/recording element..............................................................

Repeatability of indications...........................................................................

Time dependence...........................................................................................

Scales that are not marked with an accuracy class designation

Tolerance application ....................................................................................

Ratio tests......................................................................................................

Single indicating/recording element..............................................................

Repeatability of indications...........................................................................

Note: Refer to Table T.1.1. Maintenance and acceptance tolerances are the same for 

unmarked prescription and jewelers scales.

2. Sensitivity Requirement (SR)

Marked scales:

Application ....................................................................................................

G-T.1., G-T.2.

G-T.3.

G-T.4.

T.N.1.*

T.N.2.*, T.N.2.3.*

T.N.3.1.* Table 6

T.N.3.2.*

T.N.2.5.

T.N.4.2.

G-S.5.4.

T.N.4.5.*, T.N.4.5.1.*

T.1.1., Table T.1.1. 

T.N.2.5.

T.N.4.2.

G-S.5.4.

T.N.6.*
2015 NIST EPO No. 5

Part 2 – Mechanical Equal Arm

Page 20 of 22 Prescription & Jeweler Scales- Mechanical (Rev 09/14) DRAFT

H-44 General Code 

and Scales Code 

References

Pretest Determinations (Cont.):

Test load ........................................................................................................

Minimum change...........................................................................................

Unmarked scales:

Application ....................................................................................................

Test load (prescription scales).......................................................................

Test load (jewelers scales).............................................................................

Minimum change...........................................................................................

3. Minimum test weights and test loads.........................................................................

T.N.6.1.(b)*

T.N.6.2.*

T.2.1.

T.2.3.

T.2.4.

T.3.

N.3., Table 4

Test Notes:

1. Check repeatability of indications at a minimum of two different test loads during 

the test. ........................................................................................................................

2. Verify that scale returns to zero-load balance each time the entire test load is 

removed........................................................................................................................

G-S.5.4.,

N.1.9., G-UR.4.2.

3. Verify scale accuracy. 

For scales not marked with an accuracy class designation two different methods 

may be used to verify compliance with accuracy requirements, i.e., the tolerance 

testing method or the error-weight testing method. Each test method procedure 

starts with the scale in a zero-load balance condition. 

Scales marked with an accuracy class designation should always be tested using the 

error-weight method since compliance with T.N.4.4. (shift test agreement) can only 

be determined if the precise amount of error in each shift test result is known. 

Tolerance Testing: Tolerance testing determines whether or not a scale is 

performing to within applicable tolerances. This method of testing does not reveal 

the precise amount of error in a scale. To use the tolerance test method, standards 

equal to the value of tolerance are added at each test load to the pan of the lighter 

side and the balance indicator is observed. The addition of standards equal to the 

value of applicable tolerance must change the position of rest of the indicator back 

to the initial zero-load balance indication or beyond for a scale to perform within 

acceptable limits. 

Error-Weight Testing: Error-weight testing determines the precise amount of error 

in a scale. To use the error-weight method, precision field standards (i.e., error 

weights) in minimum denominations equal to 0.1 d are added at each test load to the 

pan of the lighter side in the amount necessary to cause the position of rest of the 

indicator to return back to the initial zero-load balance indication. The weight 

values of the precision field standards (i.e., error weights) are then added together 

and summed, the total result being equal to the amount of error in the scale at that 

particular test load.

Note: In cases where precision field standards in denominations equal to 0.1 d are not 

readily available, the error-weight method of testing should not be performed.
2015 NIST EPO No. 5

Part 2 – Mechanical Equal Arm

Prescription & Jeweler Scales-Electronic (Rev 09/14) Page 21 of 22

H-44 General Code 

and Scales Code 

References

Test:

Commercial weights, (i.e., weights used in connection with commercial weighing devices) are not to be used in any 

of the performance tests described herein. Rather, performance testing of prescription and jewelers scales requires 

the sole use of suitable precision field standards. Guidelines for determining the necessary accuracy level of the 

precision field standards needed to properly test prescription and jewelers scales are included in the “Equipment 

List” section of this EPO.

1. Sensitivity test at zero load and maximum test load. ...................................................

2. Ratio test .....................................................................................................................

Ratio tolerances are to be applied whenever the multiple of the lever system is 

verified using suitable precision field standards of equal value applied to opposite 

pans. 

3. Increasing-load test .....................................................................................................

N.1.4

N.1.7.

N.1.1.

For scales marked with an accuracy class, test at several points in each tolerance range including at or near the 

highest test load that can be applied to the scale without the tolerance increasing to the next tolerance range. 

For scales not marked with an accuracy class, test at several points to capacity, including test loads at or near 

one-quarter, one-half, and three-quarters scale capacity and at full capacity. 

Testing weighbeams and dials: Verify the accuracy of each weighbeam or dial at a minimum of two points, 

(i.e., at or near half and full weighbeam or dial capacity). It is recommended that additional verifications be 

performed at test loads where these elements are regularly used. 

4. Shift test (at one-half capacity test load)...................................................................... N.1.3.2.

A shift test is to be conducted separately on each pan of an equal arm balance. The test is conducted with 

mass standards equal to one-half capacity centered on one pan and standards equal to one-half capacity 

centered successively at four points equidistant between the center and the front, left, back, and right edges of 

the opposite pan. Once the results of all four positions of the first pan have been determined, remove all test 

weight, and verify return to zero-load balance. Then conduct a shift test on the pan opposite the first pan, 

using these same loading procedures, except the test load is applied to the opposite pan.
2015 NIST EPO No. 5

Part 2 – Mechanical Equal Arm

Page 22 of 22 Prescription & Jeweler Scales- Mechanical (Rev 09/14) DRAFT

1

2

3 

4

1/2 capacity test load centered

Example: Shift Test Positions - Left Pan

1 

2 

3 

4

1/2 capacity test load centered

Example: Shift Test Positions - Right Pan

H-44 General Code 

and Scales Code 

References

Equal-Arm Scales

Shift Test Patterns - Rectangular Platter

(one-half capacity test load)

The numbered boxes identify the various positions for placing the shift test load.

Equal-Arm Scales

Shift Test Patterns – Circular Platter 

(one-half capacity test load)

The numbered boxes identify the various positions for placing the shift test load.

5. Return to zero-load balance (verify each time test load is removed) ...........................

6. Commercial Weights....................................................................................................

N.1.9.

H-44 Section 2.23. 

(Weights Code)

Note: Commercial weights should be tested on a precision balance using adequate standard weights, the errors of which, when 

used without correction, do not exceed one third of the smallest tolerance to be applied to the device under test. It may not be 

appropriate to apply the requirements in Section 2.23 to verify the accuracy of commercial weights used in connection with 

prescription and jewelers’ scales in every field environment. In some instances, accuracy can only be verified in a laboratory that 

is formally accredited or recognized (i.e., a laboratory having the necessary qualifications to perform the necessary calibrations 

given the accuracy level of the standards being tested, e.g., OIML E-2, ASTM 1, etc.). In addition, it is recommended that 

commercial weights be calibrated (i.e., their accuracy verified) at a frequency interval necessary to ensure continued compliance 

with applicable tolerances and that the calibration frequency interval be based upon the collection of actual performance data. 

4

3

2

1

1/2 capacity test load 

Example: Shift Test Positions - Left Pan

4

3

2

1

1/2 capacity test load 

Example: Shift Test Positions – Right Pan