What are the main parameters of the humidity sensitive Resistors?

* Question

What are the main parameters of the humidity sensitive Resistors?

* Answer

1) Nominal resistance R0R_0

Definition: Resistance under specified conditions (typically 25 °C, 60 %RH, defined test voltage/frequency).

Typical range: 10 kΩ to 1 MΩ.

Measurement note: Prefer AC excitation (e.g., 0.1–1 V_rms, 1–10 kHz) to avoid DC polarization and self-heating.

2) Humidity operating range & resolution

Range: Commonly 20–95 %RH (or wider).

Resolution: Set by your readout/ADC and curve slope; the sensor has no fixed “step size.”

Tip: Emphasize effective resolution under real conditions (temperature, airflow), not the theoretical ADC bits.

3) Sensitivity / humidity coefficient

Definition: Relative resistance change per %RH, e.g.

S=ΔR/R₀/ΔR_H or d(logR)/dRH

Behavior: Generally monotonic, often log/exponential; use a calibrated transfer curve (linear/quadratic/log fits).

4) Linearity / nonlinearity

Definition: Max deviation from best-fit curve, usually as % of full scale.

Typical: ±1–±3 %FS. Lower nonlinearity simplifies calibration.

5) Accuracy & repeatability

Accuracy: Error vs. reference at specified points (e.g., 25 °C, several RH points).

Repeatability: Short-term standard deviation over repeats.

Typical: Accuracy ±2–±5 %RH; repeatability better than ±1 %RH.

6) Hysteresis (adsorption/desorption)

Definition: Difference between up-sweep and down-sweep at the same RH.

Typical: 1–3 %RH. Polymer elements often show more hysteresis than some ceramic/glassy types.

Tip: For applications with oscillating RH, prioritize low hysteresis.

7) Response / recovery time

Definition: t₉₀ to reach 90 % of a step change (humidifying / dehumidifying).

Typical: Seconds to tens of seconds; protective membranes slow it down.

Specify test conditions: 25 °C, airflow (e.g., 1 m/s).

8) Temperature effect (TCR at constant RH) & temperature range

TCR: Drift at fixed RH vs. temperature, in %/°C or ppm/°C.

Operating range: e.g., −20 to +60 °C; exceeding it can age the sensing film.

Compensation: Use a co-located temperature sensor and 2-D compensation (RH & T).

9) Long-term stability / drift

Definition: Change in zero/slope over time (e.g., 1000 h, 1 year).

Typical: ≤ ±1–±2 %RH/year in benign environments.

Practice: Plan periodic calibration for critical uses.

10) Excitation & maximum measuring power

Prefer AC drive; DC can cause electrolysis/ion migration → drift & hysteresis.

Limit current/voltage (e.g., ≤ 10–50 µA) to avoid self-heating.

11) Cross-sensitivity & environmental robustness

Cross-sensitivities: Organic solvents, acidic/alkaline gases, silicones, dust.

Condensation: Causes temporary errors; use hydrophobic, breathable membranes and anti-fog/anti-condensation strategies.

12) Package / protection

Forms: SMD, probe, vented packages with membranes.

Trade-off: Better protection → slower response.

Selection & application notes

Target band: If the application lives in 30–80 %RH, optimize accuracy and hysteresis in that band.

Temperature compensation: Pair with a temperature sensor; build a 2-D calibration table.

Readout circuits:

AC drive + bridge / transimpedance / divider → ADC, keeping sensor power tiny.

Or current source + sense resistor, again minimizing dissipation.

Calibration: Use saturated salt solutions at 25 °C to generate reference RH points (typical values):
LiCl ≈ 11 %, MgCl₂ ≈ 33 %, Mg(NO₃)₂ ≈ 54 %, NaCl ≈ 75 %, KCl ≈ 85 %, K₂SO₄ ≈ 97 %. Record temperature.

Maintenance: Define reconditioning/drying steps and re-calibration intervals (e.g., every 6–12 months).

Quick reference table (example values)

One-line takeaway: Specify and validate sensitivity curve, hysteresis, temperature effects, long-term drift, and safe excitation limits—with clear test/calibration conditions—and you’ll have a sensor that’s both accurate and durable in the field.