1. Balances are instruments that measure mass using gravity to act on an object. There are two main types: mechanical balances using springs or sliding weights, and electronic balances using load cells or electromagnetic force. 2. Electronic balances operate using either a load cell that detects strain to measure weight, or an electromagnetic force restoration method that precisely measures current needed to balance a lever. 3. Routine maintenance of balances includes verifying level and zero settings, cleaning weighing surfaces, and annual calibration. Faults like unstable readings may result from the balance not being leveled or calibrated correctly.

1. Balances
 The balance is an instrument
which measures the mass of
a body or substance using the
gravity force which acts on
that body
 At present, there are two
large groups of balances:
mechanical and electronic
balances.

2. Spring balance

3. Sliding weight balance.

4. Analytical balance.

5. Mechanical Balances
1.0 Spring balance
 Its function is based on a mechanical
property of springs as the force exercised
on a spring
2.0 Sliding weight
balance.
3.0 Analytical balance
 This type of balance is equipped with two
known weights which can be moved on
setting scales

6. Operation Verification
 Verify that the balance is leveled.
 Test the zero mechanism.
 Verify and adjust the sensitivity.
a) Lock the balance.
b) Place a standard weight (equivalent to the optical scale range) on the plate.
c) Position the micro setting to one (1).
d) Release the balance.
e) Adjust to the zero position.

7. Maintenance of the mechanical balance
Daily
Annually
Verify the level.
2. Verify the zero setting.
3. Verify the sensitivity
adjustment.
4. Clean the weighing plate.
1.
1.
Calibrate the balance and
document the process.
2. Disassemble and clean the
internal components.

8. Electronic balances
 The weighing plate.
 A measuring device
 Electronic controlling parts and display

9. The weighing plate.

10. load cell

11. Theory of operation
 Electronic balances are mostly used two different
technologies.
 Load cell / Strain gauge and electromagnetic force
restoration.
 Load cell principle is used for all scales with higher
limits and less accuracy like patient and baby
scales, while high quality analytical balances are used
electromagnetic force restoration principle.

12. Load cell / Strain gauge.
 A load on the pan creates as force and
the load cell gets deformed.
 Length-sensitive resistors get deformed
and change their resistance.
 Amplifier creates a voltage out of the
current through the resistances and
delivers a measurement voltage which
corresponds to the weight.
A Wheatstone bridge with three fixed
resistors and a sensor resistor. When the
bridge is in balance the output voltage is zero.

13. Electromagnetic force restoration
 The method give more precise measurements than
the load cell principle.
 The coil is floating in a magnetic field which is
created by an amplifier. The amplifier delivers
always the right current to keep the lever in
balance, regardless of the weight on the pan.
 A optical sensor detects the balance of the leaver.
 The needed current for this balancing is
proportional to the weight on the pan, the current
is also used to measure the weight. Therefore the
current flows through a precision resistor which
creates a measurement voltage.
 An A/D unit transforms the voltage into a digital
signal. The following micro processor with an
appropriate software delivers the measurement
result which is finally show on the display.
 Additional sensors like a displacement detector
and a temperature sensor are also controlling the
amplifier and ensure the accuracy of the balance.

14. Microprocessor base Electronic Balance

15. Power Supply
1.
2.
3.
4.
5.
6.
The input socket with the plug
from the external power supply.
The glass fuse.
A capacitor. Works together
with the following coil as a
filter for the DC input voltage.
Looks like a transformer, but is
just a coil for filtering out any
AC components of the
DC voltage.
Three stabilizers for three
different DC output voltages.
Capacitors for stabilizing the
output voltages.

17. Maintenance Check list
 Clean the weighing plate, Remove any contaminations.
 Check door open and closed is smooth.
 Clean the weighing chamber, externally and internally.
 Check the “Zero point display ” is stable.
 Verify that the adjustment mechanisms on the front door of the
weighing chamber works adequately
 clean, pre-weighed container for weighing (glass container or weighing
paper if possible). Note that plastic can become electromagnetically
charged and is not recommended for weighing powdered or granulated
chemicals.
 Any spill must be cleaned immediately to avoid corrosion or
contamination. Use 70% ethanol to disinfect the pan of the balance.

18. Basic fault Identification
 The weight reading is
incorrect.
 The balance’s reader is
unstable.
 Functional Error

19. 1.0 The weight reading is incorrect.
The balance was not
adjusted to zero before
the reading.
 Place the balance on zero; repeat the
The balance is
incorrectly calibrated.
 Place the balance on zero; repeat the
The balance is not
leveled.
measurement
measurement

20. 2.0 The balance’s reader is unstable.
The balance’s reader is
unstable.
 Place the balance on a stable surface
The front door of the
balance is open.
 Close the front door to measure

21. Functional Error
Readings not
reproducible (hysteresis).
 The measurement cell is dirty.
Non-linear readings
 Non-linear readings
 The measurement cell is badly assembled.
 Mechanical system is in bad condition.
The screen indicates an
overload or negative
condition without a load
being applied.
 Measuring cell damaged by overload.
 Measuring cell is inadequately assembled.