Field Inspection Manual — Non-Automatic Weighing Devices

Part 3, Section 28: STP-28: Multi-interval and Multiple Range Devices

Reference

Sections 3, 5, 6, 8, 9, 16 and 31 to 33 of the Specifications Relating to Non-automatic Weighing Devices (1998). STP-3, STP-11 and STP-13.

Purpose

This section provides additional guidelines for the examination and testing of multi-interval and multiple range devices.

Note: A weight indicator tested separately will have been set and tested for a maximum number of intervals nmax and for a minimum value of verification scale interval emin across the range.

28.1 Multi-interval devices

Multi-interval device means a device having one weighing range divided into partial weighing ranges each with different scale intervals, with the weighing range determined automatically according to the load applied (pre-determined ranges/intervals), both on increasing and decreasing loads.

The limits of error are determined on the basis of the class of the device and the value of the verification scale interval e of the partial range corresponding to the load applied.

Intervals and capacities

28.1.1 On a multi-interval device, the value of the verification scale interval of a particular range must be lower than the value of the verification scale interval of the next range

(e1 < e2 < e3, etc.).

28.1.2 Each partial range of a multi-interval device must have the same value of actual scale interval for both increasing and decreasing loads. The actual scale interval d must equal the verification scale interval e.

28.1.3 Each partial range of a multi-interval device must have the number of scale intervals n required by section 3 of the Specifications.

Example: Each partial range of a Class III device must have a minimum of 500 n and a maximum of 10 000 n. The number of scale intervals n for each partial range is determined by dividing the scale capacity for the partial range by the verification scale interval e for that partial range. See the following tables.

Examples of devices that do not satisfy section 28.1.3

Example with the total number of scale intervals over the requirements
Partial Range e n Max ÷ e
First 0 – 3 kg 1 g 3 000 3 000 ÷ 1
Second 3 – 6 kg 2 g 3 000 6 000 ÷ 2
Third 6 – 60 kg 5 g 12 000Incorrect number of scale intervals 60 000 ÷ 5
Example with the total number of scale intervals under the requirements
Partial Range e n Max ÷ e
First 0 – 1 kg 1 g 1 000 1 000 ÷ 1
Second 1 – 5 kg 5 g 1 000 5 000 ÷ 5
Third 5 – 8 kg 20 g 400Incorrect number of scale intervals 8 000 ÷ 20

Example of a device that satisfies section 28.1.3

Example with the total number of scale within the requirements
Partial Range e n Max ÷ e
First 0 – 1 kg 1 g 1 000 1 000 ÷ 1
Second 1 – 5 kg 2 g 2 500 5 000 ÷ 2
Third 5 – 8 kg 5 g 1 600 8 000 ÷ 5

Maximum Capacity of Partial Weighing Ranges (Recommendation Only — OIML R76 3.3.3)

28.1.4 With the exception of the last partial weighing range, the requirements below shall be complied with according to the accuracy class of the device.

Table 4
Class I II III IIIHD IIII
Maxi ÷ e(i + 1) ≥50 000 ≥5 000 ≥500 ≥500 ≥50

Tare features

28.1.5 The maximum tare value that may be entered shall not exceed Max1.

28.1.6 Whenever gross and tare weights fall in different weighing ranges, the net weight must always be in mathematical agreement with the gross and tare weights that are displayed and recorded (simultaneously or in sequence) (net = gross - tare).

28.1.7 The tare value must be equal to the value of the displayed scale division for all methods of tare entry (dtare = d). An attempt to enter, in the first range of a multi-interval scale, a tare value that is not equal to d must be rejected or rounded off to the nearest scale interval.

Example: A multi-interval scale has a 2 g interval in the first range and 5 g in the second range. A keyboard entry of 5 g in the first range must be either rounded to 4 g or 6 g or rejected.

28.1.8 Weight values (net or gross) must always begin with the lowest weighing range on the device regardless of the amount of keyboard or platter tare that is taken.

Example: A multi-interval scale has two ranges. The first range is 0 – 2 kg by 2 g, and the second 2 – 5 kg by 5 g. A platter tare of 1 kg is taken and the device indicates zero. The device must then indicate the net weight from 0 to 2 kg in intervals of 2 g; and from 2 up to 4 kg, in intervals of 5 g.

