Principle of Testing NAWI

Every instrument should be accurate and correct to be permitted for use in any commercial transaction, particularly at the POS. An instrument may be accurate but may not be correct. To be accurate, the instrument must meet the applicable tolerance limit (MPE) and some metrological requirements. It should pass some specific performance tests. To be correct, an instrument, should not only be accurate, it should meet some specific legal requirements also - type (design) or model approval, appropriate class for usage, descriptive markings, proper maintenance etc.

Verification Basics for Non-automatic Weighing Instruments

The Legal Metrology (General) Rules, 2011 Seventh Schedule Heading A Part II

Accuracy Class

All NAWI's are divided into 4 categories:

Accuracy classes are determined in terms of Table 17

Table 17

Class
e value in g
Verification Scale Interval
n = max/e
min capacity
I 0.001 or above 50 000 100e
II 0.001 to 0.05
0.1 or above
100 to 100 000
5 000 to 100 000
20e
50e
III 0.1 to 2
5 or above
100 to 10 000
500 to 10 000
20e
20e
IV 5 or above 100 to 1000 10e

Preliminary Checking

Descriptive Plate [Clause 7(1)]

All machines should have a permanently fixed descriptive plate with markings:

  • Manufacturer ´s Mark and Model no.
  • Accuracy Class - I / II / III / IIII
  • Verification Scale Interval e
  • Scale Interval d [when d < e ]
  • Maximum Capacity Max
  • Minimum Capacity Min

Declarations must be correct

A NAWI declares max 30 kg having e=10 g, class III with min 200 g.

Here, n = max/e = 30,000/10 = 3,000. In Table 17, the n value of class III instruments lie between 100 to 10,000. Again, a class III instrument has min value of 20e = 20 X 10 = 200 g.

Here, all conditions are satisfied.

Equipment

  • Model Approval Certificate
  • Set of Working Standard Weights, (having error not more than 1/3rd of the MPE prescribed for the machine under test.)

Metrological requirements

Essential Tests

Seventh Schedule, Part II, Clause 8 (ii)

Tests shall be carried out to verify compliance with the following requirements:

  1. Value of maximum permissible error,
  2. Maximum permissible error for net and tare values,
  3. Scale interval of the tare weighing device,
  4. Operation of the tare device shall be visible
  5. Repeatability (3 weighing on classes III and IV and 6 weighing for I and II )
  6. Eccentric loading
  7. Discrimination
Test Sequence

  1. Checking Error Limit.
  2. Repeatability
  3. Eccentricity
  4. Zero-setting
  5. Weighing Performance
  6. Discrimination
  7. Sensitivity (in selective cases)
  8. Accuracy of tare Setting
  9. Price Computation (in selective cases)

NOTE:

  • An instrument must be allowed to warm up for about half an hour testing.
  • All NAWI's should pass the required tests under normal conditions and in the sequence specified.
  • Once an instrument fails in a test, it is no use carrying out any further tests.

1. Error Limit

Maximum Permissible Error

All NAWI may have MPE as per table below.

Table 20

  Class I Class II Class III Class IV MPE ±
Load in e min to 50 000 min to 5 000 min to 500 min to 50 0.5e
Load in e 50 001 to 200 000 5 001 to 20 000 501 to 2 000 51 to 200 1e
Load in e above 200 000 20 001 to 100 000 2 001 to 10 000 201 to 1 000 1.5e

Checking Error Limit

Select at least three loads - one near minimum capacity, another two where the MPE changes over in Table 43. For example, 5e, 500e and 2000e for a class III instrument.

  • Apply the loads, one by one and check change in indication in each case.
  • If there is no change in indication, the instrument is PASSED and no further test is necessary.
  • If the load and the indicated value are not same in the ½e range, the instrument FAILS.
  • In other cases, further test is necessary.

