A sliding weight scale, often called a beam or physician scale, is a mechanical device used to measure mass accurately. Unlike digital scales, it operates by balancing counterweights against the object’s mass, making it highly reliable as it requires no external power. These scales are commonly found in medical offices and fitness centers where consistent, verifiable measurements are needed. Reading the scale involves a precise sequence of steps.
Identifying the Key Components
The scale consists of several interconnected parts that work together to achieve equilibrium. The subject stands on the main platform, which links to the balance mechanism. The most visible part is the beam, a long lever that pivots when weight is applied.
The beam holds two distinct sliding weights, called poises, that move along calibrated scales. The lower beam holds the larger poise for gross measurements, often in 50-pound increments. The upper beam holds the smaller poise for fine measurements, such as fractions of a pound.
A balance indicator, usually a pointer, signals when the scale has achieved equilibrium. A calibration knob near the zero point fine-tunes the scale’s starting balance.
Essential Preparation: Zeroing the Scale
Before any measurement is taken, the scale must be precisely calibrated, a process known as zeroing. This ensures the baseline reads exactly zero when the platform is empty, eliminating structural imbalance. First, confirm the scale is placed on a hard, level surface, as an uneven floor can skew the final reading. Next, slide both the large and small poise weights all the way to the left, positioning them squarely on the zero mark. If the balance beam is tilted, turn the calibration knob, usually located near the base, until the balance indicator rests exactly in the center of its slot. This movement adjusts a small counterweight, setting the true zero point and preparing the instrument for an accurate reading.
The Measurement Process and Calculation
The measurement process begins once the scale is zeroed and the subject steps onto the center of the platform. The balance beam will immediately rise, indicating the subject’s weight is currently unbalanced.
The first action is to determine the approximate weight using the large poise on the lower beam. Move this weight to the right, notch by notch, until the balance beam drops down, signaling the poise has passed the subject’s actual weight. Slide the large poise back one notch to the last position where the beam remained elevated. This maneuver isolates the subject’s weight to a specific range.
The final, precise adjustment is made using the small poise on the upper beam. Slowly slide the small poise along the finely marked beam until the balance indicator settles exactly in the center of its viewing window. This movement achieves mechanical equilibrium, meaning the combined mass of the two poises counters the subject’s mass on the platform.
The final step is to calculate the total weight by adding the value indicated by the large poise on the lower beam to the value indicated by the small poise on the upper beam. For example, if the large poise is resting on the 150-pound mark, and the small poise is resting on the 6.5-pound mark, the subject’s total weight is 156.5 pounds.