LimsPulse
Food & proximate analysis

Water activity meter

Measures how much of the water in a food is free to support microbial growth and chemical change, as the ratio of the food's vapour pressure to that of pure water. Read on a scale from 0 to 1.

  • Food & beverage
  • Confectionery and bakery
  • Meat and dairy
  • Pet food
  • Pharmaceutical
  • Cosmetics
Illustration of Water activity meter
LEARN & PRACTICE

Test your aw meter knowledge

Multiple-choice questions

Pick an answer to see whether you are right and why.

  1. Q1. Water activity is defined as:

    basic

  2. Q2. What is the water activity of pure water?

    basic

  3. Q3. Below about which water activity does almost no microbial growth occur?

    basic

  4. Q4. Which sensing principle is used by the most accurate water activity meters?

    intermediate

  5. Q5. Why must the water activity result always be reported with the temperature?

    intermediate

  6. Q6. A saturated sodium chloride solution at 25 °C has a water activity of about:

    intermediate

  7. Q7. Readings on a capacitance meter drift upward when testing rum-soaked cakes. What is the likely cause?

    advanced

Viva questions

Think of your answer first, then open it to compare.

  1. 1. Explain the difference between moisture content and water activity.

    Moisture content is the total amount of water (and volatiles) in the food, by mass. Water activity is how available that water is, measured as relative vapour pressure. Sugar, salt and structure bind water, so foods with similar moisture can have very different aw. Water activity, not moisture, controls microbial growth.

  2. 2. How do you verify a water activity meter before use?

    Let the meter reach its set temperature, then read one or two salt standards near the sample range (for example 0.760 and 0.984). Each reading must agree with the standard within the instrument's tolerance (about ±0.003 for dew point instruments). If not, clean the chamber, re-test with fresh standards and adjust or send for service.

  3. 3. Why is water activity used to decide whether a food needs refrigeration?

    Pathogens need available water to grow. Foods with low enough aw (often combined with low pH) cannot support pathogen growth and can be shelf-stable. US FDA rules, for example, treat foods with aw above 0.85 and pH above 4.6 as low-acid foods that need a validated process or refrigeration.

In LimsPulse · Calibration Management

Keep every aw meter in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.

See how