Introduction
A pressure sensor converts mechanical pressure (force per unit area) into an electrical signal. Selecting the correct type requires understanding the sensing physics, the application's pressure range and accuracy requirements, the media being measured, and the output format needed by the control system. This guide covers every major sensing technology with quantitative detail for engineering decisions.
Measurement Reference Types
| Type | Reference | Equation | Typical Application |
|---|---|---|---|
| Absolute | Perfect vacuum (0 Pa) | P_abs = P_process | MAP sensor, altitude, barometric |
| Gauge | Atmospheric (101.325 kPa) | P_gauge = P_abs − P_atm | Oil pressure, tyre pressure, tank level |
| Differential | Second process port | P_diff = P_port1 − P_port2 | Filter ΔP, flow measurement, level in closed vessel |
| Sealed gauge | Sealed atmospheric reference | P_sealed = P_abs − P_ref_sealed | High pressure where venting is not possible |
Sensing Technologies
1. Piezoresistive (MEMS Silicon)
The most widely used technology in automotive and industrial applications. A silicon diaphragm with boron-doped piezoresistive bridges deflects under pressure. The gauge factor (GF) of silicon is 100–150× that of metallic strain gauges.
Piezoresistive Sensitivity
ΔR/R = GF × ε. For silicon, GF ≈ 130. For a diaphragm strain ε = 100 µε at full scale, ΔR/R ≈ 1.3% → bridge output ≈ 30–100 mV at 5V excitation. Signal conditioning (INA + ADC) converts to 0.5–4.5V or digital output.
Temperature sensitivity: the piezoresistive coefficient changes 0.1–0.3%/°C with temperature. All industrial and automotive sensors include an on-chip or external temperature compensation ASIC to correct for this effect across the rated temperature range.
2. Capacitive
A pressure-sensitive diaphragm changes the gap between a fixed and moving electrode, altering capacitance: C = ε₀εᵣA/d. Capacitive sensors offer:
- Higher overload tolerance (the diaphragm bottoms out mechanically before damage)
- Lower temperature coefficient than piezoresistive silicon
- Suitable for very low pressure ranges (0–1 kPa differential) where piezoresistive sensitivity is insufficient
- Used in HVAC room pressure, barometric, and medical ventilator applications
3. Piezoelectric
Quartz or ceramic piezoelectric crystals generate charge proportional to applied force: Q = d₃₃ × F, where d₃₃ is the piezoelectric coefficient (~2.3 pC/N for quartz). Key characteristics:
- Only measures dynamic pressure changes — cannot hold a DC reading
- Extremely high frequency response (up to 1 MHz)
- Used for: combustion pressure, knock detection, acoustic emission, blast pressure
- Requires charge amplifier (high-impedance front end) — not compatible with standard voltage-input ADCs
4. Thin-Film Metallic
Strain gauges sputtered directly onto a stainless steel diaphragm. Lower GF (~2–5) than silicon, but superior long-term stability and high overload capability. Preferred for:
- High-pressure hydraulic applications (>500 bar)
- Aggressive media (wet H₂S, chlorine, highly corrosive chemicals)
- Applications requiring <0.1% FS long-term drift over 10 years
Full Technology Comparison
| Technology | Pressure Range | Accuracy | Freq. Response | Temp Range | Cost |
|---|---|---|---|---|---|
| Piezoresistive MEMS | 0–700 bar | ±0.1–0.5% FS | DC to 10 kHz | -40 to +150°C | Low–Medium |
| Capacitive | 0–100 kPa | ±0.1–0.5% FS | DC to 1 kHz | -40 to +125°C | Medium |
| Piezoelectric | Dynamic only | ±0.1% FS dynamic | DC* to 1 MHz | -200 to +600°C | High |
| Thin-film metallic | 0–2000 bar | ±0.1–0.3% FS | DC to 1 kHz | -40 to +200°C | Medium–High |
| Resonant MEMS | 0–10 bar | ±0.01% FS | DC to 100 Hz | -40 to +85°C | Very High |
Error Budget Analysis
Total measurement error (Total Error Band, TEB) comprises:
- Offset error at reference conditions: typically ±0.1–0.5% FS
- Non-linearity: ±0.1–0.5% FS — deviation from best-fit straight line
- Hysteresis: ±0.1–0.2% FS — difference between increasing and decreasing pressure readings
- Temperature coefficient of offset (TCO): ±0.01–0.05% FS/°C
- Temperature coefficient of span (TCS): ±0.01–0.03% FS/°C
- Long-term drift: ±0.1–0.5% FS per year
TEB Calculation Example
For a sensor with ±0.5% FS accuracy at 25°C, ±0.03%/°C TCO, and ±0.02%/°C TCS, operating at 125°C above the reference temperature: TEB = √(0.5² + (0.03×100)² + (0.02×100)²) = √(0.25 + 9.0 + 4.0) = ±3.7% FS. This shows that temperature effects dominate total error — specification of accuracy at 25°C alone is insufficient for field applications.
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