Road Weigh Bridge

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

Most mechanical weigh-bridges have now been converted into electronic variations that work on a number of strain gauge load cells. These cells are connected to a weight instrumentation and the whole system is treated as non-automatic weighing instrument.

Foundation of any weigh-bridge is very important. It should be precisely level, stable capable of withstanding break pressures of heavy vehicles. Unless the foundation is solid, weighing will not be accurate and longevity will suffer.

Construction Type

  • Pit mounted -
    Pit mounted weigh-bridges are level with the ground and so vehicles can enter from either side. Such weigh-bridges offer easier up-gradation or replacement. There should be adequate drainage system to prevent flooding
  • Surface Weigh-bridge
    Surface Weigh-bridges have side railings. So, vehicle can only drive through the centre of the bridge. This ensures accuracy of weighing. Approach and departures are made through removable steel ramps.

Major Components

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. Number of load cell increases with the length of the platform which may be 9 m, 15m or 18 m.
A weigh-bridge may have a capacity ranging from 30 to 100 tonnes. So, the platform should be robust enough to bear the load.

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. TYhe output is analog or digital, depending upon the nature of load cells used. The load cells with digital output is generally known as Smart Load Cells.

Arrangement of Load Cells

Ordinary Load Cell
The load cells are connected parallelly to a signal processing circuit (Summing Box)where the output of all the cells are summed up to give the net output.

In this system, the change in gain of one cell affects the gain of others. So, the calibration is very difficult.
Smart Load Cell
The smart load cells are connected serially to the processing circuit.

The output is sent to the AD (Analog to Digital) Converter through an amplifier and finally to a micro-computer which assess the overall gain of the circuit. So, the calivration can be done through a computer.

  Digitizer

  Software Interface

The above is the interface of the software used in the computer.

Verification of Road Weigh Bridge

Electronic Weigh Bridge

Electronic Weigh-bridges are treated as Non-automatic weighing Instruments. The Digitizer or Digital Indicator serves as the heart of the instruments while Computers and Printers are used for data entry and issuing weight slip.
The digitizer reads the weight directly from the Load Cell or Converter.

 

Mechanical Weighbridges

In Mechanical Weigh-bridges compound levers transfer the gravitational force to the steel yard or dial for comparison and display.
In Electronic and Hybrid instruments, a number of load cells (2X, depending on the size of the platform) are used. There is a "Converter" in between the Lad Cells and the Display Unit. The load cell is connected to the digital indicator when electricity is available, otherwise to the dial head.

Visual Checking

  • Visually inspect clearances around weigh-bridge.
  • Visually inspect weigh-bridge steel for corrosion, cracks, and broken bolts.
  • Visually inspect condition of weigh-bridge deck.

Test Weight

  • The Inspector should have a set of 1, 2, 5 and 10 kg weights.
  • The user must keep weight of 1/10th capacity of the weigh-bridge or 1 tonne whichever is less.
  • For verification, the user shall arrange weight of 1/4th of the capacity of the weigh-bridge.

Test Requirement (Steel Yard)


    

1. The Steel yard should remain horizontal at no-load.
2. Accuracy of major graduations up to the max capacity should be verified.
3. All proportional weights are to verified.
4. Sensitiveness should be tested as per Table.
5. Error should be tested at full or near full load. The error should not exceed MPE allowed.
6. Uniformity Test should also be conducted.

  • Test no 1 and 4 are not applicable to Dial type instruments.

Sensitivity Table (when Fully Loaded) [Para 4 (f)]

Capacity Sensitivity Capacity Sensitivity Capacity Sensitivity
25 t 3.5 kg 30 t 4.0 kg 40 t 5.0 kg
50 t 5.5 kg 60 t 5.5 kg 80 t 6.0 kg

Uniformity Test [ Para 4 (e)]

Place ¼ of the max load, or as near as possible at

  • At the middle of the platform, or
  • At any of the four corner points.
The Weigh-bridge should show correct weight within ±¼ of the MPE at full load.

MPE (±) applicable to S. Yard During Verification [Para 4 (d)]

Capacity 0 - ½ max load Over ½ max load
25 t 3.00 kg 6.00 kg
30 - 40t 3.50 kg 7.00 kg
50 t 4.00 kg 8.00 kg
60 t 4.25 kg 8.50 kg
80 t 5.00 kg 10.00 kg
100 t 5.75 kg 11.50 kg
* During inspection, the MPE allowed will be twice the above value.

Test Requirement (Dial Type)


A gear mechanism is used to change the scale on the dial. Each scale is requited to be verified.

MPE (±) applicable to Dial Type (During Verification)

0 to ½ max load Above ½ max
¼ d ½ d

  • d = interval between two consecutive graduation.
  • During inspection, the MPE allowed is twice the MPE (±)

Verifying Hybrid Weigh-bridges

Hybrid Weigh-bridges have two sets of display units, both of them should be verified separately and twice the verification fees is to be charged.

Preliminaries

  • Check whether the digitizer has a fixed descriptive plate.
  • Note the e / d value, maximum and minimum capacity value of the weigh-bridge.
  • Check zero setting of the machine.

Essential Tests

  • Accuracy Test
  • Eccentricity Test
  • Repeatability Test

Accuracy Test

Evaluation of error at different loads

  • At no load, L = 0
  • At 500 ¹e¹ load, L = 5 t (when ¹e¹ = 10kg)
  • Near ¼ max of capacity, L = 25 t when the capacity is 50 t.
  • Near max of capacity, L= 50 t.
  • Calculate Error by using the formula: Error E = I + ½ e - L - Δl
Load MPE
Up to 500e ½ e
Above 500e up to 2000e e
Above 2000e 1.5e

Accuracy Test: Problem with test weights

For a 50 t capacity weigh-bridge, it will be extremely difficult to find test weight of 25 tonne, let alone 50 t.

So, an alternative approach.

  • Suppose only 10,000 kg test weight is available.
  • Check the weigh-bridge at 5 t.
  • Procure an unloaded truck, which is, say 4890 kg.
  • This truck will now may act as test-weight.
  • With the combined weight of 14890 kg, accuracy at that level may be calculated. The machine is now corrected up to 15,000 kg.
  • Procure a loaded truck and say it shows a weight of 22190 kg.
  • For the moment, let us assume that the reading is correct.
  • Place the test weight of 10,000 kg and check whether the reading is 32,190 kg or the difference is within the MPE.

Eccentricity Test

Repeatablity Test

  • Test load available, in place of ½ max and max, 10,000 and 14890 kg.
  • Place them on the receptor in the following order -
    a) 10000 kg wt, note indication I and remove.
    b) 14890 kg wt, note I and remove.
    c) again 14890 kg wt, note I and remove. d) 10000 kg weight, note I.
  • Result should be : a = d and b = c.

Sealing and Stamping

After the tests and verification are over, the digitizer is stamped and sealed on the backside.

  • Stamping plate is marked A.
  • Sealing should prevent opening of the instrument.
  • Use a multi-threaded wire to seal the screws around - E B C D E.



Ensure Full View to the Operator