When the load is in ± 1e range

  • Apply the load and allow to show the stable indication of load + 1e.
  • Then add additional 0.5e load.
  • If the indication does not change, the instrument PASSES.
  • If it changes and finally stabilizes at load + 2e, it FAILS

 

  • When the indication stabilizes at load - 1e, add additional load of 0.5e.
  • If the indication changes and become stable at load value, the instrument PASSES.
  • If the indication does not change, it FAILS.

When the load value is in ± 1.5e range

  • If the stable indication is ± 1e from the load value, the instrument PASSES.
  • If the indication is more than that, it FAILS.

2. Repeatabilty

This test is designed to check whether the instrument gives consistent reading for the same load when the load is placed on the same position on the receptor for a number of times.

The difference between the maximum and minimum results of several weighings of the same load shall not be greater than the absolute value of the MPE of the instrument for that load.
[Clause 3(7)(1)]
For example, if the MPE for the load is equal to ±1e, the absolute value of this error is | ±1 | e = 1e.

Checking Repeatabilty

Determine the indicating type of the instrument

  • non-self indicating like platform instrument or equal arm instrument
  • analog indicating
  • digital indicating
Test will be different for different instruments.
  • Select a load just less than the second MPE change point. 1995e for Class III instrument, for example.

Non-self-indicating Instruments

Platform Instrument
Equal Arm Instrument
1. Set the instrument at zero. 1. Set the instrument at zero.
2. Apply the load to the load receptor. 2. Apply the load to the goods receptor.
3. Bring the indicating element to the equilibrium position using the proportional weights and/or steel yard poises and record the indication. 3. Apply standard weights to the load receptor until the indicating element is in the equilibrium position and record the weight.
4. Remove the load from the load receptor. 4. Remove the loads from both the receptors.
5. Reset the instrument to zero if the indication is not zero. 5. Reset the instrument to zero if the indication is not zero.
6. Repeat steps 2 to 5 twice more. 6. Repeat steps 2 to 5 twice more.
7. Check the difference between maximum and minimum indication does not exceed the absolute value of MPE for the applied load. If exceeds, it FAILS. 7. Check the difference between maximum and minimum indication does not exceed the absolute value of MPE for the applied load. If exceeds, it FAILS.

Analog Indicating Instruments

  1. Set the instrument to zero.
  2. Apply the load and record indication.
  3. Remove the load.
  4. Reset the instrument to zero if the indication is not zero.
  5. Repeat steps 2 to 4 twice more.
  6. Check the difference between maximum and minimum indication does not exceed the absolute value of MPE for the applied load.

Digital Indicating Instruments

  • The instrument is set to 0 and then a load is applied.
  • Add 0.5e load.
  • Add several additional loads of 0.1e one by one until the reading changes and stabilize.
  • Remove the 0.5e load. (The original load and the additional loads will remain)
  • Note the reading (R1)
  • Remove all the loads at a time and reset the indication to 0, if required.
  • Again place the load and the additional loads at a time.
  • Note the reading (R2) and remove the loads at a time.
  • Repeat the procedure for third time and note the reading (R3)
SITUATION I: If R1 = R2 = R3, the instrument PASSES.

SITUATION II: If the reading R2 or R3 differs from R1 by more than ±1e, the instrument FAILS.

SITUATION III: If the reading R2 or R3 differs from R1 by ±1e, further tests are required.

  • When the reading is R2, add additional loads of 0.1e till next changeover point is reached, that is R2 + e.
  • Let total additional load be Dl.
  • Now, calculate I2 = R2 + 1/2e - Dl.
  • Repeat the same procedure for R3 and calculate I3.
  • Rename R1 as I1.
  • Find out which has the highest and which has the lowest value and calculate the difference
  • If the difference is less than absolute value of the MPE for the load used, the instrument PASSES.
  • If the difference is greater than absolute value of the MPE for the load used, the instrument FAILS.

3. Eccentricity

The indications for different positions of a load shall meet the MPEs, when the instrument is tested according to Clause 3(7)(ii)(a) and (b).

