Electronic Analytical Balance

Figure 1.1 shows a diagram and a photo of an electronic analytical balance. The pan rides above a hollow metal cylinder that is surrounded by a coil that fits over the inner pole of a cylindrical permanent magnet. An electric current in the coil a magnetic field that supports or levitates the cylinder, the pan and indicator arm, and whatever load is on the pan. The current is adjusted so that the level of the indicator arm is in the null position when the pan is empty. Placing an object on the pan causes the pan and indicator arm to move downward, thus increasing the amount of light striking the photocell of the null detector. The increased current from the photocell is amplified and fed into the coil, creating a larger magnetic field, which returns the pan to its original null position. A device such as this, in which a small electric current causes a mechanical system to maintain a null position, is called a servo system. The current required to keep the pan and object in the null position is directly proportional to the mass of the object and is easily measured, digitized, and displayed. Electronic balances are calibrated by weighing a standard mass and adjusting the current so that the exact mass of the standard appears on the display.
 

Figure 1.1 Electronic analytical balance. (a) Block diagram. (b) Photo of electronic balance. (a) Reprinted (adapted) with permission from R. M. Schoonover, Anal. Chem., 1982, 54, 973A. Published 1982, American Chemical Society.

Figure 1.2 shows the configurations for two electronic analytical balances.  In each, the pan is tethered to a system of constraints known collectively as a cell. The cell incorporates several flexures that permit limited movement of the pan and prevent torsional forces (resulting from off-center loading) from disturbing the alignment of the balance mechanism. At null, the beam is parallel to the gravitational horizon, and each flexure pivot is in a relaxed position. Figure 1.2a shows an electronic balance with the pan located below the cell.

Figure 1.2 Electronic analytical balances. (a) Classical configuration with pan beneath the cell. (b) A top-loading design. Note that the mechanism is enclosed in a windowed case. (a) Reprinted (adapted) with permission from R. M. Schoonover, Anal. Chem., 1982, 54, 973A. Published 1982, American Chemical Society. (b) K.M. Lang. Amer. Lab., 1983, 15(3), 72. Copyright 1983 by International Scientific Communications, Inc.

  Higher precision is achieved with this arrangement than with the top-loading design shown in Figure 1.2b. Even so, top-loading electronic balances have a precision that equals or exceeds that of the best mechanical balances and additionally provides unencumbered access to the pan.

Electronic balances generally feature an automatic taring control that causes the display to read zero with a container (such as a boat or weighing bottle) on the pan. Most balances permit taring up to 100% of the capacity of the balance. Some electronic balances have dual capacities and dual precisions. These features permit the capacity to be decreased from that of a macrobalance to that of a semimicrobalance (30 g) with a corresponding gain in precision to 0.01 mg. These types of balances are effectively two balances in one.

A modern electronic analytical balance provides unprecedented speed and ease of use. For example, one instrument is controlled by touching a single bar at various positions along its length. One position on the bar turns the instrument on or off, another automatically calibrates the balance against a standard mass or pair of masses, and a third zeros the display, either with or without an object on the pan. Reliable mass measurements are obtainable with little or no instruction or practice.