Non-automatic Weighing Instruments

Seventh Schedule of the Legal Metrology (General) Rules, 2011

A weighing instrument is non-automatic when intervention of an operator is required during weighing

Concepts

Accuracy

It means the extent to which the displayed measurement agrees with the standard value for that measurement.

Precision

The precision of a measuring instrument is determined by the smallest unit to which it can measure.

It also means how close the results will be after repeated measurements.

An instrument may have high precision but low in accuracy and also vice-versa.

Error

The difference between a particular measurement and the standard is called an error.

Error is a concept that cannot be known exactly.
Error is a single value and theoretically the value of a known error can be applied as a correction to the result

Systematic and Random Error

Measurement errors are divided into two categories - systematic and random
Systematic errors are deviations in measurement when the mean of a number of measurements will differ significantly from the true value of the measured quantity.

All measurements are prone to systematic errors (also known as Zero error) arising out of many factors.: incorrect calibration and change of environment.
Random error will cause inconsistent readings when repeated measures of a constant quantity are taken.
Random errors are unpredictable.
They scatter over a range about the true value
The mean value is nor effected
They are caused by unpredictable change in environment like air buoyancy, electrical or magnetic forces

Reading on the display panel of a measuring instrument is not a single entity but includes error factors also:
Reading = T + E_s + E_r
Where T = True Value; E_s: Systematic Error and E_r: Random Error

Factors that cause error

Low capacity Load Cell
Temperature Coefficient : The sensitivity of the balance is adjusted at 38°0° C and the balance is subsequently used at 10´C.
Aging of the instrument
Non- horizontal position of the weighing instrument
Moving the weighing instrument in height
Air currents / drafts
Adsorptive layer of moisture on the surface
Electro-static and Magnetic forces
Air buoyancy

Uncertainty in measurement

Uncertainty of a measured quantity is a parameter which describe incompleteness of knowledge of the measured quantity.
For example, a weighing balance with e value of 1 g may be error free. But, it may show measurement of 200 g, when the actual measurement could have been anywhere between 199.5 to 200.5 g, wit.
This ± 0.5 g is uncertainty.

Traceability

Property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations. It maintains consistency and comparability of measurement results across technology, time and place.

Commercial Weights or Measures

Working Standards used for verification.

Secondary Standards.

Regional Standards

National Standards.

Testings and verifications are meaningless unless the standards used can be traced to the National Standard.

'e' value

Verification Scale Interval, in gram, of an instrument is called its 'e' value.
It shows the smallest division it can measure.
A digital balance shows reading like this: 15.01 g; 15.02 g; 15.03 g. It can not measure anything between 0.01 g and 0.02 g. (for example 0.015 g)
The 'e' value here is 0.01 g

Importance of 'e'

Accuracy class of a machine is primarily determined by its e value.
Maximum Permissible Error (MPE) is measured in terms of e value.
Minimum Capacity (min) of a machine is also measured in terms of e value.

'e' value - 'd' value relation

Actual Scale Interval in gm is 'd'.
In digital instruments, 'e' = 'd' unless it has an auxiliary scale.
When there is an auxiliary scale:
d < e ≤ 10d [e = 1 X 10x]

Table 19

d	0.1 g	0.2 g	0.5 g
e	1 g	1 g	1 g

'e' must be more than d but not more than '10d'
But, when d is less than 1 mg, e will always be 1 mg.

Auxiliary or Secondary Scale

Let us take a digital balance having e value of 0.01 g. Place a 15 g weight on the balance. The display will be 15.00 g.
Now keep on adding some sand very slowly. The display will not be continuous but will jump from one reading to the next.
The reading would have been more accurate, if the instrument had an e value of .001 g. The reading would be
But, when a manufacturer is not sure whether his machine could face the accuracy at that level, he may drop the last digit from the main scale and place it after a gap.
The last digit is now in the auxiliary scale and thus not subject to verification and is represented as d value.
Here, the e value is 0.01 g and d value is 0.001 g.
The e value and the d value relationship follows the pattern as given in Table 19.
An instrument cannot have an auxiliary scale unless it belongs to Class I or II (Clause 3(5)(i))

Maximum Permissible Error (MPE)

No weight or measure can be 100% accurate. The Legal Metrology (General) Rules, 2011 prescribes an error up to which a weight or measure may be allowed to deviate from the standard, to be allowed to be used for commercial purposes.

The yellow area shows the MPE limit for a flat cylindrical bullion weight of 1 g. (1.0 mg)

All categories of weights and measures have a prescribed limit for deviation, beyond which it will be treated as non-standard weight or measure. The limit is known as Maximum Permissible Error (MPE).
In weights or measures, no error in deficiency is allowed during verification but allowed during inspection.

Tolerance

Tolerance is the greatest range of variation in measurement a machine can display for a particular load.
Tolerance of a machine is set by the manufacturer which should not cross the MPE allowed.

Characteristic Curve

The relation between the load and the displayed value should be linear. In other words, the line between zero and maximum load should be the blue ideal line.

But, due to various factors, no machine is capable of showing the true weight through the entire range of its capacity. A typical very high quality machine, for instance, will deviate from the linear course by ±0.15 mg over the entire weighing range of 200 g.
There is
Zero Deviation Error (green line)
Sensitivity Error (red line) and finally,
Linearity Error which will lead to the formation of Actual Curve (Pink)

Tare Device

Tare is a device to set the weight of a load, usually the weighing container, to zero. There are two types:

Additive - setting to zero without altering the weighing range for net loads
Subtracting - setting to zero reducing the weighing range or net loads.

Tare Device Requirements

Accuracy better than 0.25 e or 0.5 d for auxiliary indicating device
operating range as indicated
not bellow or at zero point
the equilibrium is stable

Calibration

Calibration means operations required for determining the quantum of error (deviation from the standard) of a weight or measure and fixing the error to make it standard.

Verification

Verification means operations required for determining whether the quantum of error of a weight or measure is within the MPE limit set by the Acts and Rules.
Normally, this term is used by the Officers of Legal Metrology. Verification includes periodical re-verification and also calibration.

Stamping

Para 8(1)(iii) / Part II / Heading B /Seventh Schedule

All verifications / Re-verifications shall be testified by verification (punch) mark.
All components whose dismantling / replacement / maladjustment might change the accuracy of an instrument should be sealed adequately.

Inspection

Para 8(2) / Part II / Heading B / Seventh Schedule

During inspection, following test are made:

Determination of MPE (usually at ½ max load)
Eccentricity Test
whether the verification seal is intact