Silicon Piezoresistive Pressure Diaphragm Encapsulation Selection Criteria
Encapsulation selection balances gel potting low cost against oil-filled cell thermal stability to control long term zero drift and field recalibration expenses.
Volumetric Expansion Alpha_v
Volume Dilatometry
Thermodynamic equations dictate how the volume of a solid or liquid changes in response to temperature variations. The coefficient known as volumetric expansion alpha_v represents the fractional change in volume per degree of temperature change at a constant pressure. It is used to predict the expansion of fill fluids in sealed pressure transmitters during high-temperature sterilisation cycles.
Calibration Setup
Accurate measurement of this coefficient requires the use of a volume dilatometer or a thermomechanical analyzer under tightly controlled conditions. The instrument is calibrated using a reference material of high purity, such as platinum or copper, whose expansion behavior is traceably documented. During testing, the sample temperature is ramped slowly to maintain thermal equilibrium and avoid internal temperature gradients.
The resulting volume change is measured using a displacement sensor with a resolution of less than one micrometre. This precise calibration is necessary because any temperature mismatch or sensor drift introduces significant measurement errors in the calculated coefficient.
Thermal Interference
Practical sensor applications must account for temperature-induced density changes. For example, if a pressure transducer fill fluid expands, it exerts an additional pressure on the sensing diaphragm, which is known as a temperature effect on zero. This interference must be compensated for using a calibration curve stored in the transmitter’s microprocessor.
The correction algorithm uses the volumetric expansion alpha_v to subtract the thermal pressure component from the raw measurement.
Fluid Constraint
Phase transitions alter the behavior of the material during thermal cycling. If a fluid reaches its boiling point or a polymer undergoes a glass transition, the expansion coefficient changes abruptly. This transition destroys the accuracy of linear compensation algorithms.