Determine whether the load receptor on the instrument has:
  • four or less points of support;
  • more than four points of support;

Instrument with a Load Receptor with Four or Less Points of Support

On an instrument with a load receptor with four or less points of support, a load corresponding to one-third of the sum of the maximum capacity and the corresponding maximum additive tare effect shall be applied. Large standard working weights should be used in preference to several small standard weights. Apply the load centrally in the segment as shown in the figure.


  Load position

  1. Zero the instrument.
  2. Apply one-third Max plus maximum additive tare (if applicable) at one load position.
  3. Record the load and the indication.
  4. Determine if the indication is within the MPE for the load applied.
  5. Remove the load
  6. Repeat steps 1 to 5 for each segment.

Instrument with a Load Receptor with more than Four Points of Support
( Weigh-bridge, for example)

  1. Determine the number of support points.
  2. Divide the load receptor into n approximately equal segments, where n is the number of points of support.
  3. Zero the instrument.
  4. Apply 1/(n - 1) Max plus Max additive tare (if applicable) at the first load position as shown in the diagram.
  5. Record the load and the indication.
  6. Determine if the indication is within the MPE for the load applied.
  7. Remove the load.
  8. Repeat steps 3 to 7 at all the other locations in turn for all points of support.
  9. Determine if the indication is within the MPE for the load applied at every position.

4. Zero-setting

After zero setting the effect of zero deviation on the result of the weighing shall not be more than 0.25e; however, on an instrument with auxiliary indicating devices this effect shall be not more than 0.5d (Clause 4.5.ii).

If the instrument being tested has an auxiliary indicating device, activate the zero-setting device as shown below. If the indication is reading anything other than zero, the instrument does not meet the requirements of ±0.5d and has failed.

Determine, whether the instrument is

  • non-self-indicating
  • analog indicating; or
  • digital indicating

Non-self Indicating Instrument

During any test,when the entire load has been removed check visually that the instrument has returned to within ±0.25e of its equilibrium position. If it has not, apply 0.25e on the appropriate load receptor. Then:

  • If the indicator has moved through the equilibrium point the instrument PASSES; or
  • if the indicator has not moved through the equilibrium point the instrument FAILS.

Analog Indicating Instrument

The accuracy of the zero-setting device of an instrument with analogue indication can be checked at any stage during the testing of the instrument. It is essential that an instrument is set on zero before starting any test. After the completion of one of the test sequences check visually that the instrument has returned to within ±0.25e of zero.

Digital Indicating Instrument

Determine whether the instrument has non-automatic or semi-automatic or automatic zero-setting by conducting a simple test.

  • (a) Apply a load of approximately 5e.
  • (b) Zero the instrument and then remove the load.
  • (c) Wait for the automatic zero setting to occur and the indication displays zero.
If the display does not return to zero after 15 seconds, the instrument does not have automatic zero setting and it should be tested as non or semi-automatic zero-setting instrument.

Non-automatic/Semi-automatic Instrument

  1. Activate the zero-setting device.
    (a) Load the instrument using a standard weight that is within the zero-setting range (0 to 4% of Max, normally ±2% around zero).
    (b) Add additional standard weights to take the total load just below the next changeover point.
  2. Re-set the indication to zero using the zero-setting device.
  3. Apply 10e to the load receptor.
  4. Apply an additional 0.25e. If the indication:
    (i) remains unchanged go to step 5;
    (ii) changes and stabilizes at +1e from the original indication: FAIL
  5. If the indication remains unchanged in step 4, apply an additional 0.5e. If the indication:
    (i) changes and stabilizes at +1e from the original indication: PASS
    (ii) remains unchanged: FAIL

Automatic zero Indicating Instrument

  1. (a) Apply a load of approximately 5e.
    (b) Zero the instrument and then remove the load.
    (c) Wait for the automatic zero setting to occur and the indication displays zero, this should take a minimum of 5 seconds.
  2. Quickly apply 10e to the load receptor.
  3. Apply an additional 0.25e. If the indication:
    (i) remains unchanged go to step 4;
    (ii) changes and stabilizes at +1e from the original indication: FAIL
  4. If the indication remains unchanged in Step 3, apply an additional 0.5e. If the indication:
    (i) changes and stabilizes at +1e from the original indication: PASS
    (ii) remains unchanged: FAIL

5. Weighing Performance

This test procedure is used to establish the weighing performance of the instrument at several loads. When loading and unloading weights, the load shall be progressively increased and decreased. The loads shall be applied evenly distributed over the platform.

