Quick Answer
Humidity sensor calibration uses saturated salt solutions (LiCl, NaCl, K₂SO₄) as reference humidity standards at defined temperatures to generate known relative humidity levels (11.3%, 75.3%, 97.3% RH). The sensor under test is placed in the calibration chamber and its output is compared with the reference RH at 3–5 calibration points.
Key Takeaways
- Humidity sensor calibration uses saturated salt solutions (ASTM E104) or dew point hygrometer as reference.
- Calibration range: 10% to 95% RH at temperatures from +5°C to +60°C.
- Expanded uncertainty: ±1.5% RH (k=2) for capacitive sensors, ±2.5% RH for resistive sensors.
- Humidity sensors in pharma stability chambers must be calibrated every 6–12 months.
- Prism calibrates all types: capacitive RH sensors, resistive RH sensors, psychrometers, dew point meters.
Humidity Instruments We Calibrate
- Capacitive relative humidity sensors and transmitters (Vaisala, Rotronic, E+E)
- Resistive humidity sensors
- Aspirated psychrometers (wet-dry bulb)
- Hair hygrometers
- Dew point meters and chilled mirror hygrometers
- Combined temperature/humidity data loggers (Testo 175, Ebro EBI 20)
- Stability chamber built-in humidity sensors
- Cleanroom environmental monitoring sensors
Saturated Salt Solutions — The Reference for Humidity Calibration
| Salt Solution | RH at 25°C (%) | Application Range |
|---|---|---|
| Lithium Chloride (LiCl) | 11.3% | Very low humidity (dry rooms) |
| Magnesium Chloride (MgCl₂) | 32.8% | Low humidity (pharmaceutical) |
| Magnesium Nitrate (Mg(NO₃)₂) | 52.9% | Mid-range calibration |
| Sodium Chloride (NaCl) | 75.3% | General purpose, ambient |
| Potassium Sulphate (K₂SO₄) | 97.3% | High humidity (tropical, food) |
Critical Humidity Applications in Gujarat
Pharmaceutical stability chambers (ICH 40°C/75% RH for Zone IVa, 30°C/65% RH for Zone III) require ±2% RH accuracy for product shelf-life studies. Food processing facilities require humidity monitoring for baking, drying, and cold storage. Textile mills monitor humidification for yarn conditioning. All require NABL-calibrated humidity sensors to demonstrate regulatory compliance.
Frequently Asked Questions
What is the best calibration point for humidity sensors in pharma stability chambers?
For ICH Zone IVa stability chambers (40°C/75% RH), calibrate at least at 75% RH and 40°C, and verify at 65% RH (lower alarm) and 85% RH (upper alarm). Use Prism's NABL certificate for audit compliance.
How accurate can humidity sensors be calibrated to?
Modern capacitive humidity sensors can be calibrated to ±1.5% RH expanded uncertainty (k=2). Older resistive sensors and hair hygrometers may have ±3–5% RH uncertainty. Dew point meters can achieve ±0.2°C dew point accuracy.
Can humidity sensors be calibrated on-site?
Yes. Prism can perform on-site humidity sensor calibration for stability chambers, cleanrooms, and warehouses using portable humidity calibrators with reference NABL-calibrated RH standards.
How often should humidity sensors in cleanrooms be calibrated?
ISO 14644 and WHO GMP Annex 1 recommend annual calibration for cleanroom humidity sensors, with 6-month intervals for Grade A/B areas.
Does Prism calibrate dew point transmitters for compressed air systems?
Yes. Prism calibrates dew point transmitters used for compressed air quality monitoring (ISO 8573) and nitrogen blanket systems in chemical plants.
Written by
Er. Parthiv Kinariwala
Managing Director · Prism Calibration Centre · NABL CC-2480 · Ahmedabad
Er. Parthiv Kinariwala founded Prism Calibration Centre in 2004 and has over 20 years of hands-on experience in calibration engineering, NABL accreditation, and industrial compliance. His team performs 10,000+ calibrations annually from the Vatva GIDC laboratory, serving 5000+ industries across Gujarat.
Prism Calibration Centre — Vatva GIDC, Ahmedabad
Prism Calibration Centre
F-101, Rudraksh Complex 2, Phase 3, GIDC Vatva, Near Jasoda Nagar Cross Road, Ahmedabad — 382445, Gujarat, India
Phone: +91 98245 26444
Email: info@prismcalibration.com
NABL: CC-2480 · ISO/IEC 17025:2017
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