Digital Balance

A digital balance is a non-automatic weighing instrument in which a load cell converts the gravitational force on a mass into an electrical signal to show the weight digitally on the display panel.

 

Components

Load Cell

A load cell is typically an electronic device (transducer) used to convert a force into an electrical signal.

  • Normally, a load cell consists of four strain gauges in a Wheatstone bridge configuration.
  • The electrical signal output is normally in the order of a few milli-volts and requires amplification before it can be used.
  • The output of the transducer is plugged into an algorithm to calculate the force applied to the transducer.


Compressor Type

S Type

Analog / Digital Converter (ADC)

A device that converts analog signals received from the load cell into binary code understood by a micro-controller or processor. The A/D converter may be contained on a single chip or can be one circuit within a chip.

All ADCs suffer from non-linearity errors caused by their physical limitations. The output deviates from a linear function of input. Such errors are corrected by calibration, or prevented by testing.

Gain Amplifier

The output of a load cell is very weak, in the form of a few mili-volts. It requires to be amplified by a gain-amplifying device.

Digital filter

A digital filter eliminates unwanted signals caused by mechanical vibrations, electrical interferences etc.

Serial Interface

It is the device for serial communication - a process of sending data one bit at one time, sequentially, to the micro-controller. Serial communication enables data transfer at a very high speed.

Micro-Controller

A micro-controller is a computer-on-a-chip, containing a processor, memory, and input/output functions.

Peripheral Devices

  • Power Supply Unit (Step down Transformer, Diode, Heat Sink, Battery Backup and charger, On/Off switch)
  • Leveling Device
  • Control Panel (Tare, Calibrating Devices, zero setting etc.)
  • Optional (price calculating, bill printing etc.)

Mono-block Technology

Here the load cell, A/D Converter, memory, Micro-controller, etc. constitute a single block. As the technology involves no external connection between the parts; it increases efficiency and stability of the machine and also reduces the size.

Working Principle

  • A digital balance uses an analog signal from a load cell, which is converted directly to a pulse width modulated output signal. This pulse width modulated output signal represents the weight of an object on the weighing pan and is read by a micro-controller. The micro-controller then converts the signal representing the weight of the object on the pan to a weight value. The value is displayed on a LCD panel.
  • The micro-controller is responsible for calibrating an instrument i.e. correcting an error in weighing. It also controls the function of buttons on the control panel and the display panel.
  • The micro-controller also has an in-built or attached memory chip. It is a temporary storage device required during the processing of the information (signal) by the micro-controller.

Leveling and Testing

Essential tests

  • Compliance of Table 17
  • Error Calculation and Weighing Performance Test.
  • Repeatability Test
  • Eccentricity Test
  • Zero setting
  • Discrination
  • Accuracy of Tare setting
  • Price Computation

 

Levelling

A digital balance should be levelled before any use or testing.

  • Leveling is done by adjusting screws at the bottom of a balance.
  • The level indicator will show the level position.

Classification Principle

Checking Classification

Example 1

A machine has max capacity = 1 kg.  Declared "e" value = 0.1 g.

20 e 20 X 0.1 g 2 g Class III
50 e 50 X 0.1 g 5 g Class II

Example 2

A machine does not declare minimum capacity.
It simply declares - class III machine, "e" value 0.1 g, max capacity 2 kg.
Now, n = 2 000 / 0.1 = 20 000.
This should be treated as Class II machine.

Calibration

  • Calibration is a procedure that would ensure, for example, that a 100.00 gram mass would actually weigh out at 100.00 grams on the scale.
  • Calibration may be Internal or External and the procedure varies from machine to machine. Only examples are given here.

Internal Calibration

Internal calibration is most accurate but also costly.

Instruments with internal calibration facility contain internal calibration weights that are quite accurate, and can be used by the operator routinely.

External Calibration

Accuracy of external calibration depends on the accuracy of the weight used but is cheaper.

Unethical Calibration

All scrupulous manufacturers of digital balances employ a system which only allow a certain external weight within MPE for external calibration. But, others will allow any weight to be used for calibration. The calibration is done by pressing a number of keys. A password is sometimes used, sometimes not. Such devices allow a trader to cheat consumers by any quantity.