1. Pressure Sensor Calibration
The core principle is applying a known, highly accurate force and measuring the sensor's response.
Primary Standard: The Dead Weight Tester (Piston Gauge).
How it works: Precise known weights are placed on a piston of a known area. The formula Pressure = Force / Area provides a fundamentally accurate pressure value. This is a primary standard because it derives pressure from base SI units (kg, m, s).
Common Practice: Using a high-accuracy reference pressure transducer (e.g., a quartz crystal or precision digital gauge) that has itself been calibrated by a dead weight tester.
The Setup: The calibrator (pressure source), the reference standard, and the Unit Under Test (UUT) are all connected to a common port. Pressure is applied step-by-step (up and down) and the outputs of the standard and UUT are compared.
Key Considerations:
Head Pressure: The height difference between the reference and UUT can cause error if using a liquid media. They must be leveled.
Leaks: The entire system must be leak-tight, especially for low-pressure calibration.
Temperature: Affects the density of the fluid in a hydraulic tester and the performance of the sensors.
Media: The fluid (gas or liquid) must be compatible with the sensors and clean.
2. Flow Sensor Calibration
The core principle is comparing the sensor's output to a directly measured quantity of fluid over a known period of time.
Primary Standard: Gravimetric or Volumetric Calibrator.
Gravimetric: The fluid flows through the UUT and into a collection tank on a precision scale. The mass collected over a precisely measured time provides the mass flow rate. Corrections for fluid density and buoyancy are critical.
Volumetric: The fluid flows through the UUT and into a calibrated volume (e.g., a prover tank with a sight glass). The volume collected over time provides the volumetric flow rate.
Common Practice: Using a master flow meter (like a highly accurate Coriolis or ultrasonic meter) that has been calibrated against a primary standard. The UUT and master are installed in series in a flow loop.
The Setup: A flow loop with a reservoir, a pump to move the fluid, flow conditioning sections, and the master and UUT meters installed in series. The flow is varied across the desired range.
Key Considerations:
Fluid Properties: The calibration is only valid for that specific fluid at that specific temperature and pressure. Changes in density and viscosity significantly affect most flow meters.
Installation Effects: Flow meters are highly sensitive to the piping configuration upstream and downstream (elbows, valves, etc.). Field conditions often don't match the ideal calibration lab setup, leading to errors.
Pulsation & Stability: Flow must be stable and free of pulsation for an accurate calibration reading.
3. Temperature Sensor Calibration
The core principle is placing the sensor in a stable, uniform, and known temperature environment and measuring its electrical output.
Primary Standard: Fixed-Point Cells.
How it works: These cells exploit the fundamental triple points, freezing points, or melting points of ultra-pure materials (e.g., Water: 0.01°C, Gallium: 29.7646°C, Zinc: 419.527°C). These temperatures are defined by nature and are perfectly reproducible, forming the basis of the International Temperature Scale (ITS-90).
Common Practice: Using a stable temperature source (a dry-well calibrator, liquid bath, or furnace) and a reference standard thermometer (like a Standard Platinum Resistance Thermometer - SPRT) that has been calibrated at fixed points. The UUT and reference are placed side-by-side in the heat source.
The Setup: The UUT and reference sensor are inserted into a highly stable thermal source (e.g., a fluidized sand bath, an oil bath, a stirred liquid bath) that can maintain a very uniform temperature. The temperature is set to various points, and the outputs of both sensors are recorded once stabilized.
Key Considerations:
Heat Conduction: Stem conduction—heat traveling up the sensor sheath—is a major source of error. Immersion depth is critical.
Self-Heating (RTDs): The measurement current in an RTD can cause the element to heat itself slightly, leading to a reading that is higher than the actual temperature. This must be managed and specified.
Reference Junction (Thermocouples): A thermocouple measures the difference between its hot junction and cold junction. The cold junction temperature must be measured with extreme accuracy (via a thermistor or RTD) for the overall reading to be valid. This is unique to thermocouples.
Uniformity & Stability: The quality of the calibration is entirely dependent on how uniform and stable the temperature of the source is.
Conclusion: The Unifying Thread
Despite these differences, the fundamental process is the same:
Generate a known, stable physical quantity (pressure, flow, temperature) using a standard that is traceable to national/international standards.
Apply that quantity to the Unit Under Test (UUT).
Compare the output of the UUT to the known value generated by the standard.
Adjust the UUT or define its correction curve (characterize its error) through this comparison.
The essential differences lie in how that known physical quantity is generated and applied, and what environmental factors introduce the most significant errors.
