Understanding Moisture Measurement in Oil (ppm vs Water Activity – aw)

Why do ppm readings from online sensors differ from lab results?

In many applications, users compare online moisture sensor readings (ppm) with lab reports, and find differences.

This happens because:

ppm (water content) can be measured accurately only through laboratory methods such as Karl Fischer titration.

🔬 What is the most accurate way to measure ppm?

The industry-accepted method for exact moisture content is:

👉 Karl Fischer Titration

Directly measures actual water content (ppm)
Highly accurate and repeatable
Used as a reference standard

How do online moisture sensors work?

Online sensors (like those used in filtration/dehydration systems) do not directly measure ppm.

They measure:

👉 Water Activity (aw) — a thermodynamic property of moisture in oil

Then:

👉 ppm is estimated from aw using internal calibration curves based on oil type.

⚠️ Why ppm from sensors may not be accurate

Sensor-based ppm values can vary due to:

Oil type differences (mineral, synthetic, additives)
Oil ageing and contamination
Temperature variations
Presence of free or emulsified water
Sensor calibration assumptions

In high moisture conditions, ppm values from sensors can be misleading.

✅ What is Water Activity (aw)?

Water activity (aw) represents:

👉 How much moisture is active and available in the oil

Range: 0 to 1
aw = 1 → free water present
aw < 0.3 → dry oil condition

🎯 Why aw is more useful for online monitoring

Water activity is:

✔ Directly measured (not calculated)
✔ Independent of oil type variations
✔ Highly responsive to drying
✔ Ideal for real-time trend monitoring

📊 Practical Comparison
Parameter ppm (Sensor) aw (Water Activity)
Measurement type Calculated Direct
Accuracy Depends on calibration High
Reliability in high moisture Low High
Suitable for real-time monitoring Limited Excellent
Lab reference method Karl Fischer Not required

Exact ppm can only be determined by Karl Fischer titration. Online sensors estimate ppm based on water activity and calibration curves.

For best results:

Use aw for real-time monitoring and trend analysis
Use Karl Fischer for exact ppm verification when required

Online sensors measure water activity (aw) and estimate ppm using calibration curves, whereas exact ppm can only be determined through Karl Fischer titration.

Neerokleen systems utilize water activity (aw) for reliable real-time monitoring of moisture removal, ensuring accurate trend analysis even under high moisture conditions where ppm values may be misleading.