Fundamental Area
Geometric surface of a piston-cylinder assembly that converts the applied force of a mass into fluid pressure represents the fundamental metrological constant of a deadweight tester. This parameter, called the effective area, is approximately the mean of the cross sectional areas of the piston and the cylinder. It is used to calculate pressure from first principles.
Calculated Pressure
Accurate determination of this parameter is essential for primary pressure standards. During calibration, the effective area is calculated by dividing the force of the loaded masses by the known reference pressure. Any error in this area directly affects the accuracy of all subsequent measurements.
Environmental Influence
Temperature variations alter the physical dimensions of the assembly. To account for this, the effective area is adjusted using the thermal expansion coefficients of the materials. Operators must measure the temperature of the piston-cylinder during operation to apply this correction.
Measurement Traceability
Pressure-induced distortion also modifies the effective area at high operating limits. As pressure rises, the cylinder expands and the piston compresses, causing the effective area to change in a linear fashion. Metrologists use a distortion coefficient to correct for this effect, ensuring the instrument remains accurate across its entire pressure range even when operating at several hundred megapascals.
