Road Weigh Bridge

Schedule VII Heading A (NAWI) of the Legal Metrology (General) Rules, 2011

A weigh-bridge can be considered as a platform that is supported by weight-sensing elements which produce an output proportional to the load placed on the platform. It is treated as a non-automatic weighing instrument and is verified as such.

Electronic Weigh-bridge

In electronic instrument the load receptor is totally supported on load cells and the indication system is digital.

Mechanical Weigh-bridge

In mechanical instruments, the load receptor is supported on a mechanical lever system and the primary indication of mass is by a mechanical indicator which may be non-self indicating (Steel Yard) or self indicating analog display (Dial Type).

Hybrid Weight-bridge

In hybrid weigh-bridges a lever system is used for weighing and the mechanical output is fed to a load cell which converts it into electrical signal for processing. A two way switch allows the change over - either the output of the lever system is directly connected to the indicator (steel yard or dial) or the output is passed to the digitizer for processing. The mechanical and the electronic system are verified separately and the verification fee is charged separately for each system.

Major Components of a Weigh-bridge

Platform

The platform may be chequered or plain and shall be rigid and sufficiently strong for carrying maximum load.

The load cells are arranged on both the sides in pairs.

 

Load Cell

A load cell is an electronic device (transducer) that is used to convert the gravitational force into an electrical signal.
Through a mechanical arrangement, the force deforms a strain gauge. The strain gauge converts the deformation (strain) to electrical signals.

Converter

In hybrid weigh-bridges a load cell is added to a mechanical weigh-bridge lever system for gaining advantages of electronic weighing. A single load cell is inserted in the connecting rod of the weigh-bridge or in the transfer lever of the bottom work of the weigh-bridge.

There ia a switch system which allows to pass the signal to the electronic device when power is available. When the power is off, the load cell is bypassed and the lever system is connected directly to an analog display unit, normally a multi-range dial indicator.

Weigh-bridge Terminal

Electronic Unit: Digitizer, Display and Printing Unit

The output of the load cells are added in a junction box, also known as Summing Up box. Here the output of individual cells will be summed up and the net value will be sent to the digitizer that convert the signal into digital form to be fed to a computer terminal.

The Computer is used for entering vehicle data through a key board and mouse which will be visible on the monitor along with the weighing result.

The weighing result cannot be entered manually. It will be frd directly from the digitizer. The final output will be sent to the printer for issue of receipt. Many softwares are available for data entry and issue of receipt. Some softwares are even capable of calibrating load cells through the key board. Such calibration requires use of digital load cells connected in series thus eliminating the requirement of any junction box..

 

Mechanical Unit

In mechanical units, the indicator is either a steel yard (non-self indicating device) or a dial head (multi-range self-indicating analog display device)

The lever system is connected to either of the indicating devices.

 
Testing and Verification of Weigh-bridge

1. Visual inspection

  • Compliance of the instrument with the Model Approval Certificate
  • Presence of all mandatory declarations on a permanently fixed marking plate.
  • Whether previous verification seal is intact. (for re-verification and inspection only)

2. Evaluation of Error and Weighing Test

A 60 000 kg weigh-bridge has e value of 10 kg and minimum capacity of 200 kg. For weighing test and calculation of error, 5 loads (3 for inspection) are selected:

  1. 250 kg (near minimum)
  2. 4 500 kg (near first MPE change over point)
  3. 19 500 kg (near second MPE change over point
  4. 40 000 kg (2/3 max)
  5. 58 500 kg (near max)
  6. 60 100 kg (max + 10 e) to ensure the over-range blanking is correctly set. ( for electronic instruments)

 

TABLE 20 (Part)

For Class III instruments with e = 10 kg

Load MPE (±)
Up to 500 e
for 250 kg
for 4 500 kg
0.5 e
5 kg
5 kg
Above 500 e up to 2000 e
for 19 500 kg
e
10 kg
Above 2000 e
for 40 000 kg
for 58 500 kg
1.5e
15 kg
15 kg

During inspection, the MPE value will be double of above.

Test Procedure

  • Apply the load on the receptor and then keep adding 1kg (1/10 e) loads till the indication raises by 10 kg (1 e)
  • Note the Load value (L), Indication (I)
  • Note the additional load (Δl) required to raise the indication by 1 e, in this case 10 kg.
  • Calculate the Error by using the formula Error (E) = I + ½e - L - Δl
  • Compare the result with the Table and determine whether the instrument passes or fails.
  • The loads should be placed in ascending order and finally, in descending order. When the minimum weight is removed, check whether the indication has returned to ±0.25e of Zero.

