Understanding the principles of different types of weighing balances used in clinical laboratory

1. Balances
- II DMLT

2. Introduction
 Various instruments are used in clinical laboratory and their working
is based on varied sophisticated techniques.
 For accurate testing of samples and accurate results requires the use
of good instruments.
 A balance is used to find out the mass of a substance by comparing
it with known mass.
 There are 3 type of balances:
1. Physical balances
2. 2 pan analytical balance and
3. Mono – pan balances.

3. Physical balances
 They are generally open 2 pan type of
balance used
for weighing out substances, where great
accuracy is not required.
 It is used to measure the mass of the object.
 These are useful for preparation of qualitative
reagents.

4. Analytical balances
 It is a class of balance designed to measure small mass in the sub-
milligram range

5. Components
 A beam
 Knife edges, screw nuts
 Stirrups
 Scale pans
 A pointer
 Ivory scale
 Rigid supports
 Wooden platform with leveling screws
 Handle
 Central vertical pillar

6. Weighing procedure
 Adjust the leveling screws to make the pillar vertical.
 Release the beam by turning the handle.
 The pointer should swing both the sides equally.
 Otherwise adjust it by turning screw nuts at the end of the
beam.
 Place the object to be weighed in the left pan and standard
weights in the right pan.
 Adjust the weights from weight box so that the pointer swings
to both the sides equally.

7. Care and maintenance
 The beam of the balance should be arrested when not in use
and also before adding weights.
 The weights should not be allowed to lie in the pan and must
be placed in box when not in use.
 The balance should not be loaded with weight greater than
the maximum limit.
 The substance should not weighed hot.
 The cabinet should be closed before weighing
 Clean the balance after use.

8. Monopan balance
 These are electronically operated
 They work on principle of substitution of weights
 The substitution method of weighing may be used.
 In this method, counterpoise weights are added to one pan to
balance the unknown load on the other.
 Then, known weights are substituted for the unknown load.

9. Component
 A beam
 Central knife edge (fulcrum)
 A pan
 Stirrups
 Series of calibrated weights
 A fixed constant counter
weight.

10. Working
 The beam of the single pan balance is situated asymmetrically on
the central knife edge.
 The lengths of the lever arms are not equal.
 The pan is supported by a stirrup placed on the outer knife edge.
 Both knife edges are in the same plane.
 A series of calibrated weights are supported from the same end of
the beam from which the pan is suspended.
 A fixed constant counterweight on the opposite end of the beam
keeps the balance in equilibrium.

11.  The equilibrium is disturbed, when an unknown weight is
placed on the weighing pan.
 The beam deflects in the direction of the heavier side.
 To return the system to equilibrium, the corresponding mass
of calibrated weights is removed from the front of the beam.
 The removed weights give the weight of the unknown
substance.
 This is called as weighing by substitution.

12. Electrical balance
 The electronic balances consists of three basic component
systems
 1. A null detector
 2. A feedback loop to control the balancing loop and
 3. A read out device.

13. Working principle
 The null detector senses the position of the
balance beam.
 It determines the balance point at which the
system is in equilibrium.
 The magnitude of the electrical current from the null detector
is
proportional to the support weight.
 During weighing, due to weight on the pan the system’s
equilibrium is disturbed.
 To return the system to equilibrium, electromagnetic restoring
forces are used in place of standard weights.

14.  The signal from null detector is applied to control a current
through an electromagnetic.
 The compensating force required to bring the balance to
equilibrium is proportional to the weight of the pan.
 The null meter reads zero at equilibrium.
 The current required to produce the compensating force is
converted to a digital display of the weight.

15. Performance of electronic balance
 Switch on and allow the system to warm up for few minutes.
 Calibrate the equilibrium using the O/T switch on the operating
panel.
 Set the units of measurement of your choice (gram/milligram etc.).
 Place petri dish or paper on the weighing pan and press O/T to
calibrate weight of the paper.
 Now add the substance to be weighed onto the paper or petri dish.
 The weight displayed is the exact mass of the substance on the petri
dish or the paper.