28.1.9 When gross, tare and net weights are indicated or printed, there shall be mathematical agreement in all circumstances. The tare value must be rounded off to permit mathematical agreement.

Example: A multi-interval device has two ranges. The first range is 0 – 2 kg by 2 g, and the second 2 – 5 kg by 5 g. A keyboard tare of 1.998 kg is entered (in 2 g intervals) and a gross load of 2.115 kg (in 5 g intervals) is put on the platter. The net weight indicated must be either 0.116 kg or 0.118 kg (first range rounded off to 2 g interval). If the gross, net and tare weights are indicated, they must be as follows:

Gross
Tare
Net
either
2.115 kg
1.999 kg
0.116 kg
or
2.115 kg
1.997 kg
0.118 kg

28.1.10 A tare value entered in the lower range may be either rounded off to the nearest value of the interval of the higher range in which the weighing occurs or kept unchanged. In the latter case, the net weight could possibly be displayed with an interval smaller than the interval of the range in which the weighing occurs. When a device displays or prints the gross, net and tare values, in all cases the mathematical equation (tare + net = gross) must be satisfied.

Example: A multi-interval scale has a 2 g interval from 0 to 2 kg, and 5 g from 2 kg to 10 kg. A tare of 12 g is entered in the first range and the gross weight is 2 500 g (second range). The scale may display either 2 488 g (tare stored as 12 g) or 2 490 g (rounded off to 10 g).

Marking

28.1.11 The capacity Max and verification scale interval e must be conspicuously marked near the weight display (see STP-3).

Performance

28.1.12 The limits of error are determined on the basis of the class of the device and the value of the verification scale interval e of the partial range corresponding to the load applied.

The following example shows how to determine the limits of error applicable to a Class III, 15 kg weighing device with partial ranges and verification scale intervals e set as follows:

First range
0.000 – 2.000 kg × 0.001 kg (1 gram)
Second range
2.000 – 5.000 kg × 0.002 kg (2 grams)
Third range
5.000 – 15.000 kg × 0.005 kg (5 grams)

The acceptance limits of error applicable to Class III weighing devices are the following:

Table 5
Load expressed in number of verification scale intervals e Acceptance limits of error expressed in number of verification scale intervals e
0 – 500 +0.5 e
> 500 – 2 000 +1 e
> 2 000 – 4 000 +1.5 e
> 4 000 +2.5 e

The limits of error applicable to each range are therefore the following:

Table 6
  Number of verification scale intervals e Value in kilograms Limits of Error (Acceptance)
First Range (e = 1 g) 0 – 500 e
> 500 – 2 000 e
0 – 500 g
> 500 g – 2 kg
0.5 g (0.5 e)
1 g (1 e)
Second Range (e = 2 g) 0 – 500 e
> 500 – 1 000 e
> 1000 – 2 000 e
> 2000 – 2 500 e
0 – 1 kg
> 1 – 2 kg
> 2 – 4 kg
> 4 – 5 kg
N/A
N/A
2 g (1 e)
3 g (1.5 e)
Third Range (e = 5 g) 0 – 500 e
> 500 – 1 000 e
> 1000 – 2 000 e
> 2000 – 3 000 e
> 3 000 e
0 – 2.5 kg
> 2.5 – 5 kg
> 5 – 10 kg
> 10 – 15 kg
> 15 kg
N/A
N/A
5 g (1 e)
7.5 g (1.5 e)
N/A

Clarification:

  1. For any test performed on a multi-interval scale, the limit of error is a function of the verification scale interval e of the range corresponding to the test load used. Critical test points are calculated for each interval and include Maxi.
  2. The smallest value of the verification scale interval e applies to the tests to determine the maximum value of the AZSM, the maximum permissible error for the return to zero test and for the creep-return to zero test, etc. The motion detection requirement must be satisfied for each partial range. The shift test must be performed once at the specified load based upon the device Max (see STP-15 for more information).
  3. Devices equipped with keyboard or platter tare must meet the tolerances for net loads for any tare taken up to the maximum tare capacity.

28.2 Multiple Range Devices

Multiple range device means a device having two or more weighing ranges with different capacities Maxand different scale intervals e for the same load receptor, each range extending from zero to its capacity Max (two or more devices in one). The selection of the range may be manual or automatic.

