Weighing Balances

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: Physical balances 2 pan analytical balance and 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.