A Weighty Matter: Patient Scales

Abstract

Patient scales come in a variety of forms, using different technologies and methods to ultimately deliver a reasonable approximation of an object’s or person’s weight. In the medical profession, these measurements are used and recorded in a patient’s record to help monitor well-being, assess the effectiveness of treatment, and determine medication dosage. Scales started out as balances—a beam balanced on a fulcrum with some means of attaching objects to either end was as sophisticated as it got. The object to be weighed was placed on one side and known weights were added to the other side until equilibrium was found. To verify the accuracy of the balance, one had only to trade the places of the object and the weights to see if the equilibrium was once again achieved. This simple device, while very accurate, is hardly practical in the modern medical environment. Eventually the off-center balance was designed. It measured mass using proportionally smaller reference weights, but this balance still had same mechanical principle as the original balance. This highly accurate balance’s only drawback is the inability to verify accuracy. The proportional weight’s accuracy must be verifi ed against another known accurate weight. Losing just one reference weight can render this offcenter balance relatively useless. The Modern Scale The modern mechanical balance beam scale is designed with a reference weight that slides along a beam inscribed with a numerical scale representative of the mass of the object being weighed. This sliding weight, called a poise, is specifi cally calibrated to the beam it slides along. The base of the modern mechanical scale consists of a platform suspended on a rack-and-pinion system, which evenly disperses the weight over the base of the scale. Connecting rods, springs, and levers are used to attach the weighing platform to one end of the balance and the poise slides along the opposite side until the beam is balanced. This same platform system is used in analog and dial scales commonly seen in doctor’s offi ces and medical centers. In this case, the platform is connected to a calibrated coiled spring dial. As more weight is applied, the spring is coiled tighter, and the attached dial twists to a higher number on the face of the gauge.