Error Calculation

Load L Indication I Addl. Load Δl Changed Indication Error = I + ½e - L - Δl
250 250 7 260 250 + 5 - 250 - 7 = -2 kg
4 500 4 500 8 4 510 4 500 + 5 - 4 500 - 8 = -3 kg
19 500 19 510 7 19 520 19 510 + 5 - 19500 - 7 = 8 kg
40 000 40 020 8 40 030 40 020 + 5 - 40 000 - 8 = 17 kg
58 500 58 520 6 58 530 58 520 + 5 - 58 500 - 6 = 19 kg

Weighing test using substitute weights

In the above example, standard weight of 60 0000 kg will be required which may be difficult to procure. The Rule allows up to 50% max capacity, 30 000 kg of substitution weights. But, before using substitution weights they will have to be tested.

  • Calculate the maximum test weight available and requirement of substitution weight.
  • For example, 35 000 kg of test load available and 25 000 kg of substitution material will be required
  • Place the 25 000 kg of test load on the receptor and keep on adding additional loads 1 kg (0.1 e) to make the indication jump by 1e. Note that indication.
  • Remove the load but keep 10e (100 kg) of load in electronic instruments to avoid zero tracking) and the additional weights.
  • Put on substitution weights till the same indication is reached. Now, substitute weight of 24 900 kg is available.
  • If different quantity of substitution material is required, the same procedure may be repeated.

3. Repeatability Test

Test purpose

The test is required to check whether an instrument gives consistent results for the same load placed on the same position on the receptor for a number of times.

Test requirement

  • Take a load of 2/3 max. (The Rule requires that tests be made at 1/2 load and at max load and during loading and unloading.)
  • The difference between the highest and the lowest indication should not be more than the absolute value of MPE (when MPE is ±1.5e, the absolute value is 1.5e or 15 kg) for that load.
  • Note whether the instrument is non-self indicating (steel yard), analog indicating (dial) or digital indicating (electronic). The test procedure will be different for different types.

Test procedure

Non-self Indicating
Analog Indicating
  1. Zero the instrument and then apply the load on the receptor.
  2. Set the indicating element to the equilibrium position by adjusting proportional weight and or poises.
  3. Note the indication.
  4. Remove the load and reset the instrument to zero if the indication is not at zero.
  5. Repeat the exercise two more times.
  6. Calculate if the difference between the highest and the lowest reading is within MPE value.
  1. Zero the instrument and then apply the load on the receptor.
  2. Note the indication.
  3. Remove the load and reset the instrument to zero if the indication is not at zero.
  4. Repeat the exercise two more times.
  5. Calculate if the difference between the highest and the lowest reading is within MPE value.
Digital Indicating Devices
  • Select two loads - 30 000 kg (½ max) and 60 000 kg (max).
  • Place them on the receptor in the following order -
    a) 30 0000 kg wt, note the indication I1.
    b) Add another 30 000 kg wt and note the indication I2.
    c) Remove 30 000 kg wt and note indication I3
    d) Again add 30 0000 kg wt and note I4
    e) Remove 30 000 kg wt and note indication I5
  • Result should be I1 = I3 = I5 and I2 = I4

4. Eccentricity Test

Test Procedure

The indications for different positions of a load on the receptor should meet the MPE for that load.

Test Requirements

  • Find out the number (n) of support points. and divide the load receptor into n number of more or less equal segments.( 6 in the figure)
  • Mark the segments 1 to 6, starting from bottom left and running clockwise.
  • Calculate the load required for the testing: Max/(n-1) or 60 000 / 5 = 12 000 kg.
  • Apply the load at position 1 and record the Load (L), indication (I) and additional load (Δl) required to increase the indication by 1 e.
  • Caculate the error by using the formula and note if it is within the MPE for that load.
  • The procedure should be repeated for all positions.

5. Discrimination Test

Test Purpose

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

Test Procedure

  • The test procedure will be different for different types of instrument.
  • Three different loads should be used - near min, ½ max and near max.
  • Calculate the MPE for each load from Table 20.
  • Zero the instrument and pace the testing load on the receptor and then
  • Non-self Indicating
    Analog Indicating
    Digital Indicating
    1. Bring the instrument to the equilibrium position.
    2. An extra load equal to the MPE value for that load should be applied or removed gently.
    3. This should make the indicator move by at least 5 mm.
    1. Put a small amount of load to to make an indication (I1)
    2. An extra load equal to the MPE value for that load should be applied or removed gently.
    3. Note the new indication (I2)
    4. The difference (I2 - I1) should not be less than 0.7 times of the extra load.
    1. Keep applying additional standard loads of 0.1 e until the indication changes and stabilizes.
    2. Note the indication.
    3. add another load of 1.4 e gently.
    4. The indication should increase by 1 e.

Sealing and Stamping

In electronic instruments, the digitizer is sealed while there is a sealing point just below the dial of analog indicating instruments.

The descriptive marking plate should be stamped.

Ensure Full View to the Operator

  • The entire platform should be in the full view of the operator.
  • He should ensure that the vehicle has been placed at the centre of the platform and no wheels are straying outside the platform.
  • He should not allow any object (other than the commodity to be weighed) or person to be on the vehicle or platform while weighing.