While reviewing flyers and brochures of water activity instruments, one could get confused about which factors are relevant when considering which instrument you should buy. Since some of the information on these flyers can be misleading or even false, we thought it might be a good idea to get an unbiased review by one of the world’s leading water activity scientist, Dr. Brady Carter from Carter Scientific Inc.

Out of many factors that could influence you when purchasing a water activity instrument, he has distilled down the most important ones for you to consider while evaluating a water activity meter:

  1. Accuracy
  2. Maintenance
  3. Difficulty/Cost of Verification
  4. Sensor Hysteresis
  5. Adjustable Equilibrium
  6. Detect Equilibrium
  7. Measuring Volatiles
  8. Temperature Control
  9. Data Control

Depending on the product, there are other factors that you should consider. But the above criteria apply to all applications.

1. Accuracy

When measuring water activity, you want your results to be accurate and precise. If you cannot rely on the integrity of your results, measuring water activity is pointless.

2. Maintenance

You should always include the running costs of the instrument in your purchasing decision. This includes the time and labour required to maintain the performance of the water activity measuring device. Does the instrument become easily contaminated and have to be cleaned often and with great effort?

3. Difficulty/Cost of Verification

An important factor when deciding on which water activity instrument will work the best for you is how easy it can be verified to ensure accurate measurements. Salt standards are used to verify the instruments performance, but some standards are consumables and can drive up costs. Does the instrument you are considering use consumable standards or reusable saturated salt slurries?
Video coming soon.

4. Sensor Hysteresis

The impact of repeated wetting and drying of the water activity measuring sensor should also be considered when purchasing a water activity instrument. Hygroscopic polymer sensors suffer hysteresis that shifts the calibration during repeated wetting and drying cycles while the electrolytic sensor does not. Which type of sensor is in the instrument you are considering?
Video coming soon.

5. Adjustable Equilibrium

Water activity measurement depends on measuring the relative humidity of a headspace that is in vapor equilibrium with the sample. This is a time dependent process with each sample having its own unique time constant for achieving equilibrium. You can only shorten that time by either making a prediction or taking the measurement before true equilibrium is reached, but this makes the results unreliable. Does the instrument you are considering allow for stringent equilibrium settings to ensure true and reliable results?
Video coming soon.

6. Detect Equilibrium

The vapor equilibrium that is required to measure water activity is determined by tracking the rate of water activity change. Some systems do not allow the user to make adjustments to rate that determines equilibrium while other systems not only allow the rate to be adjusted, but also show a graph of the equilibrium process so you can visually see when equilibrium has been achieved. Does the instrument you are considering allow you to adjust the equilibrium settings and watch a graph of the results during testing?
Video coming soon.

7. Measuring Volatiles

Sample that contain volatiles other than water can prove difficult to measure for water activity as the volatiles interfere the sensor. Some instruments simply cannot measure these types of sample while other are able to use filters to protect the sensor and still allow for water activity measurement. Is the system you are considering able to use filters to measure samples with volatiles?
Video coming soon.

8. Temperature Control

Water activity is temperature dependent and all water activity tests results must be reported with a temperature. Water activity instrumentation should have temperature control to prevent temperature change as a source of variation and to monitor the water activity at various temperatures. Does the instrument you are considering have temperature control?
Video coming soon.

9. Data Control

Water activity testing is an important monitoring tool to meet governmental regulations. Data integrity is critical for meeting audit requirements and 21 CFR Part 11 Compliance. Is the instrument you are considering Part 11 compliant?
Video coming soon.

We hope we were able to give you some tips on what you should pay attention to when purchasing a water activity meter. If you have any questions or want to learn more about measuring water activity for your product, please do not hesitate to contact us.