Day in and day out we keep on listening to exhortations on protection of consumer rights and related regulatory campaigns to make it happen. There is hardly any forum be it political, social, voluntary, governmental or non-governmental, where this subject is not broached.
This reminds me of an anecdote when I was travelling in connection with my work. I was driving my car at a comfortable and permissible speed. Before I could cross the State border, traffic cops signalled me to stop and I did. Reason cited was violation of speed limit. Before I could be handed over a challan, I tried to reason out that something was amiss, as the speed being followed was very much within permissible limits as per speedometer and my driving experience. Enforcement official appeared helpless as his gauging meter indicated overspeed. Somehow appealing his conscience worked. Being a good Samaritan, he reluctantly agreed that there appeared some mismatch between his visual observation and the gadget reading. I was let go.
Any indication on a digital or analog meter is no guarantee that it is a true measurement. Errors can accrue on account of various factors, like the design parameters, accuracy of sensors, operating method, environment, temperature, maintenance, age and many more if not taken care of at the right stage in conformance to prescribed specifications/guide lines.
Let us take example of Weighing instruments used for measurement of mass. Department of Consumer Affairs is the regulatory, Licensing, certifying and stamping authority for measuring instruments. Legal Metrology General (Rules) 2011 stipulate various parameters for Construction, metrological characteristics, technical requirements, and testing procedures. Objective is to protect consumer interests and ensure that transactions happen within the permitted limits of accuracy and errors. Rules framed are generally in line with the suggestions/recommendations form OIML.
The International Organization of Legal Metrology (OIML) is an intergovernmental treaty whose mission is to enable economies to put in place effective legal metrology infrastructures that are mutually compatible and internationally recognized, for all areas for which governments take responsibility, such as those which facilitate trade, establish mutual confidence and harmonize the level of consumer protection worldwide.
Weighing instruments comprise of various components. The most critical being Load cell that senses the applied load and transfers the output to instrumentation for display of the measured mass.
For better understanding it is important to know the weighing terminology as under: –
As per OIML R60: 2000
T.2.2.1- Load cell
Force transducer which, after taking into account the effects of the acceleration of gravity and air buoyancy at the location of its use, measures mass by converting the measured quantity (mass) into another measured quantity (output).
Note: Load cells equipped with electronics including amplifier, analog-to-digital converter (ADC), and data processing device (optionally) are called digital load cells.
T.2.2.2- Indicator
Electronic device of an instrument that may perform the analog-to-digital conversion of the output signal of the load cell, and which further processes the data, and displays the weighing result in units of mass.
T.2.4 – Displaying device (of a weighing instrument)
Device providing the weighing result in visual form.
T.3.2.2 – Actual scale interval, d
The difference between two consecutive indicated values, for digital indication.
T.3.2.3 – Verification scale interval, e
Value, expressed in units of mass, used for the classification and verification of an instrument.
T.3.2.5- Number of verification scale intervals, n
Quotient of the maximum capacity and the verification scale interval:
n = Max / e
T.5.1.3 – Digital indication
Indication in which the scale marks are composed of a sequence of aligned figures that do not permit interpolation to fractions of the scale interval.
T.5.5.4 – Maximum permissible error, mpe
Maximum difference, positive or negative, allowed by regulation between the indication of an instrument and the corresponding true value, as determined by reference standard masses or standard weights, with the instrument being at zero at no-load, in the reference position.
4.2 Accuracy classes
Load cells shall be ranked, according to their overall performance capabilities, into four accuracy classes whose designations are – Class A; Class B; Class C; Class D.
Table 1 Maximum number of load cell verification intervals (nmax) according to accuracy class.
| Class A | Class B | Class C | Class D | |
| Lower limit | 50 000 | 5 000 | 500 | 100 |
| Upper limit | Unlimited | 100 000 | 10 000 | 1 000 |
The maximum number of load cell verification intervals, nmax, into which the load cell measuring range can be divided in a measuring system shall be within the limits fixed in Table 1 above.
Legal Metrology (General) Rules 2011 is silent about load cell accuracy specifications but it appears from above that measuring range in a measuring system may not be beyond the limits as shown in the table 1 above.
As per OIML R 76-1: 2006 (E) and Legal Metrology (General) Rules 2011 Schedule 7 Part II
Classification of instruments
The verification scale interval, number of verification scale intervals and the minimum capacity, in relation to the accuracy class of an instrument, are given in Table 2 below.
Table 2
Accuracy class | Verification scale interval, e | Number of verification scale intervals, N=Max/e | Number of verification scale intervals, N=Max/e | Minimum capacity,Min (Lower limit |
| minimum | maximum | |||
| Special (I) | 0.001 g ≤ e* | 50 000** | 100 e | |
| High (II) | 0.001 g ≤ e ≤ 0.05 g | 100 | 100 000 | 20 e |
| High (II) | 0.1 g ≤ e | 5 000 | 100 000 | 50 e |
| Medium (III) | 0.l g ≤ e ≤ 2 g | 100 | 10 000 | 20 e |
| Medium (III) | 5 g ≤ e | 500 | 10 000 | 20 e |
| Ordinary (IIII) | 5 g ≤ e | 100 | 1 000 | 10 e |
- Most of the scales used for ‘legal for trade’ purpose are certified for min. class III
- Class I weighing instruments are most accurate and Class IIII instruments lesser.
- Only Class I and II weighing instruments can have d different from e. e.g., a Class I weighing instrument of 5 kg may have d as 0.1 g but e as 0.2 g or 0.5 g or 1 g.
- Class 111 and 1111 have d equal to e.
Maximum permissible errors as per Legal Metrology (General) Rules 2011. Seventh schedule Part 11 NAWI are described in Table 20
Table 20
| Maximum permissible errors on verification/ reverification | For Loads m expressed in verification scale interval e | |||
| Class 1 | Class 11 | Class111 | Class 1V | |
| +/- 0.5e | Min ≤ m ≤ 50000 | Min ≤ m {5000 | Min ≤ m ≤ 500 | Min ≤ m ≤ 50 |
| +/- 1e | 50000 < m ≤ 200000 | 5000 ≤ m ≤ 20000 | 500 ≤ m ≤ 2000 | 50 < m ≤ 200 |
| +/- 1.5 e | 200000 < m | 20000 < m ≤ 100000 | 2000 < m ≤ 10000 | 200 < m ≤ 1000 |
Inference
Load sensing and load measuring device are different. Indicator is part of the load measuring device on which the direct reading of the load sensed is obtained after due processing. Discrimination thresh hold being, the least weight added or removed from the load receptor to cause perceptible change in the indicator. This means load sensed and how accurately it is done by the load cells, gets displayed in the measuring device.
On examining table 1 and 2 above, it is abundantly clear that load cell accuracy class A, B, C and D directly correspond to weighing instrument accuracy class 1,11,111 and 1111 respectively in terms of number of divisions. Therefore, load cells in a weighing system can deliver accuracy in terms of number of divisions, equal to the measuring instrument divisions.
Considering the critical role of load cells in delivering the right and stipulated accuracy of weighing instruments in transaction and protection, it is highly important to have minimum specifications laid down, in line with the international practice and recommendations from OIML in this regard. Table 1 above can serve as guide line in this regard. Consumer protection and right to accuracy are buzzwords of the Department of Consumer affairs, Rights advocates and Manufacturers together. This indicates will of all stake holders who matter in the process of delivering accuracy to consumers. The matter has not received its due priority, may be for the reason that it has not been brought to the notice of regulators. It is worth while to speak in one voice and collaborate with the government to make it happen sooner than later.
Vijay Bhat 24th January 2023
Director General