When loading and unloading the instrument it must not be allowed to zero track. This is achieved by maintaining a suitable load on the instrument when loading and unloading.

  • Select at least five different loads so that following loads may be covered by adding them suitably in increasing order - Minimum capacity, 1st MPE changeover point, 2nd MPE changeover point, 1/2 maximum and maximum capacity
  • Zero the instrument.
  • Apply the loads one by one increasing from minimum to maximum.
  • Note, if the indication is within the MPE after each load is added.
  • When the maximum weight is reached, an additional weight of 10e is added to see if the over-range blinking is set correctly.
  • Remove the load in decreasing order and see if the indication is within the MPE after a load is removed.
  • After the last load is removed check if the indication has returned to within ±0.25e of zero.

6. Discrimination

This test is done to check if the instrument is capable of sensing a small change in load.

The actual scale interval for a class 1 or 2 digital instrument may be d and not e. As the majority of instruments to be verified will be class 3 or 4 where e = d, e is used in the procedure. If the instrument to be tested has d ≠ e then e becomes d in this instance for the whole procedure.

Determine whether the instrument is:

  • non-self-indicating;
  • analogue indicating; or
  • digital indicating

Non-self-indicating Instrument

An extra load equivalent to 0.4 times the absolute value of the MPE for the applied load when gently placed on or withdrawn from the instrument at equilibrium shall produce a visible movement of the indicating element [Clause 3(8)(i)].

Indicator Displacement Table

Visible movement
Accuracy Class
1 mm Class I or II
2 mm Class III or IV (with max up to 30 kg)
5 mm Class III or IV (with max above30 kg)

  • Zero the instrument.
  • Apply the load to the load receptor.
  • Bring the instrument to its equilibrium position.
  • Gently apply an extra load of 0.4 times the absolute value of the MPE for the applied load on the load receptor.
  • Observe if there is a visible amount of movement of the indicator.
  • Remove the load.
  • Determine whether the instrument has passed or failed.

Analogue Indicating Instrument

An extra load equivalent to the absolute value of the MPE for the applied load when gently placed on or withdrawn from the instrument at equilibrium shall cause a permanent displacement of the indicating element corresponding to not less than 0.7 times the extra load [Clause 3(8)(ii)(a)]

1. Zero the instrument.
2. Apply a load to the load receptor and, bring the indication to a mark by applying a small amount of extra material to the load receptor.
3. Record the initial indication as I1.
4. Gently apply an extra load equal to the absolute value of the MPE for the applied load on the load receptor.
5. Record the new indication as I2.
6. Calculate the difference in the two indications; I2 - I1.
7. Ensure that the change in indication determined in step 6 is greater than or equal to 0.7 times the extra load added in step 4.
8. Determine whether the instrument has passed or failed.

Digital Indicating Instrument

An additional load equal to 1.4 times the verification scale interval, when gently placed on or withdrawn from the instrument at equilibrium shall change the initial indication by one actual scale interval [Clause 3(8)(ii)(b)

1. Zero the instrument.
2. Apply a load to the load receptor.
3. Apply additional standards weights of 0.1e until the indication changes up and stabilizes.
4. Record this indication.
5. Gently apply a load of 1.4e. The indication should increase by 1e to the next scale interval.
6. Determine whether the instrument has passed or failed.

7. Sensitivity

The sensitivity test is only performed on non-self-indicating instruments.

An extra load equivalent to the absolute value of the MPE for the applied load, shall be placed on the instrument at equilibrium and shall cause a permanent displacement of the indicating element should follow the "Indicator Displacement table, at least.