For the purposes of determining limits of error, each range is considered a separate device. The limits of error for each range are determined on the basis of the value of the verification scale interval e of that range.

Number of intervals and capacity

28.2.1 Each range of a multiple range device must have the number of scale intervals n as required by section 3 of the Specifications.

28.2.2 For each range, the actual scale interval d must equal the verification scale interval e.

Zero setting mechanism

28.2.3 The zero setting feature in any weighing range must also be effective in the greater weighing ranges, if switching to a greater weighing range is possible while the device is loaded. The deviation from zero must be no more than 0.25 e. This is applicable to manual selection or automatic changing of weighing ranges.

Selection of the weighing range

28.2.4. Manual selection of the weighing ranges is allowed from:

  • a smaller to a greater weighing range at any load; and
  • a greater to a smaller weighing range when there is no load on the platter and the indication is zero or at a negative net value. The tare operation must be cancelled and zero must be set to + 0.25 e1, both automatically.

28.2.5. Automatic selection of the weighing range is allowed from:

  • a smaller to the following greater weighing range when the load exceeds the maximum gross weight of the range being operated; and
  • a greater to a smaller weighing range when there is no load on the platter and the indication is zero or at a negative net value. The tare operation must be cancelled and zero must be set to + 0.25 e1, both automatically.

Tare features

28.2.6 The tare value may only be transferred from one weighing range to another one with a larger verification scale interval e.

28.2.7 The tare operation shall also be effective in the greater weighing range, if switching to a greater weighing range is possible when the device is loaded.

28.2.8 The tare value must be equal to the value of the displayed scale division for all methods of tare entry (dtare = d). An attempt to enter, in the lower range of a multiple range scale, a tare value that is not equal to d must be rejected or rounded off to the nearest actual scale interval d.

Example: A scale has a 2 g interval in the first range and 5 g in the second range. A keyboard entry of 5 g in the first range must be either rounded to 4 or 6 g or rejected.

28.2.9 A tare value entered in the lower range must be rounded off to the nearest actual scale interval d of the higher range in which the net weighing occurs.

Example: An automatic multiple range scale has a 2 g interval from 0 to 2 kg (lower range), and 5 g from 0 to 10 kg (higher range). A tare of 13 g is entered in the lower range and the gross weight is 2 500 g (second range). The tare value must be rounded to 15 g and the scale must display 2 485 g (rounded off to 15 g).

28.2.10 In all cases, the net weight must be in mathematical agreement with the gross and tare weights that are displayed and recorded (net = gross - tare).

Indication of the range in use

28.2.11 The range in which the device operates shall automatically and clearly be indicated for both the operator and the consumers.

Clarification:

If a scale has a decimal point and a different number of decimal places in each weighing range, the position of the decimal point and the number of digits following is an adequate definition of the weighing range in use. If the weighing range does not utilize a decimal point and differing numbers of decimal places (e.g. scale divisions are 20, 50 and 100 lb), another method must be provided to indicate the weighing range in use. A display of the capacity by division with an annunciator located near the weight display such as the following is acceptable:

Range in use
Range 1
5 × 0.002 kg
Range 2
20 × 0.005 kg

Marking

28.2.12 Each weighing capacity by division must be clearly indicated near the weight display. If the manufacturer chooses to indicate which weighing range is in operation using the method recommended above (with annunciator), no other marking of capacity by division is required.

Performance

28.2.13 Each range is considered a separate device. The limits of error for each range are determined on the basis of the value of the verification scale interval of that range.

Clarification:

  1. Each weighing range of a multiple range scale is considered to be an individual scale and must be evaluated accordingly.
  2. After returning to zero from any load greater than Max1, and immediately after switching to the lowest weighing range (manually or automatically), the indication near zero must not vary by more than e1 during the following 5 minutes.

Revisions

The purpose of revision 2 is to:

  • update requirements for multi-interval eccentricity testing in 28.1.12, clarification 2.
  • clarify formula in 28.1.4 by adding brackets where appropriate.

The purpose of revision 1 was to:

  • add section 28.1.4, Maximum Capacity of Partial Weighing Ranges (OIML R76 3.3.3) Recommendation Only.
  • correct "d must equal e" references.
  • add table data for calculating n in section 28.1.3.
  • correct general formatting issues.
  • correct references to Specifications Relating to Non-automatic Weighing Devices (1998).
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