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Water activity meter reading over 1

  1. Oct 14, 2015 #1


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    Good evening. I recently was taught how to use our new water activity meter, and I noticed something strange.
    This meter uses changes in the reflectance of a mirror.

    Whenever we'd bring the sample up to temperature, the measurement would reach a low point, then at about 0.5° below our set temperature of 25°, the measurement would hit about 1.02 or 1.03. I'm curious if this is a quirk of the meter itself, or if there is a physical phenomenon underlying this. The underlying sample was a tomato paste with aw measured at about 0.99.
  2. jcsd
  3. Oct 19, 2015 #2
    When you say 'water activity' are you referring to the refractive index of water?
  4. Oct 20, 2015 #3


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    Nope, water activity is a measure of the saturation of the water vapor, used in food sciences: https://en.wikipedia.org/wiki/Water_activity

    Not that I knew it, had to google.
  5. Oct 20, 2015 #4


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    Which device are you using to get those measurements?
    How accurate is the measurement in general?
    Could be a simple accuracy issue, but it is odd that the device software doesn't filter it.
  6. Oct 20, 2015 #4


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    Science Advisor

    Well, that's pretty much the definition of activity in thermodynamics.

    Sounds like a dew point measurement.
    Last edited: Oct 20, 2015
  7. Oct 30, 2015 #5


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    Thank you for all the posts here.
    I've decided the readings above one are probably from hysteresis, probably due to a difference in frequency of sampling temperature and mirror fogginess.
    The makers of the device (it's an Aqualab 4) could assume that people would only use the result at a stable temperature.
    Since the device has a target temperature, it simply waits for the measurement to stabilize to a certain accuracy within range.

    Perhaps they display a preliminary value for convenience of the end user.
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