Non-automatic Weighing Instruments

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

A weighing instrument may be automatic or non-automatic. It is non-automatic when intervention of an operator is required during weighing:
(i) to deposit or remove the load to be measured from the receptor
(ii) to obtain the result.

Categories

A non-automatic weighing instrument may have graduated scale for display the weighing result. The display also may be analog using a linear scale or a circular scale or may be digital. Such instruments (A, B and C) are covered under Heading A of the Schedule.

Instruments (D) with un-graduated scale, beam scales and counter machines are covered under heading B and have totally different metrological requirements.

Measurement Principle

Measuring instruments determine the mass of a body by using the action of gravity on this body.
Mechanical weighing instruments use principle of lever or elasticity (spring) to move an indicator to display the weighing result on a scale.
Electronic weighing instruments use load cells which transforms gravitional force into electrical signals to display the weighing result on an electronic panel.

Categories of Mechanical instruments

Depending on the nature of indicating devices, the mechanical instruments are farther divided into:
(i) Non-self indicating and
(ii) Self or semi-self indicating - analog or digital.

Non-self indicating instruments

In non-self indicating weighing instruments the equilibrium position is obtained totally by the operator. They include a variety of mechanical lever balances, such as mobile counter scale, platform scale, stationary weighing-bridge etc.

Measurement procedure
1. Load is applied on load receptor
2. Weights are applied to reach a rough equilibrium.
3. Complete equilibrium is reached by moving the Sliding poises
4. Load = total weight value + reading of the sliding

Non-self Indicating


Platform Machine

Equal Arm Machine

Self Indicating - Analog


Spring balance
(Graduated Scale)


Counter Machine
(Un-graduated Scale).

Self Indicating - Digital


Digital Scale.

Self or semi-self indicating instruments

In self indicating instruments, the weighing result is obtained directly on a scale and no intervention of the operator is required.

In semi-self indicating a little intervention of the operator is necessary.

Measurement Procedure of semi-self indicating instruments
In this type of instruments, the display range of the scale is very narrow and there are weights inside the instruments which can be removed by an outside knob.
1. Load is applied on the load receptor
2. Inside weights are removed by a knob to bring the reading within the scale range.
3. Load = Removed value (knob)+ reading on the indicator
Example: 30 g (knob value) + 3.5 g (reading on the scale) = 35.5 g (load value)

More on categories of NAWIs

  • Self or semi-self indicating instruments may have analog or digital display
  • Only Class I or Class II instruments may have an auxiliary scale while
  • Weighing instruments may be Multi-range or Multi-interval
  • An instrument shall have a zero setting device which may be non-automatic, semi-automatic or automatic.
  • The indicating system of a weighing instrument should be identified carefully as testing procedures vary with the type of indicating device of a non-automatic weighing instrument.
  • Semi-self indicating instruments and instruments with automatic zero-setting devices are rarely found now-a-days.
 
Multi-range Instruments

A weighing instrument may have two or more weighing ranges with different maximum capacities and different scale intervals for the same load receptor, each range extending from zero to its maximum capacity. The selection of the weighing range may be determined manually through a pushbutton or switch or may automatically change to the higher capacity range once the load exceeds the capacity of the lower range. Such instruments are known as Multi-range instruments. Each range is treated as a separate instrument.

Multi-interval Instruments

A weighing instrument may have one weighing range that is divided into partial weighing ranges (segments). Each weighing range (segment) is defined by its interval size, its minimum capacity, and its maximum capacity. The selection of the appropriate weighing segment is determined automatically according to the load applied, both on increasing and decreasing loads. Such instruments are known as multi-interval instruments

Concepts

Accuracy

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

Precision

Precision is something different from accuracy. The precision of a measuring instrument is determined by the smallest unit to which it can measure. An instrument can measure up to 0.001g but that measurement may not be accurate and may even be wide of the mark. So, an instrument may have high precision but low in accuracy and also vice-versa.

Precision also means how close the results will be after repeated measurements. Ideally, if something is weighed for 5 times, the result should be same every time. Bu, result will differ slightly every time it is measured with a low precision instrument.


Accuracy and Precision

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 etc.
  • 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 + Es + Er
Where T = True Value; Es: Systematic Error and Er: Random Error
Absolute and Relative Error

Absolute error is simply the amount of physical error in a measurement while Relative error shows how large the error is.

Absolute Error =
Quantity Measured - Actual Quantity.

Relative Error =
(Quantity Measured - Actual Quantity)/Actual quantity.

A person buys 500 g of sugar but he actually gats 490 g, while another person buys 100 g of sugar and gets 90 g. In both cases, the absolute error is same, 10 g. But in the first case, the Relative Error =
(500-490)/490 = 0.01 (negligible).
But in the second case, the Relative Error = (100-90)/90 =0.1. In percentage, this comes to 10%, which is unacceptable.

In Legal Metrology, only the Relative error in percentage is taken into account.

Factors that cause error

  • Low capacity Load Cell
  • Temperature Coefficient : The sensitivity of the balance is adjusted at 38°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, wt. 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.

Testings and verifications are meaningless unless the standards used can be traced to the National Standard. The national standards, in turn, are traced to international standards which is acceptable to all countries.

 
Trace the standard

Commercial Weights or Measures

Working Standards used for verification.

Secondary Standards.

Regional Standards

National Standards.

'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 0.02g. (for example 0.015 g)
The 'e' value here is 0.01 g

Importance of 'e' value

  • 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]
  • '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.

 

Table 19

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

e, d relation example

Auxiliary or Secondary Scale

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.

  • 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.


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

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 leads to 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

Tare devises require

  • 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

Verification

 

Calibration

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.

 

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.

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). The The MPE allowed will be twice the normal value.
  • Eccentricity Test
  • whether the verification seal is intact

Read more ......

Testing and Sealing of NA Weighing Instruments