What is the Principle Behind Analytical Balances?
The analytical lab balance is available for 60 years now; and it has been steadily evolving during that period. Leading equipment manufacturers have come up with continuous invention to improve its performance. The aim is to increase the precision and accuracy of the analytical lab balance and to make it more reliable for researchers.
The earliest analytical balance, manufactured in 1945 had a single pan. In 1971, the first major advancement took place, when the nanogram balance made precision weighing possible. This balance was actually used to weigh the rocks from the moon, which were brought back by Neil Armstrong. Then, in 1974, Mettler produced its PT1200 scale, the industry’s first fully electronic precision balance. It had a capacity of 1,200 grams and sensitivity of 0.01 grams. Then further advancement continues. The up-to-date analytical balances need just the touch of a key for automatic motorized leveling; and are top-loading with a motorized draft shield and a resolution of 61 million digits.
Equal arm balances have become practically obsolete and almost all analytical balances today are electronic. These balances can be either single pan or two pans. Single pan balances do not function on the traditional principle of balancing a material to be weighed against standard masses. Its mass is computed through load cells following the principles of electromagnetic force compensation.
This is how it works. When the test sample is put on the analytical balance pan (usually on a weighing paper), a displacement of the load cell occurs; and the vertically downward force exerted by the “weight” displaces the coil, causing the load cell to generate a current and return a compensation circuit. This current is converted to a voltage and uses electronic circuitry and appropriate software to attain proper calibration and display the measured weight.
Two-pan analytical balances use a precision chain and a multi-weight carrier activated by dials. The precision chain, in conjunction with internal weights on a carrier, has similar function with traditional weights. The difference is that the personnel does not need to open the glass enclosure to add weights, as long as they are less than 100 grams. Therefore, once the measured weight is within 100 milligrams of the desired weight, he just dials in the incremental weight required. This obviates the necessity of opening the balance case to arrest the beam . He also does not need to physically add additional weights, then release the beam and observe its swing before closing the case. This can save time for repeated weighing operations.
The principle employed by some direct reading analytical balances is constant load balance. It has the beam, ring weights and pan at one end, where the objects to be weighed are placed. The balance weight, load, is at the other end. When a substance is placed on the pan, it disturbs the balance that is, it disrupts the equilibrium. Therefore, in order to reestablish balance equilibrium, the corresponding amount of weight is removed. The microscale in the balance then reads the projection and provides the right value. In order to increase the efficiency and reduce vibrations of the balance, the operating knob is generally located in front on the base of the balance. It will also allow the personnel to comfortably work on the balance for longer.
This article was written by Firoze Hirjikaka, a retired Civil/Structural Engineer with a graduate degree from London University. He explores his passion for scientific & engineering equipment at Tovatech a leading American supplier of analytical balances. For more information on this article visit the Tovatech site from any of the above links.