The sensitivity tests shall be carried out by placing extra loads with a slight impact, in order to eliminate the effects of discrimination threshold

1. Zero the instrument.
2. Apply a load to the load receptor.
3. Bring the instrument to its equilibrium position.
4. Apply an extra load equal to the absolute value of the MPE for the applied load.
5. Measure and record the linear distance between the middle points of this reading and the reading without the extra load as the permanent
displacement of the indication.
6. Determine whether the instrument has passed or failed.

8. Accuracy of Tare setting

This test is only required at first time verification only

A tare device shall permit setting the indication to zero with accuracy better than:

  • ± 0.25e for electronic instruments and any instrument with analogue indication;
  • ± 0.5d for mechanical instruments with digital indication and instruments with auxiliary indicating devices.
The operation of the tare device shall be visible on the indication panel with the sign "net".

For any tare load applied, the MPE for the remaining weighing capacity is the same as if no tare was being used

1. Load the instrument using a weight that exceeds the tare capacity and ensure the tare facility is not functional.
2. Activate the tare-setting device in the following way:
(a) Load the instrument using a weight that is within the tare-setting range.
(b) Add additional weights to take the total load just below the next changeover point.

3. Re-set the indication to zero using the tare-setting device.

4. Check the accuracy of tare-setting in the following way:
(a) Apply 10e to the load receptor.
(b) Apply an additional 0.25e. If the indication:

  • remains unchanged go to step 4(c);
  • changes up and stabilizes +1e from the original indication: FAIL
(c) If the indication remains unchanged in step 4(b), apply an additional 0.5e. If the indication:
  • changes up and stabilizes +1e from the original indication: PASS
  • remains unchanged: FAIL
(d) Remove 10e, 0.25e and 0.5e to bring the indication back to zero.

5. Check the tare weighing device in the following way:
(a) Ascertain whether the instrument has additive or subtractive tare.
(b) Determine the instruments remaining capacity.
(c) Add a load equal to full remaining capacity.
(d) Determine if the indication is within the MPE.

9. Price Computation

This test is only required at the initial verification

On a price-indicating instrument the supplementary primary indications are unit price and total price and, if applicable, number, unit price and total price for non-weighed articles, prices for non-weighed articles and price totals.

The price computation check is performed to check that the price calculating function is able to compute the total price. Conduct the check over a range of loads, and preferably during the weighing test.
1. Apply a load to the load receptor and input an appropriate unit price.
2. Calculate the total price from the unit price and indicated weight.
3. Compare the calculated total price with the indicated price.
4. Repeat the test at least four more times.
5. Determine whether the instrument has passed or failed.

Sealing and Stamping

After all the testing is done and the instrument passes all tests, it is essential that the instrument be secured through sealing and stamping. Wilful or otherwise calibration of an instrument after a competent authority does the verification can change the metrological characteristic of the instrument without being visible. Sealing is mainly done to

  • Prevent dismantling or maladjustment of components that might alter metrological characteristics of the instrument.
  • To control access to the calibration point of the instrument.

Using Paper Seal

The viable method is to put destructible self-adhesive type paper or thin plastic seal or sticker. Such seals, if attempted to remove, becomes defaced and any attempt of tempering can be easily detected. The adhesive should be strong enough so that a seal cannot be removed without tearing it out. Such seals may carry

  • A serial number, which may be noted in the Verification Certificate.
  • All other marks put by punches now.

Mechanical Sealing

When a mechanical device like screw drivers can calibrate an instrument, then lead and wire seal or paper seal seals may be used to prevent access to this point.

Stamping

  • The plate bearing the descriptive markings, fixed to the instrument, should be stamped.
  • Stamping should be made on the soft metal plug provided on Dial Type Weigh-bridges
  • On Weigh-bridge machines, other than dial type, the soft plug will be on the indicating lever or steel yard and this be stamped.
  • In other machines, the soft plug should be on a conspicuous part of the beam or body.
  • In all cases, the soft plugs should be made irremovable