Relative humidity looks simple but suffers temperature coupling, hysteresis, contamination, and self-heating. Capacitive polymer RH sensors dominate IAQ and industrial monitoring, yet factory calibration cannot replace periodic field verification. Senseiot shares calibration practices from buildings, warehouses, and greenhouses in this end-to-end workflow.
RH Basics and Error Sources
RH is the ratio of partial vapor pressure to saturation pressure at the same temperature. Capacitive sensors track polymer dielectric change; outputs are 0–100% RH plus temperature for dew-point math.
Key errors: hysteresis (especially wet-to-dry), self-heating under continuous power, film contamination, and PCB condensation from poor layout.
Confirm non-condensing range (often 0–95% RH) before calibrating—exceeding limits can permanently drift the film.
Lab Calibration: Saturated Salts
Sealed saturated salts produce stable RH—NaCl ~75.3% at 25 °C, MgCl₂ ~32.8%. Equilibrate 4–24 h, compare reading, store correction factors.
Low cost and repeatable for production sampling. ±0.1 °C bath error can cause ~0.5% RH error.
Use two or three points to fit linear/quadratic correction. Senseiot modules accept I2C cal coefficients—see the Product Catalog.
Precision: Generators and Dew-Point References
Humidity generators sweep 10–90% RH with chilled-mirror references at ±0.5% RH uncertainty—pharma and metrology grade.
Stabilize → compare DUT → record curve → load LUT/polynomial → verify mid-point (e.g., 50% RH).
Climate chambers calibrate dozens of IoT nodes in parallel. Bulk services via Request a Quote.
Field Verification and Maintenance
Compare fixed sensors every 6–12 months with portable references at the same location. Clean or replace if drift exceeds ±3% RH.
Field salt jars are trend-only, not traceable. Calibrate under representative conditions—not right after humidifier startup.
Mount away from direct mist for stable readings—see Industry Applications mounting guides.
T-RH Compensation and Firmware
Factory T-RH compensation still fights board self-heating. Use intermittent power, filtering, and dew-point consistency checks.
Flag invalid data when RH is out of 0–100% or dew point disagrees with RH/T.
Median fusion across three probes resists single-point contamination in Industry Application warehouses.
Contamination Recovery and Replacement
Contamination shows slow high-RH response, high low-RH offset, or failure to return below ~30%. Distilled water rinse only if allowed—no solvents.
Zero >5% RH in dry air or repeat drift within a month means replace. Log serial numbers and environment.
Senseiot supplies interchangeable industrial RH probes—Product Catalog.
Documentation and Quality Systems
Log date, reference ID, ambient T/RH, before/after readings, operator—for ISO 9001/GMP retain 3–5 years.
Cloud calibration events support remote audit; sign OTA coefficient updates.
Bulk SOP or metrology liaison—contact Senseiot via Request a Quote.