Hospital beds equipped with integrated scales measure the total weight on the frame but are designed to provide only the patient’s weight. This equipment, often found in Intensive Care Units (ICU) and other areas where patients are immobile, allows healthcare providers to monitor body weight changes without moving the person. Precise weight tracking is necessary for accurate medication dosing, especially for weight-dependent drugs, and for monitoring fluid balance in conditions like heart failure or kidney failure. The technology achieves this measurement by systematically removing the bed’s own weight from the overall reading through a specific process called “taring” or “zeroing.”
The Basic Mechanism of Bed Scales
Hospital bed scales rely on specialized sensors known as load cells, typically built into the bed’s frame near the legs or corners. Four load cells are the most common configuration used. A load cell is a transducer that converts mechanical force—the downward pressure of weight—into an electrical signal.
The mechanical force causes a slight deformation in the load cell’s metallic component, which is lined with strain gauges. This deformation changes the electrical resistance of the gauges, causing a proportional change in the output voltage. A microprocessor then processes this electrical signal and translates the data into a measurable weight value. This system measures the entire vertical load, including the patient, the mattress, and the bed frame itself.
Isolating the Patient’s Weight
The process that isolates the patient’s weight from the total load is called taring, or zeroing the scale. Before the patient is placed in the bed, a healthcare worker will press the “tare” or “zero” button on the control panel. This action instructs the internal computer to recognize the current total weight—the bed frame, the mattress, the sheets, and any standard pillows—as the zero point.
By setting this initial force as the baseline, the scale effectively subtracts the weight of the bed and its standard accessories from all subsequent measurements. When the patient is in the bed, the scale measures the new, total force and displays only the difference between the total force and the established zero point, which is the patient’s weight. This calculation allows the reading on the screen to represent the patient’s body mass alone.
If non-standard items are added to or removed from the bed after the initial taring, the reading will become inaccurate until the scale is re-tared. For instance, if a heavy blanket or a specialized pressure-relieving overlay is placed on the bed, the scale must be zeroed again with these items in place. The tare weight must be updated whenever the components on the bed change, ensuring the displayed weight remains the result of the total measured force minus the tare weight.
Factors Affecting Measurement Accuracy
Achieving an accurate weight reading requires careful adherence to procedure, as several practical issues can compromise the scale’s precision. Patient movement is one of the most common factors, as any shifting or moving while the measurement is being taken causes the load to fluctuate across the sensors, resulting in an unstable and invalid reading. To ensure a reliable number, the patient must be completely still and centered on the mattress while the scale stabilizes.
External physical contact with the bed frame can also introduce significant errors into the measurement. Any external item that is touching the bed but is not part of the tare weight will register as additional patient mass. This includes IV poles leaning against the frame, drainage bags resting on the floor and connected to the patient, or a nurse leaning on the side rail during the reading.
Inadequate calibration is another source of error, as studies show bed scale weights can sometimes be inaccurate compared to standing scale weights. Regular maintenance and calibration of the load cells are necessary to ensure the system’s sensitivity remains within acceptable medical tolerances. For the most accurate result, all external items must be removed, the bed should be in a flat or standardized position, and the scale must be properly tared before a weight is taken.